BORONIC ESTER AND ACID COMPOUNDS, SYNTHESIS AND USES

Abstract: Disclosed herein is a method for reducing the rate of degradation of proteins in an animal comprising contacting cells of the animal with certain boronic ester and acid compounds. Also disclosed herein are novel boronic ester and acid compounds, their synthesis and uses.

Description: Boronic Ester and Acid Compounds, Synthesis and Uses Cross-Reference to Related Applications This application is a continuation-in-part of U.S. application no. 08/442,581, filed May 16, 1995, which is a continuation-in-part of U.S. application no. 08/330,525, filed October 28, 1994, now abandoned, the contents of which are incorporated herein by reference. Background of the Invention 1. Field of the Invention The present invention relates to boronic ester and acid compounds, their synthesis and uses. 2. Description of Related Art The synthesis of N-terminal peptidyl boronic ester and acid compounds, in general and of specific compounds, has been described previously (Shenvi et al. USPN 4.499,082 issued February 12, 1985; Shenvi et al. USPN 4,537,773 issued August 27, 1985; Siman et al. WO 91/13904 published September 19, 1991 ; Kettner et al., J. Biol. Chem. 259(24):15\06-15U4 (1984)). These compounds have been shown to be inhibitors of certain proteolytic enzymes (Shenvi et al. USPN 4,499,082 issued February 12, 1985; Shenvi et al. USPN 4,537,773; Siman et al. WO 91/13904 published September 19, 1991; Kettner et al, J. Biol. Chem. J f24f . 15106-15114 (1984)). A class of N-terminal tri- peptide boronic ester and acid compounds has been shown to inhibit the growth of cancer cells (Kinder et al. USPN 5, 106,948 issued April 21, 1992). A broad class of N-terminal tri-peptide boronic ester and acid compounds and analogs thereof has been shown to inhibit renin (Kleeman et al. USPN 5,169,841 issued December s. 1992). In the cell, there is a soluble proteolytic pathway that requires ATP and involves covalent conjugation of the cellular proteins with the small polypeptide ubiquitin ("Ub") (Hershko et al, A. Rev. Biochem. 67:761-807 (1992); Rechsteiner et al, A. Rev. Cell. Biol. 5:1-30 (1987)). Thereafter, the conjugated proteins are hydrolyzed by a 26S proteolytic complex containing a 20S degradative particle called the proteasome (Goldberg, Eur. J. Biochem. 203:9-23 (1992); Goldberg et al, Nature 357:375-379 (1992)). This multicomponent system is known to catalyze the selective degradation of highly abnormal proteins and short-lived regulatory proteins. The 20S proteasome is composed of about 15 distinct 20-30 kDa subunits. It contains three different peptidase activities that cleave specifically on the carboxyl side of the hydrophobic, basic, and acidic amino acids (Goldberg et al, Nature 357:375-379 (1992); Goldberg, Eur. J. Biochem. 203:9-23 (1992); Orlowski, Biochemistry 29:10289 (1990); Rivett et al, Archs. Biochem. Biophys. 218:\ (1989); Rivett et al, J. Biol. Chem. 264:12,215-12,219 (1989); Tanaka et al, New Biol. 4:1-11 (1992)). These peptidase activities are referred to as the chymotrypsin-like activity, the trypsin-like activity, and the peptidylglutamyl hydrolyzing activity, respectively. Various inhibitors of the peptidase activities of the proteasome have been reported (Dick et al, Biochemistry 50:2725-2734 (1991); Goldberg et al, Nature 557:375-379 (1992); Goldberg, Eur. J. Biochem. 203:9-23 (1992); Orlowski, Biochemistry 29: 10289 (1990); Rivett et al, Archs. Biochem. Biophys. 2/5:1 (1989); Rivett etal, J. Biol Chem. 264:12,215-12,219 (1989); Tanaka etal, New Biol. 4:1-11 (1992); Murakami etal, Proc. Natl. Acad. Sci. U.S.A. 55:7588-7592 (1986); Li et al, Biochemistry 50:9709-9715 (1991); Goldberg, Eur. J. Biochem. 203:9-23 (1992); Aoyagi et al, Proteases and Biological Control, Cold Spring Harbor Laboratory Press (1975), pp. 429-454. Stein et al, U.S. patent application serial number 08/212,909 filed March 15, 1994, describe the use of peptide aldehydes to 1 ) reduce the rate of loss of muscle mass in an animal by contacting cells of the muscle with a peptide aldehyde proteasome inhibitor, 2) reduce the rate of intracellular protein breakdown in an animal by contacting cells of the animal with a peptide aldehyde proteasome inhibitor, and 3) reduce the rate of degradation of p53 protein in an animal by administering to the animal a peptide aldehyde proteasome inhibitor. Palombella et al. , PCT application serial number PCT US95/03315, filed March 17, 1995, describe the use of peptide aldehydes to reduce the cellular content and activity of NF-κB in an animal by contacting cells of the animal with a peptide aldehyde inhibitor of proteasome function or ubiquitin conjugation. The transcription factor NF-κB and other members of the rel family of protein complexes play a central role in the regulation of a remarkably diverse set of genes involved in the immune and inflammatory responses (Grilli et al, International Review of Cytology 143:1-62 (1993)). NF-κB exists in an inactive form in the cytoplasm complexed with an inhibitor protein, IκB. In order for the NF-KB to become active and perform its function, it must enter the cell nucleus. It cannot do this, however, until the IκB portion of the complex is removed, a process referred to by those skilled in the art as the activation of, or processing of, NF-KB. In some diseases, the normal performance of its function by the NF- KB can be detrimental to the health of the patient. For example, NF-κB is essential for the expression of the human immunodeficiency virus (HIV). Accordingly, a process that would prevent the activation of the NF-κB in patients suffering from such diseases could be therapeutically beneficial. Goldberg and Rock, WO 94/17816, filed January 27, 1994, describe the use of proteasome inhibitors to inhibit MHC-I antigen presentation. The ubiquitination/proteolysis pathway is shown to be involved in the processing of internalized cellular or viral antigens into antigenic peptides that bind to MHC-I molecules on an antigen presenting cell. Accordingly, inhibitors of this pathway would be useful for the treatment of diseases that result from undesired response to antigen presentation, including autoimmune diseases and transplant rejection. Cyclins are proteins that are involved in cell cycle control in eukaryotes. Cyclins presumably act by regulating the activity of protein kinases, and their programmed degradation at specific stages of the cell cycle is required for the transition from one stage to the next. Experiments utilizing modified ubiquitin (Glotzer et al, Nature 549:132-138 (1991); Hershko et al, J. Biol. Chem. 266:376 (1991)) have established that the ubiquitination/proteolysis pathway is involved in cyclin degradation. Accordingly, compounds that inhibit this pathway would cause cell cycle arrest and would be useful in the treatment of cancer, psoriasis, restenosis, and other cell proliferative diseases. Summary of the Invention The present invention provides previously unknown peptidyl boronic acid ester and acid compounds. The present invention also provides methods of using amino acid or peptidyl boronic ester and acid compounds, in general, as inhibitors of proteasome function. In a first embodiment, the present invention provides novel boronic acid and ester compounds having formula (la) or (2a), as set forth below. An additional aspect of the present invention is related to the discovery that amino acid and peptidyl boronic acids and esters, in general, are potent and highly selective proteasome inhibitors and can be employed to inhibit proteasome function. Inhibition of proteasome function has a number of practical therapeutic and prophylactic applications. In a second embodiment, the present invention provides a method for reducing the rate of muscle protein degradation in a cell comprising contacting said cell with a proteasome inhibitor having formula (lb) or (2b) as defined below. This aspect of the present invention finds practical utility in inhibiting (reducing or preventing) the accelerated breakdown of muscle proteins that accompanies various physiological and pathological states and is responsible to a large extent for the loss of muscle mass (atrophy) that follows nerve injury, fasting, fever, acidosis, and certain endocrinopathies. In a third embodiment, the present invention provides a method for reducing the activity of NF-κB in a cell comprising contacting the cell with a proteasome inhibitor of the formula (lb) or (2b), as set forth below. The inhibitors employed in the practice of the present invention are capable of preventing this activation. Thus, blocking NF-κB activity is contemplated as possessing important practical application in various areas of medicine, e.g., inflammation, sepsis, AIDS, and the like. In a fourth embodiment, the present invention provides a method of reducing the rate of degradation of p53 protein in a cell comprising administering to the cell a proteasome inhibitor of the formula (1 b) or (2b), as set forth below. In a fifth embodiment, the present invention provides a method for inhibiting cyclin degradation in a cell comprising contacting said cells with a proteasome inhibitor of the formula (lb) or (2b), as set forth below. Inhibiting cyclin degradation is contemplated as possessing important practical application in treating cell proliferative diseases, such as cancer, restenosis and psoriasis. In a sixth embodiment, the present invention provides a method for inhibiting the growth of a cancer cell, comprising contacting said cell with a proteasome inhibitor of the formula (la) or (2a), as set forth below. In a seventh embodiment, the present invention provides a method for inhibiting antigen presentation in a cell comprising administering to the cell a proteasome inhibitor of the formula (lb) or (2b), as set forth below. In an eighth embodiment, the present invention provides a method for inhibiting inducible NF-κB dependent cell adhesion in an animal comprising administering to said animal a proteasome inhibitor of the formula (lb) or (2b), as set forth below. In a ninth embodiment, the present invention provides a method for inhibiting HIV replication in an animal comprising administering to said animal a proteasome inhibitor of the formula (lb) or (2b), as set forth below. In a tenth embodiment, the present invention provides an approach for inhibiting cytolytic immune responses. The proteasome inhibitors of formula (lb) or (2b) can be used to inhibit the processing of internalized cellular or viral antigens into antigenic peptides that bind to MHC-I molecules in an animal, and are therefore useful for treating autoimmune diseases and preventing rejection of foreign tissues, such as transplanted organs or grafts. In an eleventh embodiment, the present invention provides pharmaceutical compositions that comprise compounds of formula (la), (lb), (2a) or (2b) in an amount effective to inhibit proteasome function in a mammal, and a pharmaceutically acceptable carrier or diluent. Brief Description of the Figures Figure 1. Three day cumulative urinary 3-methylhistidine. Figure 2. NF- B binding activity. Figure 3. Inhibition by MG-273. Description of the Preferred Embodiments A first aspect of the present invention is directed to novel subsets of boronic acid and ester compounds having formula (la) or (2a) below. Novel compounds of formula (la) include the following: P— N-l-B 1 — X 1 j— CH— X 2 — CH— B(Z 1 )(Z 2 ) I I R 1 R2 R 3 (la) or a pharmaceutically acceptable salt thereof; wherein P is hydrogen or an amino-group-protecting moiety as further defined herein; B\ at each occurrence, is independently one of N or CH; X 1 , at each occurrence, is independently one of-C(O)-NH-, -CH 2 -NH-, -CH(OH :H 2 -, -CH(OH)-CH(OH)-, -CH(OH)-CH 2 -NH-, -CH=CH-, -C(OXCH 2 -, -SO 2 -NH- -SO 2 -CH 2 - or -CH(0HH H 2 -C(0)-NH-, provided that when B 1 is N, then the X 1 attached to said B 1 is -C(O)-NH-; X 2 is one of -C(O)-NH-, -CH(OH)-CH 2 - -CH(OH)-CH(OH)-, -C(O)-CH 2 -, -SO 2 -NH-, -SO 2 -CH 2 - or -CH(OH)-CH 2 -C(O)-NH-; R is hydrogen or alkyl, or R forms together with the adjacent R', or when A is zero, forms together with the adjacent R 2 , a nitrogen-containing mono-, bi- or tri-cyclic, saturated or partially saturated ring system having 4-14 ring members, that can be optionally substituted by one or two of keto, hydroxy, alkyl, aryl, aralkyl, alkoxy or aryloxy; R 1 , at each occurrence, is independently one of hydrogen, alkyl, cycloalkyl, aryl, a 5-1 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 5 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; R 2 is one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 5 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; R 3 is one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 5 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; R 5 , in each instance, is one of aryl, aralkyl, alkaryl, cycloalkyl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -W-R 6 , where W is a chalcogen and R 6 is alkyl, where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; Z 1 and Z 2 are independently one of alkyl, hydroxy, alkoxy, or aryloxy, or together Z 1 and Z 2 form a moiety derived from a dihydroxy compound having at least two hydroxy groups separated by at least two connecting atoms in a chain or ring, said chain or ring comprising carbon atoms, and optionally, a heteroatom or heteroatoms which can be N. S, or O; and A is 0, l, or 2. Other novel boronic acid and ester derivatives include compounds having a single amino acid side-chain. These compounds have the following formula: and pharmaceutically acceptable salts thereof; wherein Y is one of R 8 -C(O)-, R 8 -SO 2 -, R 8 -NH-C(O)- or R 8 -O-C(O)-, where R 8 is one of alkyl, aryl, alkaryl, aralkyl, any of which can be optionally substituted, or when Y is R 8 -C(O)- or R 8 -SO 2 -, then R 8 can also be an optionally substituted 5-10 membered, saturated, partially unsaturated or aromatic heterocycle; X 3 is a covalent bond or -C(O)-CH 2 -; R 3 is one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R S , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; R 5 , in each instance, is one of aryl, aralkyl, alkaryl, cycloalkyl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -W-R 6 , where W is a chalcogen and R 6 is alkyl, where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; and Z 1 and Z 2 are independently alkyl, hydroxy, alkoxy, aryloxy, or together form a moiety derived from dihydroxy compound having at least two hydroxy groups separated by at least two connecting atoms in a chain or ring, said chain or ring comprising carbon atoms, and optionally, a heteroatom or heteroatoms which can be N, S, or O; provided that when Y is RN2(O)-, R 8 is other than phenyl, benzyl or C, - C 3 alkyl. Alternatively, the group Y in formula (2a) above, can be: P is one of R 7 -C(O)-, R 7 -SO 2 -, R 7 -NH-C(O)- or R 7 -O-C(O)-; R 7 is one of alkyl, aryl, alkaryl, aralkyl, any of which can be optionally substituted, or when Y is R 7 -C(O)- or R 7 -SO 2 -, R 7 can also be an optionally substituted 5-10 membered saturated, partially unsaturated or aromatic heterocycle; and R 1 is defined above as for formula (la). Pharmaceutical compositions that comprise compounds of formula (la) or (2a) in an amount effective to inhibit proteasome function in a mammal, and a pharmaceutically acceptable carrier or diluent are within the scope of the present invention. A second aspect of the present invention lies in the discovery that boronic acid and ester derivatives of amino acids and peptides, in general, as well as isosteric variations thereof, inhibit proteasome function. Thus, the present invention also relates to the use of proteasome inhibitors having formula (1 b) or (2b) for reducing the rate of proteasome dependent intracellular protein breakdown, such as reducing the rate of muscle protein degradation, reducing the rate of degradation of p53 protein, and inhibiting cyclin degradation, and for inhibiting the activity of NF-κB in a cell. Finally, the present invention relates to the use of proteasome inhibitors having formula (lb) or (2b) for treating specific conditions in animals that are mediated or exacerbated, directly or indirectly, by proteasome functions. These conditions include inflammatory conditions, such as tissue rejection, organ rejection, arthritis, infection, dermatoses, inflammatory bowel disease, asthma, osteoporosis, osteoarthritis and autoimmune disease such as lupus and multiple sclerosis: cell proliferative diseases, such as cancer, psoriasis and restenosis; and accelerated muscle protein breakdown that accompanies various physiological and pathological states and is responsible to a large extent for the loss of muscle -10- mass (atrophy) that follows nerve injury, fasting, fever, acidosis, and certain endocrinopathies. Proteasome inhibitors of formula (lb) include: or a pharmaceutically acceptable salt thereof; wherein P 10 is hydrogen or an amino-group-protecting moiety; B" is independently one of N or CH; X", at each occurrence, is independently one of-C(O)-NH-, -CH 2 -NH-, -CH(OH)-CH 2 -, -CH(OH)-CH(OHX, -CH(OH)-CH 2 -NH-, -CH=CH-, -C(O>-CH 2 -, -SO 2 -NH- -SO 2 -CH 2 - or -CH(OH)-CH 2 -C(O)-NH-, provided that when B u is N, then X" is -C(O)-NH; X 12 is one of -C(O)-NH- -CH(OH)-CH 2 -, -CH(OH)-CH(OH)-, -C(O)-CH 2 -, -SO 2 -NH- -SO 2 -CH 2 - or -CH(OH)-CH 2 -C(O)-NH-; R 10 is hydrogen or alkyl, or R 10 forms together with the adjacent R 1 ' , or when A' 0 is zero, forms together with the adjacent R 12 , a nitrogen-containing mono-, bi- or tri-cyclic, saturated or partially saturated ring system having 4-14 ring members, that can be optionally substituted by one or two of keto, hydroxy, alkyl, aryl, aralkyl, alkoxy or aryloxy; R n , at each occurrence, is independently one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 15 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; R 12 and R 13 are each independently one of hydrogen, alkyl, cycloalkyl, aryl, a 5- 10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 15 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted, where R 15 is aryl, aralkyl, alkaryl, cycloalkyl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle, or -chalcogen-alkyl, where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; Z 1 ' and Z 12 are independently alkyl, hydroxy, alkoxy, aryloxy, or Z 1 ' and Z 12 together form a moiety derived from a dihydroxy compound having at least two hydroxy groups separated by at least two connecting atoms in a chain or ring, said chain or ring comprising carbon atoms, and optionally, a heteroatom or heteroatoms which can be N, S, or O; and A 10 is 0, 1, or 2 Proteasome inhibitors of formula (2b) include: or pharmaceutically acceptable salts thereof; wherein Y 10 is one of R 8 -C(O)-, R 8 -SO 2 - R 8 -NH-C(O)- or R -O-C(O)-, where R 8 is one of alkyl, aryl, alkaryl, aralkyl, any of which can be optionally substituted, or when Y is R 8 -C(O)- or R 8 -SO 2 -, then R 8 can also be an optionally substituted 5-10 membered, saturated, partially unsaturated or aromatic heterocycle; X' 3 is a covalent bond or -C(O)-CH 2 -; R 13 is one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or-CH 2 -R 15 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; R 15 , in each instance, is one of aryl, aralkyl, alkaryl, cycloalkyl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -W-R 16 , where W is a chalcogen and R 16 is alkyl, where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; and Z" and Z' 2 are independently alkyl, hydroxy, alkoxy, aryloxy. or together form a moiety derived from a dihydroxy compound having at least two hydroxy -12- groups separated by at least two connecting atoms in a chain or ring, said chain or ring comprising carbon atoms, and optionally, a heteroatom or heteroatoms which can be N, S, or O. Alternatively, the group Y in formula (2b) can be: P is one of R 7 -C(OX, R 7 -SO 2 -, R 7 -NH-C(O)- or R 7 -O-C(O)-; R 7 is one of alkyl, aryl, alkaryl, aralkyl, any of which can be optionally substituted, or when Y is R 7 -C(O)- or R 7 -SO 2 -, R 7 can also be an optionally substituted 5-10 membered saturated, partially unsaturated or aromatic heterocycle; and R 1 is as defined for formula (la) above. Preferred embodiments of the aforementioned methods of use employ compounds of formula (la) and formula (2a) as defined above. Pharmaceutical compositions comprising an effective amount of the proteasome inhibitors of formula (2a) or (2b), in combination with any conventional pharmaceutically acceptable carrier or diluent, are included in the present invention. The term "amino-group-protecting moiety," as used herein, refers to terminal amino protecting groups that are typically employed in organic synthesis, especially peptide synthesis. Any of the known categories of protecting groups can be employed, including acyl protecting groups, such as acetyl, and benzoyl; aromatic urethane protecting groups, such as benzyloxycarbonyl; and aliphatic urethane protecting groups, such as tert- butoxycarbonyl. See, for example, The Peptides, Gross and Mienhoffer, eds., Academic Press, New York (1981), Vol. 3, pp. 3-88; and Green, T.W. & Wuts, P.G.M.. Protective Groups in Organic Synthesis, 2nd edition, John Wiley and Sons, Inc., New York (1991). Preferred protecting groups include aryl-, aralkyl-, heteroaryl- and heteroarylalkyl- carbonyl and sulfonyl moieties. As used herein, the term "heterocycle" is intended to mean a stable 5- to 7- membered monocyclic or 7- to 10-membered bicyclic heterocyclic moieties that are either saturated or unsaturated, and which consist of carbon atoms and from 1 to 4 heteroatoms independently selected from the group consisting of N, O and S, wherein the nitrogen and sulfur heteroatoms can optionally be oxidized, the nitrogen can optionally be quatemized, and including any bicyclic group in which any of the above-defined heterocyclic rings is fused to a benzene ring. The heterocyclic ring can be attached to its pendant group at any heteroatom or carbon atom that results in a stable formula. The heterocyclic rings described herein can be substituted on carbon or on a nitrogen atom if the resulting compound is stable. Examples of such heterocycles include, but are not limited to, pyridyl, pyrimidinyl, furanyl, thienyl, pyrrolyl, pyrazolyl, imidazolyl, tetrazolyl, benzofuranyl, benzothiophenyl, indolyl, indolenyl, quinolinyl, isoquinolinyl, benzimidazolyl, piperidinyl, pyrrolidinyl, 2-pyrrolidonyl, pyrrolinyl, tetrahydrofuranyl, tetrahydroquinolinyl, tetrahydroisoquinolinyl, decahydroquinolinyl or octahydroisoquinolinyl, azocinyl, triazinyl, 6H-l,2,5-thiadiazinyl, 2H,6H-l,5,2-dithiazinyl, thiophene(yl), thianthrenyl, furanyl, pyranyl, isobenzofuranyl, chromenyl, xanthenyl, phenoxathiinyl, 2H-pyrrolyl, pyrrole, imidazolyl, pyrazolyl, isothiazoiyl, isoxazolyl, pyridinyl, pyrazinyl, pyrimidinyl, pyridazinyl, indolizinyl, isoindolyl, 3H-indolyl, indolyl, lH-indazolyl, purinyl, 4H-quinolizinyl, isoquinolinyl, quinolinyl, phthalazinyl, naphthyridinyl, quinoxalinyl, quinazolinyl, cinnolinyl, pteridinyl, 4aH-carbazolyl, carbazolyl, β-carbolinyl, phenanthridinyl, acridinyl, phenanthrolinyl, phenazinyl, phenothiazinyl, furazanyl, phenoxazinyl, isochromanyl, chromanyl, pyrrolidinyl, imidazolidinyl, imidazolinyl, pyrazolidinyl, pyrazolinyl, piperazinyl, indolinyl, isoindolinyl, quinuclidinyl, mo holinyl or oxazolidinyl. Also included are fused ring and spiro compounds containing, for example, the above heterocycles. The term "substituted", as used herein, means that one or more hydrogens of the designated moiety are replaced with a selection from the indicated group, provided that no atom's normal valency is exceeded, and that the substitution results in a stable compound. When a substituent is keto (i.e., =O), then 2 hydrogens attached to an atom of the moiety are replaced. By "stable compound" or "stable formula" is meant herein a compound that is sufficiently robust to survive isolation to a useful degree of purity from a reaction mixture and formulation into an efficacious therapeutic agent. The term "heteroaryl" as employed herein refers to groups having 5 to 14 ring atoms; 6, 10 or 14 π electrons shared in a cyclic array; and containing carbon atoms and 1, 2 or 3 oxygen, nitrogen or sulfur heteroatoms (where examples of heteroaryl groups are: thienyl, benzo[b]thienyl, naphtho[2,3-b]thienyl, thianthrenyl, furyl, pyranyl, isobenzofuranyl, benzoxazolyl, chromenyl, xanthenyl, phenoxathiinyl, 2H-pyrrolyl, pyrrolyl, imidazolyl, pyrazolyl, pyridyl, pyrazinyl, pyrimidinyl, pyridazinyl, indolizinyl, isoindolyl, 3H-indolyl, indolyl, indazolyl, purinyl, 4H-quinolizinyl, isoquinolyl, quinolyl, phthalazinyl, naphthyridinyl, quinazolinyl, cinnolinyl, pteridinyl, 4aH-carbazolyl, carbazolyl, β-carbolinyl, phenanthridinyl, acridinyl, perimidinyl, phenanthrolinyl, phenazinyl, isothiazoiyl, phenothiazinyl, isoxazolyl, furazanyl and phenoxazinyl groups). The terms "substituted heteroaryl" or "optionally substituted heteroaryl," used in reference to R 1 , refer to heteroaryl groups, as defined above, having one or more substituents selected from halogen, C,^ alkyl, C,. 6 alkoxy, carboxy, amino, C,. 6 alkylamino and/or di(C,. 6 )alkylamino. The term "aryl" as employed herein by itself or as part of another group refers to monocyclic or bicyclic aromatic groups containing from 6 to 12 carbons in the ring portion, preferably 6-10 carbons in the ring portion, such as phenyl, naphthyl or tetrahydronaphthyl. The term "substituted aryl" as employed herein includes aryl groups, as defined above, that include one or two substituents on either the phenyl or naphthyl group selected from C,. 6 alkyl, C 3 . g cycloalkyl, C,^alkyl(C 3 . g )cycloalkyl, C 2 . 8 alkenyl, C 2 . g alkynyl. cyano, amino, C,. 6 alkylamino, di(C,. 6 )alkylamino, benzylamino. dibenzylamino, nitro, carboxy, carbo(C,. 6 )alkoxy, trifluoromethyl, halogen, C 1-6 alkoxy, C^^ aryl(C w )alkoxy, hydroxy, C,. 6 alkylthio, C,^ alkylsulfinyl, C alkylsulfonyl, ^o aryl, C^o arylthio, C^^arylsulfinyl and/or C 6 . 10 arylsulfonyl. The term "alkyl" as employed herein includes both straight and branched chain radicals of up to 12 carbons, preferably 1-8 carbons, such as methyl, ethyl, propyl, isopropyl, butyl, t-butyl, isobutyl, pentyl, hexyl, isohexyl, heptyl, 4,4- dimethylpentyl, octyl, 2,2,4-trimethylpentyl, nonyl, decyl, undecyl and dodecyl. The term "substituted alkyl" as employed herein includes alkyl groups as defined above that have one, two or three halo substituents, or one C,^ alkyl(C 6.10 )aryl, halo(C 6-10 )aryl, C 3 . g cycloalkyl, C,. 6 alkyl(C 3 . g )cycloalkyl, C 2 . g alkenyl, C 2 . g alkynyl, hydroxy and/or carboxy. The term "cycloalkyl" as employed herein includes saturated cyclic hydrocarbon groups containing 3 to 12 carbons, preferably 3 to 8 carbons, which include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclodecyl and cyclododecyl, any of which groups can be substituted with substituents such as halogen, C,. 6 alkyl, alkoxy and/or hydroxy group. The term "aralkyl" or "arylalkyl" as used herein by itself or as part of another group refers to C^alkyl groups as discussed above having an aryl substituent, such as benzyl. The term "halogen" or "halo" as used herein by itself or as part of another group refers to chlorine, bromine, fluorine or iodine with chlorine being preferred. For medicinal use, the pharmaceutically acceptable acid and base addition salts, those salts in which the anion does not contribute significantly to toxicity or pharmacological activity of the organic cation, are preferred. Basic salts are formed by mixing a solution of a boronic acid (Z 1 and Z 2 are both OH) of the present invention with a solution of a pharmaceutically acceptable non-toxic base, such as, sodium hydroxide, potassium hydroxide, sodium bicarbonate, sodium carbonate, or an amino compound, such as choline hydroxide, Tris, bis-Tris, N- methylglucamine or arginine. Water-soluble salts are preferable. Thus, suitable salts include: alkaline metal salts (sodium, potassium etc.), alkaline earth metal salts (magnesium, calcium etc.), ammonium salts and salts of pharmaceutically acceptable amines (tetramethylammonium, triethylamine, methylamine, dimethylamine, cyclopentylamine, benzylamine, phenethylamine, piperidine monoethanolamine, diethanolamine, tris(hydroxymethyl)amine, lysine, arginine and N-methyl-D-glucamine). The acid addition salts are obtained either by reaction of an organic base of formula (la) or (2a) with an organic or inorganic acid, preferably by contact in solution, or by any of the standard methods detailed in the literature available to any practitioner skilled in the art. Examples of useful organic acids are carboxylic acids such as maleic acid, acetic acid, tartaric acid, propionic acid, fumaric acid, isethionic acid, succinic acid, cyclamic acid, pivalic acid and the like; useful inorganic acids are hydrohalide acids such as HCl, HBr, HI; sulfuric acid; phosphoric acid and the like. Preferred acids for forming acid addition salts include HCl and acetic acid. The boronate esters of boronic acid compounds of the present invention are also preferred. These esters are formed by reacting the acid groups of the boronic acid with a hydroxy compound. Preferred hydroxy compounds are dihydroxy compounds, especially pinacol, perfluoropinacol, pinanediol, ethylene glycol, diethylene glycol, 1,2-cyclohexanediol, 1,3-propanediol, 2,3-butanediol, glycerol or diethanolamine. The P moiety of the proteasome inhibitor of formula (la) is preferably one of K 7 -C(θy, R 7 -SO 2 - R 7 -NH-€(O)- or R 7 -O-C(O)-, and R 7 is one of alkyl, cycloalkyl, aryl, aralkyl, heteroaryl or heteroarylalkyl, the ring portion of any of which can be optionally substituted, or if Y is R 7 -C(O)- or R 7 -SO 2 -, then R 7 can also be a saturated or partially unsaturated heterocycle. More preferably, P is one of R 7 -C(O)- or R 7 -SO 2 -, and R 7 is one of aryl, aralkyl, heteroaryl or heteroarylalkyl, any of which can be optionally substituted, or a saturated or partially unsaturated heterocycle. Where R 7 is alkyl, it is preferably straight chained or branched alkyl of from 1 to 6 carbon atoms, more preferably 1-4 carbon atoms. Useful values include methyl, ethyl, propyl, butyl, isopropyl, isobutyl and tert-butyl, with methyl being most preferred. Additionally, where R 7 is alkaryl, aralkyl or heteroarylalkyl, the alkyl moiety thereof is also preferably one having from 1 to 4 carbon atoms, and most preferably 1 carbon atom. Where R 7 is aryl, it is preferably aryl of from 5 to 10 carbon atoms, more preferably 6 to 10 carbon atoms. Where R 7 is heteroaryl, one or more of the carbon atoms of the aforementioned aryl is replaced by one to three of O, N, or S. The aryl and heteroaryl moieties may, if desired, be ring substituted. Useful ring substituents include one or two of hydroxy, nitro, trifluoromethyl, halogen, alkyl, alkoxy, cyano, C^^aryl, benzyl, carboxyalkoxy, amino, and guanidino. Preferred substituents include halogen, C,^ alkyl, C M alkoxy, phenyl and benzyl. Additionally, where R 7 is alkaryl, aralkyl or heteroarylalkyl, the above statements equally apply. Useful R 7 aryl and aralkyl groups include phenyl, 4-tolyl, benzyl, phenethyl, naphthyl, and naphthylmethyl. Preferred heteroaryl groups are quinolinyl, quinoxalinyl, pyridyl, pyrazinyl, furanyl or pyrrolyl. Useful values of R 7 heteroaryl include 8-quinolinyl, 2-quinoxalinyl, 2-pyrazinyl, 3-furanyl, 2-pyridyl, 3-pyridyl and 4- pyridyl. Preferred saturated or partially saturated heterocycle moieties are 5-, 6-, 9- and 10- membered heterocycles having one, two or three ring heteroatoms selected from O, S or N. A useful value is N-morpholinyl. Preferred cycloalkyl moieties include C 3 ., 0 cycloalkyl. Useful values include cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl and cyclononyl. Especially preferred values of P are 2-pyrazinecarbonyl, 8-quinolinesulfonyl and N-morpholinoyl. As noted above, A in formula (la) and (lb) can be either 0, 1 or 2. Thus, when A is zero, the residue within the brackets is not present and the inhibitor is a dipeptide. Similarly, where A is 1 , the amino acid or isosteric residue within the brackets is present and the inhibitor is a tripeptide. Where A is 2, the inhibitor is a tetrapeptide. Most preferably, A is zero. It is preferred that R 1 , R 2 , and R 3 in formula (la) and (lb) are each independently one of hydrogen, C,. g alkyl, C 3 . 10 cycloalkyl, ., 0 aryl, a 5-, 6-, 9- or 10- membered heteroaryl group, or -CH 2 -R 5 , and more preferably C,. g alkyl or — CH 2 — R 5 wherein R 1 , R 2 , R 3 and R 5 are optionally substituted. More preferably, R 1 , R 2 and R 3 are each independently one of C alkyl, e.g., methyl, ethyl, propyl, butyl, isopropyl, isobutyl, sec-butyl and t-butyl, or — CH 2 — R 5 , where R 5 is one of cycloalkyl, aryl or heterocycle. R 5 is preferably one of C 6.!0 aryl, . , 0 ar(C,. 6 )alkyl, C,. 6 alk(C 6 . 10 )aryl, C 3 . 10 cycloalkyl, C , . , alkoxy, C ,. g alkylthio or a 5-, 6-, 9- or 10- membered heteroaryl group. The ring portion of any of said aryl, aralkyl, alkaryl or 5-, 6-, 9- or 10- membered heteroaryl groups of R\ R 2 , R 3 and R 5 can be optionally substituted by one or two substituents independently selected from the group consisting of C, .6 alkyl, C 3 .„ cycloalkyl, C,^ alkyl(C 3 . g )cycloalkyl, C 2 . g alkenyl, C 2 . g alkynyl, cyano, amino, C,^ alkylamino, di(C,^)alkylamino, benzylamino, dibenzylamino, nitro, carboxy, aryl(C,. 6 )alkyl, C^oaryKC^alkoxy, hydroxy, C,. 6 alkylthio, C, -6 alkylsulfinyl, C, .6 alkylsulfonyl, .,,, arylthio, .,,, arylsulfinyl, C^ arylsulfonyl, .,,, aryl, C, .6 alkyl(C < l0 )aryl, and halo(C 6 . 10 )aryl. It is more preferred that at least one of R' and R 2 is isobutyl or — CH 2 — R 5 , and most preferred that R 2 is — CH 2 — R 5 . It is preferred that R 5 is C 6 ., 0 aryl, a 5-, 6-, 9- or 10- membered heteroaryl group having one to three heteroatoms independently selected from O, N and S. Most preferably, R 2 is isobutyl, 6-quinolinylmethyl, 3-indolylmethyl, 4- pyridylmethyl, 3-pyridylmethyl, 2-pyridylmethyl, benzyl, 1 -naphthylmethyl, 2- naphthylmethyl, 4-fluorobenzyl, 4-benzyloxybenzyl, 4-(2'- pyridylmethoxy)benzyl or benzylnaphthylmethyl. Preferably, R 3 is C,. 12 alkyl, more preferably C,^ alkyl, most preferably C 4 alkyl, such as isobutyl. Where R 1 , R 2 or R 3 is a substituted alkyl, it is preferably C M alkyl substituted with at least one cycloalkyl group, preferably a C M cycloalkyl group. Where R 1 , R 2 , R 3 , or R 5 is substituted aryl or substituted heterocycle, it is preferably substituted with at least one C M alkyl group. Where R 1 , R 2 , R 3 or R 5 is cycloalkyl, it is preferably C 5-6 cycloalkyl, e.g., cyclopentyl or cyclohexyl, and can be optionally substituted with at least one .,,, aryl group or at least one alkyl group, preferably a C M alkyl group. Where R 5 is -W-R 6 , W is a chalcogen, preferably oxygen or sulfur, more preferably sulfur; and R 6 is alkyl, preferably C M alkyl, e.g., methyl, ethyl, propyl, butyl, or isomers thereof. Preferred values of R include hydrogen or C,. g alkyl, more preferably C M alkyl. Useful values of R include methyl, ethyl, isopropyl, isobutyl and n-butyl. Additionally, R can form together with the adjacent R 1 , or when A is zero, form together with the adjacent R 2 , a nitrogen-containing mono-, bi- or tri-cyclic, saturated or partially saturated ring system having 4-14 ring members, and can be optionally substituted by one or two of keto, hydroxy, aryl, alkoxy or aryloxy. It is preferred that the ring system be chosen from one of: The nitrogen in each of the above formulae is attached to P in formula ( 1 a) and the open valence carbon is attached to either X 1 or X 2 . It is preferred that Z 1 and Z 2 are each independently one of C alkyl, hydroxy, C l-6 alkoxy, and ., 0 aryloxy; or together Z 1 and Z 2 preferably form a moiety derived from a dihydroxy compound selected from the group consisting of pinacol, perfluoropinacol, pinanediol, ethylene glycol, diethylene glycol, 1 ,2- cyclohexanediol, 1 ,3-propanediol, 2,3-butanediol, glycerol or diethanolamine, or other equivalents apparent to those skilled in the art. Useful values include methyl, ethyl, propyl and n-butyl. Most preferably, Z 1 and Z 2 are hydroxy. A preferred embodiment of the invention is directed to a subgenus of compounds having formula (la) above, where P is R 7 -C(O)- or R 7 -SO : -. and R 7 is one of quinolinyl, quinoxalinyl, pyridyl, pyrazinyl, furanyl or pyrrolyl, and when P is R 7 -C(O)-, R 7 can also be N-morpholinyl. A preferred group of compounds of this embodiment are compounds of formula (la) wherein P is one of quinolinecarbonyl, pyridinecarbonyl, quinolinesulfonyl, quinoxalinecarbonyl, quinoxalinesulfonyl, pyrazinecarbonyl, pyrazinesulfonyl, furancarbonyl, furansulfonyl or N-morpholinylcarbonyl; A is zero; X 2 is -C(O)-NH-; R is hydrogen or C,. g alkyl; R 2 and R 3 are each independently one of hydrogen, C,. g alkyl, C 3-I0 cycloalkyl, C^oaryl, C 6 ., ( ^r(C ,. 6 )alkyl, pyridylmethyl, or quinoliny lmethy 1; and Z 1 and Z 2 are both hydroxy, C,. 6 alkoxy, or C 6.)0 aryloxy, or together Z' and Z 2 form a moiety derived from a dihydroxy compound selected from the group consisting of pinacol, perfluoropinacol, pinanediol, ethylene glycol, diethylene glycol, 1,2-cyclohexanediol, 1,3-propanediol, 2,3-butanediol, glycerol or diethanolamine. Even more preferred are those compounds wherein: P is 8-quinolinecarbonyl, 8-quinolinesulfonyl, 2-quinoxalinecarbonyl, 2-quinoxalinesulfonyl, 2-pyrazinecarbonyl, 2-pyrazinesulfonyl. 3- pyridinecarbonyl. 3-pyridinesulfonyl, 3 -furancarbonyl, 3-furansulfonyl or N-morpholinecarbonyl; R is hydrogen; R 3 is isobutyl; R : is isobutyl. 1-naphthylmethyl. 2-naphthylmethyl, 3-pyridylmethyl, 2-pyridylmethyl 6- quinolinylmethyl, 3-indolylmethyl, benzyl, 4-fluorobenzyl, 4-hydroxy benzyl, 4-(2'-pyridylmethoxy)benzyI, 4-(benzyloxy)benzyl, benzylnaphthylmethyl or phenethyl; and Z 1 and Z 2 are both hydroxy, or together Z l and Z 2 form a moiety derived from a dihydroxy compound selected from the group consisting of pinacol, perfluoropinacol, pinanediol, ethylene glycol, diethylene glycol, 1 ,2-cyclohexanediol, 1 ,3-propanediol, 2,3-butanediol, glycerol or diethanolamine. Another preferred embodiment of the present invention is directed to compounds of formula (la) where A is zero. These compounds possess unexpectedly high potency and selectivity as inhibitors of proteasome function. A third preferred subgenus of compounds are compounds of formula ( 1 a) where one of R 1 , R 2 or R 3 corresponds to an amino acid side-chain corresponding to tyrosine or an O-substituted tyrosine derivative, formed by reacting the hydroxyl group of the tyrosine side-chain with a compound having a reactive functional group. This subgenus includes compounds having the formula (la), wherein at least one R 1 , R 2 or R 3 is: where R 9 is one of hydrogen, alkyl, cycloalkyl, aryl, aralkyl, heteroaryl or heteroarylalkyl, wherein the alkyl is optionally substituted with one of C,^ alkyl, halogen, monohalo (C l-6 ) alkyl, and trifluoromethyl; and wherein said cycloalkyl, aryl, aralkyl, heteroaryl and heteroarylalkyl groups can be optionally substituted with one or two of C._ 6 alkyl, C 3 . g cycloalkyl, C 1-6 alkyl(C 3 . g )cycloalkyl, C 2 . g alkenyl, C 2-g alkynyl, cyano, amino, C,^ alkylamino, di(C,. 6 )alky lamino, benzylamino, dibenzylamino, nitro, carboxy, carbo(C,. 6 )alkoxy, trifluoromethyl, halogen, C,. 6 alkoxy, C^ 10 ryl, C O aryl(C,. 6 )alkyl, C^oar C^alkoxy, hydroxy, C, .6 alkylthio, C^alkylsulfinyl, C,. 6 alkylsulfonyl, C 6 . 10 arylthio, C^^arylsulfinyl, C 6 . 10 arylsulfonyl, C,. 6 alkyl(C 6-l0 )aryl, and halo(C 6 . l0 )aιyl; and A' and A 2 are independently one of hydrogen, C^alkyl, halogen, monohalo(C 1-6 )alkyl, or trifluoromethyl. The group -O-R 9 is in either the ortho- or para- position, with para- being preferred. The groups A' and A 2 can be at any remaining positions on the phenyl ring. It is preferred that R 9 is one of C,. g alkyl, C 3 . 10 cycloalkyl, C 6 ., 0 aryl, ^ar (C,. 6 )alkyl, 5- to 10- membered heteroaryl or 5- to 10-membered heteroaryl(C,. 6 )alkyl. Useful values of R 9 include benzyl, phenethyl, pyridyl, pyridy lmethy 1, furanylmethyl pyrrolymethyl, pyrrolidylmethyl, oxazolylmethyl and imidazoly lmethy 1. The ring portion of any of said aryl, aralkyl, alkaryl or 5-, 6-, 9- or 10- membered heteroaryl groups of R 1 , R\ R 3 and R 5 can be optionally substituted by one or two substituents independently selected from the group consisting of C,. 6 alkyl, C 3-g cycloalkyl, C,. 6 alkyl(C 3 . g )cycloalkyl, C 2 . g alkenyl, C 2 . g alkynyl, cyano, amino, C,^ alkylamino, di(C,. 6 )alky lamino, benzylamino, dibenzy lamino, nitro, carboxy, carbo(C,^)alkoxy, trifluoromethyl, halogen, C,. 6 alkoxy, C^^aryl, C 6 . 10 aryl(C, .6 )alkyl, C 6. , 0 aryl(C l . 6 )alkoxy, hydroxy, C, -6 alkylthio, C,. 6 alkylsulfinyl, C,. 6 alkylsulfonyl, ., 0 arylthio, .,,, arylsulfinyl, C^^ arylsulfonyl, C^ aiyl, C , .6 alky C^^aryl, and halo(C 6 . l0 )aryl. A preferred class of compounds of this embodiment are compounds of formula (la) wherein: A is zero; P is one of R 7 -C(O)-, R 7 -SO 2 - R 7 -NH-C(O)- or R 7 -O-C(OX; R 7 is one of quinolinyl, quinoxalinyl, pyridyl, pyrazinyl, furanyl or pyrrolyl, or when P is R 7 -C(O)-, R 7 can also be N-morpholinyl; X 2 is -C(O)-NH-; R 3 is C,. 6 alkyl; R 2 is: where A 1 and A 2 are independently one of hydrogen, C,. 6 alkyl, halogen, monohalo(C,. 6 )alkyl or trifluoromethyl; and R 9 is one of hydrogen, C phenyl, benzyl, phenethyl or pyridylmethyl; and Z 1 and Z 2 are both hydroxy, C,. 6 alkoxy, or .ioaryloxy, or together Z 1 and Z 2 form a moiety derived from a dihydroxy compound selected from the group consisting of pinacol, perfluoropinacol, pinanediol, ethylene glycol, diethylene glycol, 1 ,2-cyclohexanediol, 1,3-propanediol, 2,3-butanediol, glycerol or diethanolamine. Even more preferred are compounds of formula (la) wherein: A is zero; P is 8-quinolinecarbonyl, 8-quinolinesulfonyl, 2-quinoxalinecarbonyl, 2-quinoxalinesulfonyl, 2-pyrazinecarbonyl, 2-pyrazinesulfonyl, 3- pyridinecarbonyl, 3-pyridinesulfonyl, 3-furancarbonyl, 3-furansulfonyl or N-morpholinecarbonyl; X 2 is -C(O)-NH-; R 3 is isobutyl; R 2 is: where A ' and A 2 are independently one of hydrogen, methyl, ethyl, chloro, fluoro, or trifluoromethyl; and R 9 is one of hydrogen, methyl, ethyl, butyl, phenyl, benzyl, phenethyl or pyridylmethyl; and Z 1 and Z 2 are both hydroxy, or together Z 1 and Z 2 form a moiety derived from a dihydroxy compound selected from the group consisting of pinacol, perfluoropinacol, pinanediol, ethylene glycol, diethylene glycol, 1 ,2-cyclohexanediol, 1 ,3-propanediol, 2,3-butanediol, glycerol or diethanolamine. A fourth preferred subgenus of compounds includes compounds of formula (la) wherein one of the amino acid side-chains, preferably the side-chain defined by R 2 , is an unnatural amino acid selected from naphthy lmethy 1, pyridylmethyl and quinolinylmethyl, with quinolinylmethyl being most preferred. Thus, this subgenus includes compounds of formula (la), wherein at least one R 1 , R 2 or R 3 is naphthylmethyl, pyridylmethyl or quinolinylmethyl; provided that the compound is other than isovaleryl-phenylalanine-norvaline-[(naphthylmethyl), (4,4,5,5-tetramethyl-l,3,2-dioxaborolane-2-yl)]methylamide or (3-t- butylsulfonyl)propionyl-norvaline-(l -naphthyl, dihydroxyboryl)methylamide. A fifth preferred subgenus includes compounds of formula (la) where R, together with R 1 , or with R 2 when A is zero, forms a nitrogen containing heterocycle. This subgenus includes compounds having formula (la), wherein: R forms together with the adjacent R', or when A is zero, forms together with the adjacent R 2 , a nitrogen-containing mono-, bi- or tri-cyclic, saturated or partially saturated ring system having 4-14 ring members, and one or two optional substituents selected from the group consisting of keto, hydroxy, aryl, alkoxy and aryloxy; when A is 2, the R' that is not adjacent to N-R is one of hydrogen, alkyl, cycloalkyl, aryl, heterocycle or -CH 2 -R 5 ; and when A is 1 or 2, R 2 is one of hydrogen, alkyl, cycloalkyl, aryl, heterocycle or -CH 2 -R 5 , where R 5 is defined as above. A preferred class of compounds of this embodiment of the invention are those wherein: A is zero; P is hydrogen; X 2 is -C(O}-NH-; and R forms together with the adjacent R 2 , one of the nitrogen-containing ring systems shown in the above structures; R 3 is C^alkyl; and Z 1 and Z 2 are both hydroxy, C, -6 alkoxy, or C 6 ., 0 aryloxy, or together Z 1 and Z 2 form a moiety derived from a dihydroxy compound selected from the group consisting of pinacol, perfluoropinacol, pinanediol, ethylene glycol, diethylene glycol, 1,2-cyclohexanediol, 1,3-propanediol, 2,3-butanediol, glycerol or diethanolamine. The hydrochloride salts of these compounds are also especially preferred. Even more preferred are those compounds wherein R forms together with the adjacent R 2 , a nitrogen-containing ring system having one of the structures shown above; R 3 is isobutyl; and Z 1 and Z 2 are both hydroxy, or together Z 1 and Z 2 form a moiety derived from a dihydroxy compound selected from the group consisting of pinacol, perfluoropinacol, pinanediol, ethylene glycol, diethylene glycol, 1,2-cyclohexanediol, 1,3-propanediol, 2,3-butanediol, glycerol or diethanolamine. Examples of suitable proteasome inhibitors include without limitation the following compounds, as well as pharmaceutically acceptable salts and boronate esters thereof: N-(4-morpholine)carbonyl-β-(l -naphthyl)-L-alanine-L-leucine boronic acid, N-(8-quinoline)sulfonyl-β-(l -naphthyl)-L-alanine-L-leucine boronic acid, N-(2-pyrazine)carbonyl-L-phenylalanine-L-leucine boronic acid, L-proline-L-leucine boronic acid, N-(2-quinoline)carbonyl-L-homophenylalanine-L-leucine boronic acid, N-(3-pyridine)carbonyl-L-phenylalanine-L-leucine boronic acid, N-(3-phenylpropionyl)-L-phenylalanine-L-leucine boronic acid, N-(4-morpholine)carbonyl-L-phenylalanine-L-leucine boronic acid, N-(4-morpholine)carbonyl-(0-benzyl)-L-tyrosine-L-leucine boronic acid, N-(4-morpholine)carbonyl-L-tyrosine-L-leucine boronic acid, and N-(4-moφholine)carbonyl-[O-(2-pyridylmethyl)]-L-tyrosine-L-leucine boronic acid. Preferred compounds having formula (2a) include compounds where Y is one of R 8 -C(O)-, R 8 -SO 2 -, R 8 -ΝH-C(O)- or R 8 -O-C(O)-, and R 8 is one of ., 0 aryl, C M0 ar(C,^)alkyl, or a 5-10 membered heteroaryl, any of which can be optionally substituted, or when P is R 8 -C(O)-, R 8 can also be N-moφholinyl; provided that when Y is R -C(O)-, then R 8 is other than phenyl, benzyl or C,. 3 alkyl. Where R 8 is alkyl, it is preferably alkyl of from 1 to 4 carbon atoms, e.g., methyl, ethyl, propyl, butyl, or isomers thereof. Additionally, where R 8 is alkaryl or aralkyl, the alkyl moiety thereof is also preferably one having from 1 to 4 carbon atoms. Where R 8 is aryl, it is preferably aryl of from 6 to 10 carbon atoms, e.g., phenyl or naphthyl, which may, if desired, be ring substituted. Additionally, where R s is alkaryl, aralkyl, aryloxy. alkaryloxy, or aralkoxy, the aryl moiety thereof is also preferably one having from 5 to 10 carbon atoms, most preferably 6 to 10 carbon atoms. Preferably, the R 8 moiety is a saturated, partially unsaturated or aromatic heterocycle, more preferably an isomeric pyridine ring or moφholine ring. Y is most preferably one of: where R" is C^ alkyl. In an additional preferred embodiment of the present invention, the Y moiety of the proteasome inhibitor of formula (2a) is an isosteric amino acid replacement of formula (3a): where R' is as defined for formula (la) above. Useful and preferred values of R' are the same as those defined for formula (la) above; and P is one of R 7 -C(O)-, R 7 -SO 2 -, R 7 -NH-C(O)- or R'-O-CiO , and R 7 is one of alkyl, aryl, alkaryl, aralkyl, any of which can be optionally substituted, or when Y is R 7 -C(O)- or R 7 -SO 2 -, R 7 can also be an optionally substituted 5-10 membered saturated, partially unsaturated or aromatic heterocycle. Useful and preferred values of R 7 , when R 7 is one of alkyl, aryl, alkaryl, aralkyl, any of which are optionally substituted are as defined for formula (la) above. When R 7 is optionally substituted 5-10 membered saturated, partially unsaturated or aromatic heterocycle, preferred and useful values are as defined for heteroaryl, unsaturated and partially saturated heterocycle of the R 7 of formula (la). In this aspect of the invention Y is most preferably: In either embodiment of the compounds of formula (2a), useful and preferred values of R 3 are the same as for formula (la) above. In formula (la) and (lb), X 1 represents a peptide bond or an isostere that can be used as a peptide bond replacement in the proteasome inhibitors to increase bioavailability and reduce hydrolytic metabolism. As noted above, X 1 can be one of -C(O)NH-, -CH 2 -NH-, -CH(OH)-CH(OH)-, -CH(OH)-CH 2 — CH(OH)-CH 2 -NH-, -CH=CH- -C(O)-CH 2 -, -SO 2 -NH- -SO 2 -CH 2 - or -CH(OH)-CH 2 -C(O)-NH-. Preferably, X 1 is — C(O>— NH— . Introduction of these X 1 moieties into the proteasome inhibitors results in the following wherein R x and R y have the same definitions as R 1 and R 2 , above and P, Z 1 , Z 2 and R 3 are defined as above for formula (la). tide bond keto methylene isostere -28- reduced peptide bond hydroxyethylene isostere dihydroxyethylene isostere hydroxyethylamine isostere alkene isostere sulfonamide isostere sulfone methylene isostere Thus, for example, if Z-Leu-Leu-Leu-B(OH) 2 is found to undergo rapid hydrolytic metabolism to produce Z-Leu-OH and H 2 N-Leu-Leu-B(OH) 2 , the hydroxyethylene isostere can be prepared to eliminate this reaction: Another group of compounds of the present invention are aza-peptide isosteres. This is the result of the replacement of the α-carbon atom of an amino acid with a nitrogen atom, e.g., wherein R, represents R 1 , R y represents R 2 , P, Z\ Z 2 and R 3 are defined as above for formula (la) and (lb). When P and R are both H, formula (1) will exist in equilibrium with a cyclic formula (4), which is considered to be covered by the current invention: The above-described boronic ester and acid compounds include both D and L peptidyl configurations. However, L configurations are preferred. The present invention relates to a method for reducing the rate of muscle protein degradation in a cell comprising contacting the cell with a proteasome inhibitor described above. More specifically, the present invention relates to a method for reducing the rate of loss of muscle mass in an animal comprising contacting cells of the muscle with a proteasome inhibitor described above. The present invention also relates to a method for reducing the activity of NF-κB in a cell comprising contacting the cell with a proteasome inhibitor described above. More specifically, the present invention also relates to a method for reducing the activity of NF- B in an animal comprising contacting cells of the animal with a proteasome inhibitor described above. The present invention also relates to a method for reducing the rate of proteasome-dependent intracellular protein breakdown comprising contacting cells with a proteasome inhibitor described above. More specifically, the present invention also relates to a method for reducing the rate of intracellular protein breakdown in an animal comprising contacting cells of the animal with the proteasome inhibitor described above. The present invention further relates to a method of reducing the rate of degradation of p53 protein in a cell comprising administering to the cell a proteasome inhibitor described above. More specifically, the present invention further provides a method of reducing the rate of degradation of p53 protein in an animal (preferably, an animal subjected to DNA damaging drugs or radiation) comprising administering to said animal a proteasome inhibitor described above. The present invention further relates to a method for inhibiting cyclin degradation in a cell comprising contacting said cells with a proteasome inhibitor described above. More specifically, the present invention relates to a method for inhibiting cyclin degradation in an animal comprising contacting cells of said animal with a proteasome inhibitor described above. The present invention also provides a method for treating cancer, psoriasis, restenosis, or other cell proliferative diseases in a patient comprising administering to the patient a proteasome inhibitor described above. The present invention also relates to a method for inhibiting antigen presentation in a cell comprising administering to the cell a proteasome inhibitor described above. More specifically, the present invention relates to a method for inhibiting antigen presentation in animal comprising administering to the animal a proteasome inhibitor described above. The present invention further provides a method for inhibiting inducible NF-κB dependent cell adhesion in an animal comprising administering to said animal a proteasome inhibitor described above. The present invention also provides a method for inhibiting HIV infection in an animal comprising administering to said animal a proteasome inhibitor described above. The "animals" referred to herein are preferably mammals. Both terms are intended to include humans. Preferably, the methods described above deliver the proteasome inhibitor by either contacting cells of the animal with a proteasome inhibitor described above or by administering to the animal a proteasome inhibitor described above. The compounds of the present invention inhibit the functioning of the proteasome. This proteasome-inhibition activity results in the inhibition or blocking of a variety of intracellular functions. In particular, inhibition of proteasome function inhibits the activation or processing of transcription factor NF-κB. NF-κB plays a central role in the regulation of a diverse set of genes involved in the immune and inflammatory responses. Inhibition of proteasome function also inhibit the ubiquitination/proteolysis pathway. This pathway catalyzes selective degradation of highly abnormal proteins and short-lived regulatory proteins. The ubiquitination proteolysis pathway also is involved in the processing of internalized cellular or viral antigens into antigenic peptides that bind to MHC-I molecules. Thus, the proteasome inhibitors of the present invention can be used in reducing the activity of the cytosolic ATP-ubiquitin- dependent proteolytic system in a number of cell types. The inhibitors can be used in vitro or in vivo. They can be administered by any number of known routes, including orally, intravenously, intramuscularly, subcutaneously, intrathecally, topically, and by infusion (Platt et al, U.S. Patent No. 4,510, 130; Badala ente et al, Proc. Natl Acad. Sci. U.S.A. 56:5983-5987 (1989); Staubli et al, Brain Research 444Λ53-15S (1988)) and will generally be administered in combination with a physiologically acceptable carrier (e.g., physiological saline). The effective quantity of inhibitor given will be determined empirically and will be based on such considerations as the particular inhibitor used, the condition of the individual, and the size and weight of the individual. It is to be expected that the general end-use application dose range will be about 0.01 to 100 mg per kg per day, preferably 0.1 to 75 mg per kg per day for an effective therapeutic effect. The present invention relates to a method of inhibiting (reducing or preventing) the accelerated or enhanced proteolysis that occurs in atrophying muscles and is known to be due to activation of a nonlysosomal ATP-requiring process in which ubiquitin plays a critical role. Inhibition of the ATP-ubiquitin-dependent pathway is a new approach for treating the negative nitrogen balance in catabolic states. This can be effected through use of an inhibitor of the present invention, resulting in reduction of loss of muscle mass in conditions in which it occurs. Excessive protein loss is common in many types of patients, including individuals with sepsis, burns, trauma, many cancers, chronic or systemic infections, neuromotor degenerative disease, such as muscular dystrophy, acidosis, or spinal or nerve injuries. It also occurs in individuals receiving corticosteroids, and those in whom food intake is reduced and/or absoφtion is compromised. Moreover, inhibitors of the protein breakdown pathway could possibly be valuable in animals, e.g., for combating "shipping fever", which often leads to a major weight loss in cattle or pigs. The accelerated proteolysis evident in atrophy of skeletal muscles upon denervation or fasting is catalyzed by the nonlysosomal ATP-dependent degradative pathway. It has been shown that in a variety of catabolic states (e.g., denervation, fasting, fever, certain endocrinopathies or metabolic acidosis) muscle wasting is due primarily to accelerated protein breakdown and, in addition, that the increased proteolysis results from activation of the cytosolic ATP-ubiquitin-dependent proteolytic system, which previously had been believed to serve only in the rapid elimination of abnormal proteins and certain short-lived enzymes. The discovery that this pathway is responsible for the accelerated proteolysis in these catabolic states is based on studies in which different proteolytic pathways were blocked or measured selectively in incubated muscles, and the finding of increased mRNA for components of this pathway (e.g., for ubiquitin and proteasome subunits) and increased levels of ubiquitin-protein conjugates in the atrophying muscles. The nonlysosomal ATP-ubiquitin- dependent proteolytic process increases in muscle in these conditions and is responsible for most of the accelerated proteolysis that occurs in atrophying muscles. There is a specific increase in ubiquitin mRNA, induction of mRNA for proteasome and increased ubiquitinated protein content in atrophying muscles that is not seen in non-muscle tissue under the same conditions. The inhibitors of the present invention can be used to reduce (totally or partially) the nonlysosomal ATP-dependent protein degradation shown to be responsible for most of the increased protein degradation that occurs during fasting, denervation, or disuse (inactivity), steroid therapy, febrile infection, and other conditions. One approach to testing drug candidates for their ability to inhibit the ATP-ubiquitin-dependent degradative process is to measure proteolysis in cultured cells (Rock, et al, Cell 78:761 (1994)). For example, the degradation of long-lived intracellular proteins can be measured in mouse C2C12 myoblast cells. Cells are incubated with 35 S-methionine for 48 hours to label long-lived proteins and then chased for 2 hours with medium containing unlabeled methionine. After the chase period, the cells are incubated for 4 hours in the presence or absence of the test compound. The amount of protein degradation in the cell can be measured by quantitating the trichloroacetic acid soluble radioactivity released from the pre-labeled proteins into the growth medium (an indicator of intracellular proteolysis). Inhibitors can also be tested for their ability to reduce muscle wasting in vivo. Urinary excretion of the modified amino acid 3-methyl histidine (3-MH) is probably the most well characterized method for studying myofibrillar protein degradation in vivo (see Young and Munro, Federation Proc. 57.229-2300 (1978)). 3-Methylhistidine is a post-translationally modified amino acid which cannot be reutilized for protein synthesis, and it is only known to occur in actin and myosin. It occurs in actin isolated from all sources, including cytoplasmic actin from many different cell types. It also occurs in the myosin heavy chain of fast-twitch (white, type II) muscle fibers, but it is absent from myosin of cardiac muscle and myosin of slow-twitch (red, type I) muscle fibers. Due to its presence in actin of other tissues than skeletal muscle, other tissues will contribute to urinary 3-MH. Skeletal muscle has been estimated to contribute 38-74% of the urinary 3-MH in normal rats and 79-86% of the urinary 3-MH in rats treated with corticosterone (100 mg/kg/day subcutaneously) for 2-4 days (Millward and Bates, Biochem. J. 274:607-615 (1983); Kayali, et al, Am. J. Physiol 252.E621 -E626 (1987)). High-dose glucocorticoid treatment is used to induce a state of muscle wasting in rats. Treating rats with daily subcutaneous injections of corticosterone (100 mg/kg) causes an increase of approximately 2-fold in urinary 3-MH. The increase in excretion of 3-MH is transient, with a peak increase after 2-4 days of treatment and a return to basal values after 6-7 days of treatment (Odedra, et al, Biochem. J. 214:617-627 (1983); Kayali, et al, Am. J. Physiol 252.E621-E626 (1987)). Glucocorticoids have been shown to activate the ATP-ubiquitin-dependent proteolytic pathway in skeletal muscle (Wing and Goldberg, Am. J. Physiol. 264.E66S-E676 (1993)) and proteasome inhibitors are therefore expected to inhibit the muscle wasting that occurs after glucocorticoid treatment. The proteasome inhibitors can be administered alone or in combination with another inhibitor or an inhibitor of another pathway (e.g., a lysosomal or Ca -dependent pathway) responsible for loss of muscle mass. Use of proteasome inhibitors as agents that selectively protect normal cells from DNA damage during radiation and chemotherapy treatment of tumors The inhibitors of the present invention will block the degradation of the tumor suppressor protein p53. This protein is degraded by the ATP ubiquitin dependent proteolysis by the proteasome (see Scheffner et al, Cell 75:495-505 (1993)). Studies of p53 knockout mice indicate an important role for p53 in reducing incidence of tumors (Donehower et al, Nature 356:215-221 (1992)). In normal cells expressing wild type, unmutated p53, the basal levels of p53 are very low due to very rapid degradation of p53 protein. However, expression of p53 protein in normal cells is stimulated in response to radiation and drugs that induce DNA damage (Kastan et al, Cancer Res. 57:6304-631 1 (1991)). These induced high levels of wild type, unmutated p53 induce arrest of normal cell proliferation at the Gl stage of the cell cycle (Kastan et al., supra; Kuerbitz, PNAS 89:7491-7495 (1992)). This arrest of cell proliferation permits repair of damaged DNA. By contrast, in tumor cells expressing mutant forms of p53, DNA damaging drugs or radiation do not induce cell cycle arrest (Kastan et al, supra; Kastan et al, Cell 77:587-597 (1992)). Consequently, tumor cells are selectively damaged by radiation and cytotoxic drugs. The selective arrest response of normal cells by inducing p53 suggests that enhancing the p53 response can allow the treatment of the tumor with higher/more prolonged tumoricidal doses of radiation or antineoplastic drugs. The idea that induction of p53 by a non toxic agent as an adjunct to radiotherapy has been reported previously (Lane, Nature 358: 15- 16 ( 1992), but a method for reducing it to practice was not described. The use of proteasome inhibitors provides a method for augmenting the expression of p53 in normal cells by preventing its degradation by the proteasome. An example of this would be the systemic administration of proteasome inhibitor at a sufficient dose to inhibit p53 degradation by the proteasome during the treatment of the tumor with cytotoxic drugs or radiation. This will prolong and increase the levels of p53 expression in normal cells and will enhance the arrest of normal cell proliferation, reducing their sensitivity to higher doses of radiation or cytotoxic drugs. Administration of proteasome inhibitors would therefore permit exposing the tumor to higher doses of radiation, enhancing the killing of tumor cells. Thus, proteasome inhibitors can be used as adjuvants to therapy with tumoricidal agents, such as radiation and cytotoxic drugs. Topical application of proteasome inhibitors to enhance p53 expression in skin The expression of p53 in normal skin is induced by exposure of the skin to UV irradiation, which inhibits DNA replication that is needed for cell division (Maltzman et al. Mol. Cell. Biol. 4: 1689 (1984); Hall et al., Oncogene 5.203-207 (1993)). This protects normal skin from chromosomal DNA damage by allowing time for DNA repair before DNA replication. Defects in the p53 response pathway, such as seen with Ataxia Telangiectasia, result in increased susceptibility to ionizing radiation-induced skin tumors (Kastan et al, Cell 77:587-597 (1992)). It is well established that exposure of normal individuals increases the risk for many kinds of skin cancers. This risk can be diminished by UV filtering chemicals in skin creams. Another approach would be to promote the resistance of the DNA in skin cells to UV damage by the topical application of agents that enhance the skin's expression of p53 in response to UV light. Inhibiting p53 degradation by the topical application of proteasome inhibitors provides a method to enhance the p53 response. One preferred embodiment of the present invention is the topical application of proteasome inhibitors to reduce the acknowledged risk of skin cancers that results from the treatment of psoriasis using UV light, which is often combined with psoralens or coal tar. Each of these agents can induce DNA damage. Use of proteasome inhibitors to reduce the activity ofNF-κB NF-κB exists in an inactive form in the cytoplasm complexed with an inhibitor protein, IκB. In order for the NF-κB to become active and perform its function, it must enter the cell nucleus. It cannot do this, however, until the IκB portion of the complex is removed, a process referred to by those skilled in the art as the activation of, or processing of, NF-κB. In some diseases, the normal performance of its function by the NF-κB can be detrimental to the health of the patient. For example, as mentioned above, NF-κB is essential for the expression of the human immunodeficiency virus (HIV). Accordingly, a process that would prevent the activation of the NF-κB in patients suffering from such diseases could be therapeutically beneficial. The inhibitors employed in the practice of the present invention are capable of preventing this activation. Thus, blocking NF- KB activity could have important application in various areas of medicine, e.g., inflammation, through the inhibition of expression of inflammatory cytokines and cell adhesion molecules, (ref. Grilli et al, International Review of Cytology 143: 1 -62 ( 1993)) sepsis, AIDS, and the like. More specifically, the activity of NF-KB is highly regulated (Grilli et al, International Review of Cytology 143: 1-62 (1993); Beg et al, Genes and Development 7:2064-2070 (1993)). NF-κB comprises two subunits, p50 and an additional member of the rel gene family, e.g., p65 (also known as Rel A). In most cells, the p50 and p65 are present in an inactive precursor form in the cytoplasm, bound to IκB. In addition, the p50 subunit of NF-κB is generated by the proteolytic processing of a 105 kD precursor protein NF-κB, (pi 05), and this processing is also regulated. The sequence of the N-terminal 50 kD portion of pi 05 is similar to that of p65 and other members of the rel gene family (the rel homology domain). By contrast, the C-terminal 55 kD of pi 05 bears a striking resemblance to IκB-α (also known as MAD3). Significantly, unprocessed pi 05 can associate with p65 and other members of the rel family to form a p65/pl05 heterodimer. Processing of pi 05 results in the production of p50, which can form the transcriptionally active p50/p65 heterodimer. The C-terminal IκB-α- homologous sequence of pi 05 is rapidly degraded upon processing. There is another rel-related protein, NF-κB 2 (pi 00), that is similar to pi 05 in that it, too, is processed to a DNA binding subunit, p52 (Neri et al. Cell (57:1075 (1991); Schmid et al, Nature 352:733 (1991); Bours et al, Molecular and Cellular Biology 72:685 (1992); Mercurio et al, DNA Cell Biology 77:523 (1992)). Many of the structural and regulatory features of pi 00 are similar to pi 05. In addition, the pi 00 protein can also form a heterodimer with p65 and other rel family members. In summary, the transcriptional activity of heterodimers consisting of p50 and one of the many rel family proteins, such as p65, can be regulated by at least two mechanisms. First, the heterodimers associate with IκB-α to form an inactive ternary cytoplasmic complex. Second, the rel family members associate with pi 05 and pi 00 to form inactive complexes. The ternary complex can be activated by the dissociation and destruction of IκB-α, while the p65/pl05 and p65/pl00 heterodimer can be activated by processing pi 05 and pi 00, respectively. The dissociation of IκB-α can be induced by a remarkably large number of extracellular signals, such as lipopolysaccharides, phorbol esters, TNF-α, and a variety of cytokines. The IκB-α is then rapidly degraded. Recent studies suggest that pi 05 and pi 00 processing can also be induced by at least some of these extracellular signals. Studies have demonstrated that pi 05 or a truncated form of p 105 (p60Tth) can be processed to p50 in vitro (Fan et al, Nature 354:395-398 (1991)). Certain of the requirements and characteristics of this in vitro processing reaction (e.g., ATP/Mg *+ dependency) implicated the involvement of the ubiquitin-mediated protein degradation pathway (Goldberg, Eur. J. Biochem. 203:9-23 (1992), Hershko et al, Annu. Rev. Biochem. 67:761-807 (1992)). The proteasome is required for the processing of pi 05 to p50. pl05/p607>A proteins are not processed in mammalian cell cytoplasmic extracts depleted of proteasome activity. However, addition of purified 26S proteasomes to these depleted extracts restores the processing activity. Additionally, specific inhibitors of the proteasome block the formation of p50 in mammalian cell extracts and in vivo. Also, mammalian pi 05 is processed to p50 in Saccharomyces cerevisiae in vivo, and a mutant deficient in the chymotrypsin- like activity of the proteasome showed a significant decrease in pi 05 processing. p60Tth is ubiquitinated in vitro and this ubiquitination is a pre-requisite for pi 05 processing. As mentioned above, the C-terminal half of the pi 05 (pl05C) is rapidly degraded during the formation of p50 and the sequence of pl05C is remarkably similar to that of IκB. IκB-α is rapidly degraded in response to NF-κB inducers and this degradation has been shown to be necessary for the activation (Mellits et al, Nucleic Acids Research 21 (22) :5059-5066 (1993); Henkel et al, Nature 365: 182-185 (1993); Beg et al. Molecular and Cellular Biology 75^:3301-3310 (1993)). IκB-α degradation and the activation of NF-κB are also blocked by inhibitors of proteasome function or ubiquitin conjugation (Palombella et al. , Cell 75:773-785 (1994)). Accordingly, the proteasome plays an essential role in the regulation of NF-κB activity. First, the proteasome is required for the processing of pi 05 and possibly pi 00. The degradation of the inhibitory C-terminus can also require the proteasome. Second, the proteasome appears to be required for the degradation of IκB-α in response to extracellular inducers. The present invention relates to a method for reducing the activity of NF- KB in an animal comprising contacting cells of the animal with inhibitors of proteasome function. Compounds can be tested for their ability to inhibit the activation of NF-κB by means of a DNA binding assay (Palombella, et al, Cell 78:773 (1994)). Whole-cell extracts are prepared from untreated or TNF-α treated cells that have been pretreated for 1 hour with the test compound. The DNA binding activity of NF-κB is measured by an electrophoretic mobility shift assay using the PRDII probe from the human IFN-β gene promoter. As an indirect measure of NF-κB activation, the cell-surface expression of E-selectin, I-CAM-1, and V-CAM-1 on primary human umbilical vein endothelial cells (HUVECs) can be determined by means of a cell surface fluorescent immuno-binding assay. Because E-selectin, I-CAM- 1 , and V-C AM- 1 are under the regulatory control of NF-κB, inhibition of NF-κB activation results in reduced levels of these adhesion molecules on the cell surface. Compounds can also be tested for their ability to inhibit a delayed-type hypersensitivity response in mice. Contact hypersensitivity is a manifestation of an in vivo T-cell mediated immune response (Friedmann, Curr. Opinion Immunology, 7:690-693 (1989)). Although the exact molecular mechanisms that regulate the cellular interactions and vascular changes involved in the response remain obscure, it is clear that the process is dependent upon the inteφlay of soluble mediators, adhesion molecules, and the cytokine network (Piguet, et al, J. Exp. Med. 173:673-679 (1991); Nickoloff, et al. J. Invest. Dermatol 9 . 151S-157S (1990)). NF-κB, by mediating events such as the production of cytokines and the induction and utilization of cell-surface adhesion molecules, is a central and coordinating regulator involved in immune responses. The compounds of formula (lb) or (2b) can be used to treat chronic or acute inflammation that is the result of transplantation rejection, arthritis, rheumatoid arthritis, infection, dermatosis, inflammatory bowel disease, asthma, osteoporosis, osteoarthritis and autoimmune disease. Additionally, inflammation associated with psoriasis and restenosis can also be treated. The term "treatment of inflammation" or "treating inflammation" is intended to include the administration of compounds of the present invention to a subject for piuposes which can include prophylaxis, amelioration, prevention or cure of an inflammatory response. Such treatment need not necessarily completely ameliorate the inflammatory response. Further, such treatment can be used in conjunction with other traditional treatments for reducing the inflammatory condition known to those of skill in the art. The proteasome inhibitors of the invention can be provided as a "preventive" treatment before detection of an inflammatory state, so as to prevent the same from developing in patients at high risk for the same, such as, for example, transplant patients. In another embodiment, efficacious levels of the proteasome inhibitors of the invention are administered so as to provide therapeutic benefits against the secondary harmful inflammatory effects of inflammation. By an "efficacious level" of a composition of the invention is meant a level at which some relief is afforded to the patient who is the recipient of the treatment. By an "abnormal" host inflammatory condition is meant an level of inflammation in the subject at a site which exceeds the norm for the healthy medical state of the subject, or exceeds a desired level. By "secondary" tissue damage or toxic effects is meant the tissue damage or toxic effects which occur to otherwise healthy tissues, organs, and the cells therein, due to the presence of an inflammatory response, including as a result of a "primary "inflammatory response elsewhere in the body. Amounts and regimens for the administration of proteasome inhibitors and compositions of the invention can be determined readily by those with ordinary skill in the clinical art of treating inflammation-related disorders such as arthritis, tissue injury and tissue rejection. Generally, the dosage of the composition of the invention will vary depending upon considerations such as: type of pharmaceutical composition employed; age; health; medical conditions being treated; kind of concurrent treatment, if any, frequency of treatment and the nature of the effect desired; extent of tissue damage; gender; duration of the symptoms; and, counter indications, if any, and other variables to be adjusted by the individual physician. A desired dosage can be administered in one or more applications to obtain the desired results. Pharmaceutical compositions containing the proteasome inhibitors of the invention can be provided in unit dosage forms. Thus, the proteasome inhibitors are useful for treating such conditions as tissue rejection, arthritis, local infections, dermatoses, inflammatory bowel diseases, autoimmune diseases, etc. The proteasome inhibitors of the present invention can be employed to prevent the rejection or inflammation of transplanted tissue or organs of any type, for example, heart, lung, kidney, liver, skin grafts, and tissue grafts. Compounds of the present invention inhibit the growth of cancer cells. Thus, the compounds can be employed to treat cancer, psoriasis, restenosis or other cell proliferative diseases in a patient in need thereof. By the term "treatment of cancer" or "treating cancer" is intended description of an activity of compounds of the present invention wherein said activity prevents or alleviates or ameliorates any of the specific phenomena known in the art to be associated with the pathology commonly known as "cancer." The term "cancer" refers to the spectrum of pathological symptoms associated with the initiation or progression, as well as metastasis, of malignant tumors. By the term "tumor" is intended, for the puφose of the present invention, a new growth of tissue in which the multiplication of cells is uncontrolled and progressive. The tumor that is particularly relevant to the invention is the malignant tumor, one in which the primary tumor has the properties of invasion or metastasis or which shows a greater degree of anaplasia than do benign tumors. Thus,"treatment of cancer" or "treating cancer" refers to an activity that prevents, alleviates or ameliorates any of the primary phenomena (initiation, progression, metastasis) or secondary symptoms associated with the disease. Cancers that are treatable are broadly divided into the categories of carcinoma, lymphoma and sarcoma. Examples of carcinomas that can be treated by the composition of the present invention include, but are not limited to: adenocarcinoma, acinic cell adenocarcinoma, adrenal cortical carcinomas, alveoli cell carcinoma, anaplastic carcinoma, basaloid carcinoma, basal cell carcinoma, bronchiolar carcinoma, bronchogenic carcinoma, renaladinol carcinoma, embryonal carcinoma, anometroid carcinoma, fibrolamolar liver cell carcinoma, follicular carcinomas, giant cell carcinomas, hepatocellular carcinoma, intraepidermal carcinoma, intraepithelial carcinoma, leptomanigio carcinoma, medullary carcinoma, melanotic carcinoma, menigual carcinoma, mesometonephric carcinoma, oat cell carcinoma, squamal cell carcinoma, sweat gland carcinoma, transitional cell carcinoma, and tubular cell carcinoma. Sarcomas that can be treated by the composition of the present invention include, but are not limited to: amelioblastic sarcoma, angiolithic sarcoma, botryoid sarcoma, endometrial stroma sarcoma, ewing sarcoma, fascicular sarcoma, giant cell sarcoma, granulositic sarcoma, immunoblastic sarcoma, juxaccordial osteogenic sarcoma, coppices sarcoma, leukocytic sarcoma (leukemia), lymphatic sarcoma (lympho sarcoma), medullary sarcoma, myeloid sarcoma (granulocitic sarcoma), austiogenci sarcoma, periosteal sarcoma, reticulum cell sarcoma (histiocytic lymphoma), round cell sarcoma, spindle cell sarcoma, synovial sarcoma, and telangiectatic audiogenic sarcoma. Lymphomas that can be treated by the composition of the present invention include, but are not limited to: Hodgkin's disease and lymphocytic lymphomas, such as Burkitt's lymphoma, NPDL, NML, NH and diffuse lymphomas. The compounds of formulae (lb) and (2b) appear to be particularly useful in treating metastases. Amounts and regimens for the administration of proteasome inhibitors and compositions of the invention can be determined readily by those with ordinary skill in the clinical art of treating cancer-related disorders such as the primary phenomena (initiation, progression, metastasis) or secondary symptoms associated with the disease. Generally, the dosage of the composition of the invention will vary depending upon considerations such as: type of composition employed; age; health; medical conditions being treated; kind of concurrent treatment, if any, frequency of treatment and the nature of the effect desired; extent of tissue damage; gender; duration of the symptoms; and, counter indications, if any, and other variables to be adjusted by the individual physician. A desired dosage can be administered in one or more applications to obtain the desired results. Pharmaceutical compositions containing the proteasome inhibitors of the invention can be provided in unit dosage forms. The present invention will now be illustrated by the following examples, which are not intended to be limiting in any way. Examples Most compounds of formulas (la), (lb), (2a) or (2b) were prepared according to the general reaction sequence depicted in Scheme 1. R 2 and R 3 are as defined above for formulas (lb) and (2b). PG represents an amino-group- protecting moiety. The general procedures employed for each compound are summarized in Table I, and detailed descriptions of these procedures are provided in the Examples. Syntheses that do not conform to the general reaction sequence are described in full in the Examples. (lS,2S,37?,5S)-Pinanediol leucine boronate trifluoroacetate salt was prepared as previously reported (Kettner, C.A.; Shenvi, A.B. J. Biol. Chem. 259:15106 (1984)). N-Protected (Boc-, Cbz-, or Fmoc-) amino acids were commercially available or were prepared from the corresponding free amino acid by standard protection methods, unless otherwise described in the Examples. l-Ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC), benzotriazol-l-yloxytris(dimethylamino)phosphonium hexafluorophosphate (BOP reagent), or O-(lH-benzotriazol-l-yl)- NNN'N'-tetramethyluronium tetrafluoroborate (TBTU) were employed as coupling reagents (Sheehan, J.C. et al, J. Am. Chem. Soc. 57:2492 (1965); Castro, B., et al, Synthesis 11:751 (1976); Tetrahedron Lett. 30: 1927 ( 1989)). All compounds were characterized by proton nuclear magnetic resonance (ΝMR) spectroscopy. The purity of the products was verified by thin layer chromatography and by high performance liquid chromatography (ΗPLC). Scheme I Boronic acid De protection (4) (6) -50- " A = NaIO 4 , NH 4 OAc, acetone-water; B = -BuB(OH) 2 , IN HCl, MeOH-hexane. See Examples for detailed descriptions of procedures. Example 1; N-(4-Morpholine)carbonyl- -(l-naphthyl)-L-alanine-L- leucine boronic acid [MG-273] A. (lS,2S,3R,5S)-Pinanediol N-Boc-$-(l-naphthyl)-L- alanine-L-leucine boronate To a solution of (lS,2S,3Λ,5S)-pinanediol leucine boronate trifluoroacetate salt (664 mg, 1.76 mmol) and N-Boc-β-(l-naphthyl)-L-alanine (555 mg, 1.76 mmol) in DMF (10 mL) at 0°C was added l-ethyl-3-(3- dimethylaminopropyl)carbodiimide hydrochloride (EDC) (404 mg, 2.11 mmol), l-hydroxybenzotriazole monohydrate (HOBT) (285 mg, 2.11 mmol), and N-methylmoφholine (ΝMM) (0.3 mL, 2.64 mmol). The mixture was allowed to warm to room temperature and stir overnight. The reaction was quenched with water (100 mL), and the mixture was extracted with CH 2 C1 2 (4 x 25 mL). The combined organic layers were washed with 5% aqueous HCl and saturated aqueous ΝaHCO 3 , dried over anhydrous MgSO 4 , filtered, and concentrated to give a yellow oil. Water was added and the resultant gummy precipitate was extracted with ether (3 x 25 mL). The organic layer was dried (anhydrous MgSO 4 ), filtered, and concentrated to afford the title compound (202 mg) as a white foam. B. (IS,2S,3R,5S)-Pinanediol -(l-Naphthyl)-L-alanine- L-leucine boronate trifluoroacetate salt To a solution of the product of Example 1A (930 mg, 1.38 mmol) in CH 2 C1 2 (10 mL) at 0°C was added trifluoroacetic acid (5 mL) and thioanisole (1 mL). The reaction mixture was allowed to warm to room temperature. After 4 h, the reaction mixture was concentrated to dryness and dried in vacuo. The residue was used in the next reaction without further purification. (lS,2S,3R,5S)-PinanediolN-(4-morpholine)carbonyl- -(l-naphthyl)-L-alanine-L-leucine boronate 4-Moφholinecarbonyl chloride (50 mL, 0.42 mmol) and triethylamine (150 mL, 1.08 mmol) were added to a solution of the product of Example IB (0.25 g, 0.36 mmol) in CH 2 C1 2 (6 mL). After 24 h, additional moφholinecarbonyl chloride (50 mL) and triethylamine (150 mL) were added. After 2 days total reaction time, the reaction mixture was diluted with EtOAc, washed with 1 N HCl and saturated aqueous ΝaHCO 3 , dried over MgSO 4 , filtered, and concentrated. Purification by flash chromatography (elution with 1 :2 EtOAc hexanes and 4:4:1 hexanes/EtOAc/MeOH) afforded the title compound (124 mg). D. N-(4-Morpholine)carbonyl-$-(l-naphthyl)- L-alanine-L-leucine boronic acid To a stirred solution of the product of Example IC (124 mg, 0.21 mmol) in acetone (10 mL) was added aqueous NH 4 OAc (0.1 N, 5 mL, 1.0 mmol), followed by ΝaIO 4 (120 mg, 0.21 mmol). The reaction mixture was stirred at room temperature for 72 h, and then the acetone was evaporated. The aqueous layer was acidified to pH 3 with IN HCl and extracted with EtOAc (3 x 20 mL). The combined organic layers were dried over anhydrous MgSO 4 , filtered, and concentrated. The residue was purified by flash chromatography (elution with 1 :1 hexane/EtOAc, 2:2:1 hexanes/EtOAc/MeOH, and l :l :few drops MeOH:EtOAc:HOAc) to give the title compound (29 mg). Example 2: N-Cbz-L-Leucine-L-leucine boronic acid [MG-274] A. (lS,2S,3R,5S)-PinanediolN-Cbz-L-leucine-L-leucine boronate B enzotri azo l- 1 -y loxytri s(di methyl ami no)phosphoni um hexafluorophosphate (BOP reagent, 827 mg, 1.87 mmol) was added in one portion to a mixture of (lS,2S,37?,5S -pinanediol leucine boronate trifluoroacetate salt (595 mg, 1.58 mmol), N-Cbz-L-leucine (500 mg, 1.87 mmol) in acetonitrile (30 mL) at room temperature. The mixture was stirred at room temperature for 2 hours. The reaction was quenched with brine (50 mL) and the mixture was extracted with EtOAc (3 x 50 mL). The combined organic layers were washed with aqueous 5% HCl, saturated aqueous ΝaHCO 3 , and saturated aqueous NaCl, and then dried (anhydrous MgSO 4 ), filtered, and concentrated. The residue was purified by silica gel chromatography (elution with 20-30% acetone/hexanes) to afford the title compound (539 mg). B. N-Cbz-L-Leucine-L-leucine boronic acid By a procedure analogous to that described in Example ID, the compound of Example 2A above (539 mg) was deprotected by treatment with sodium metaperiodate (1.2 g, 5.61 mmol) and aqueous NH 4 OAc (0.1 N, 10 mL, 1.0 mmol) to provide the title compound as a white solid (154 mg). Example 3: -(l-Naphthyl)-L-alanine-L-leucine boronic acid hydrochloride salt [MG-302J and -(l-Naphthyl)-L-alanine- L-leucine boronic acid [MG-303J (lS,2S,3R,5S)-Pinanediol $-(l-naphthyl)-L-alanine- L-leucine boronate hydrochloride salt To a solution of (lS,2S,37?,5S)-pinanediol β-(l-naphthyI)-L-alanine-L- leucine boronate trifluoroacetate salt (prepared as described in Example IB, 536 mg, 0.93 mmol) in ether (2 mL) was added 10 mL of IN HCl. The mixture was sonicated for several minutes. Ether was allowed to slowly evaporate. The resultant crystals were collected, washed with H 2 O and ether, and dried in vacuo to provide the title compound (300 mg). B. -(l-Naphthyl)-L-alanine-L-leucine boronic acid hydrochloride salt; and β-(l-Naphthyl)-L-alanine-L-leucine boronic acid To the product of Example 3 A (290 mg, 0.58 mmol) in a mixture of hexane (4 mL), MeOH (4 mL), and 1 Ν HCl ( 1.3 mL) was added -BuB(OH) 2 (71 mg, 0.70 mmol). The reaction mixture was stirred for 72 h at room temperature. The MeOH-H 2 O layer was washed with hexanes, and the MeOH was evaporated. The aqueous solution was made basic with ΝaOH and washed with ether-EtOAc (1 :1). The aqueous layer was lyophilized to give 640 mg of a yellow solid. The solid was dissolved in MeOH, 4N HCl in 1 ,4-dioxane was added, and the solution was filtered to remove a white solid. The filtrate was concentrated and the residue was purified by reverse phase HPLC (elution with CH 3 CΝ-H 2 O) to afford 45 mg of MG-302 and 10 mg of MG-303. Example 4 N-(4-Morph oline) carbonyl-(0-benzyl)- -tyrosin e-L-leucin e boronic acid [MG-306J A. N-Boc-O-Benzyl-L-tyrosine A suspension of O-benzyl-L-tyrosine (3.12 g, 11.5 mmol) in a mixture of 1 ,4-dioxane (14 mL) and water (14 mL) was treated, in order, with triethylamine (5.0 mL, 35.9 mmol) and a solution of (Boc) 2 O (2.86 g, 13.1 mmol) in 1,4-dioxane (12 mL). After 19 h, the reaction mixture was diluted with water (140 mL) and washed with ether. The aqueous layer was acidified with IN citric acid (35 mL) and extracted with CH 2 C1 2 (2 x 100 mL). Additional citric acid (15 mL) was added to the aqueous layer, which was again extracted with CH 2 C1 2 ( 100 mL). The combined organic extracts were dried (MgSO 4 ), filtered, and concentrated to give the crude product (4.5 g), which was used directly in the next reaction. B. (lS,2S,3R,5S)-Pinanediol N-Boc-(0-benzyl)-L-tyrosine-L- leucine boronate To a stirred and cold (0°C) solution of (lS,2S,37?,55)-pinanediol β-(l-naphthyl)-L-alanine-L-leucine boronate trifluoroacetate salt (prepared as described in Example IB, 3.03 g, 7.98 mmol), N-Boc-O-benzyl-L-tyrosine (2.97 g, 7.99 mmol), and TBTU (3.35 g, 8.84 mmol) in anhydrous DMF (30 mL) was added by syringe pump, at the rate of 1.9 mL/h, DIEA (4.2 mL, 24.1 mmol). After the addition was complete, the mixture was allowed to warm to room temperature over 30 min, and then it was added dropwise to 30 mL of rapidly stirring water. Additional water was added and the mixture was filtered. The collected solid was dissolved in MeOH, concentrated to near dryness and again added to rapidly stirring water (300 mL). The resultant white solid was collected by suction filtration, washed with water, frozen, and lyophilized to provide the title compound (4.49 g). (lS,2S,3R,5S)-Pinanediol (0-benzyl)-L-tyrosine-L-leucine boronate The product of Example 4B (4.47 g, 7.23 mmol) was dissolved in CH 2 C1 2 (40 mL) and cooled to 0°C. A solution of 4NHC1 in dioxane (40 mL, 0.16 mol) was added and the reaction mixture was stirred at room temperature for 1.5 h. Concentration afforded a yellow solid, which was triturated with hexane-ether (1 :1, 100 mL). Filtration afforded the title compound (3.65 g) as a pale yellow solid. D. (lS,2S,3R,5S)-Pinanediol N-(4-morpholine)carbonyl- (O-benzyl)-L-tyrosine-L-leucine boronate By a procedure analogous to that described in Example 1 C, the product of Example 4C (2.53 g, 4.56 mmol) was treated with 4-moφholinecarbonyl chloride (0.75 mL, 6.43 mmol) to provide the title compound (2.35 g) as a pale yellow solid. E. N-(4-morpholine)carbonyl-(0-benzyl)-L-tyrosine-L-leucine boronic acid The product of Example 4D (0.39 g, 0.62 mmol) was deprotected according to the procedure described in Example 3B to provide the title compound (146 mg) as a white solid. Example 5: N-Methyl-N-Cbz-L-leucine-L-leucine boronic acid [MG-268J A. N-Methyl-N-Cbz-L-leucine To a solution ofN-Cbz-leucine (1.38 g, 5.2 mmol) in THF (15 mL) at 0°C was added methyl iodide (2.5 mL, 40.1 mmol). Sodium hydride (60% dispersion in oil, 0.6 g, 15 mmol) was added cautiously, and the resultant mixture was stirred at room temperature for 24 h. The reaction mixture was diluted with EtOAc (25 mL) and water (2 mL) was added dropwise. The mixture was concentrated to dryness, and the residue was partitioned between ether (15 mL) and water (50 mL). The organic layer was extracted with saturated aqueous NaHCO 3 (25 mL), and the combined aqueous extracts were acidified to pH 2 with 3N HCl. The product was extracted with EtOAc (3 x 25 mL), dried over MgSO 4 , filtered, and concentrated to afford the title compound (1.41 g) as a yellow solid. B. (lS,2S,3R,5S)-PinanediolN-methyl-N-Cbz-L-leucine-L-leucine boronate By a procedure analogous to that described in Example 1 A, the product of Example 5A (85.1 mg, 0.30 mmol) was coupled with (lS,2S,37?,5_S)-pinanediol leucine boronate trifluoroacetate salt (105 mg, 0.28 mmol) in the presence of EDC (64 mg, 0.33 mmol), HOBT (45 mg, 0.33 mmol), and ΝMM (37 mg, 0.37 mmol) to provide, after purification by flash chromatography (elution with 3:2 hexanes/acetone), the title compound (85 mg). C. N-Methyl-N-Cbz-L-leucine-L-leucine boronic acid By a procedure analogous to that described in Example ID, the product of Example 5B (85 mg, 0.16 mmol) was deprotected by treatment with ΝaIO 4 (104 mg, 0.485 mmol) and aqueous NH 4 OAc (0.1N, 5 mL, 0.5 mmol) in 10 mL of acetone to provide, after purification by flash chromatography (elution with 4:4:2 hexanes/acetone/MeOH), the title compound (21 mg). Example 6 N-(4-Morpholine)carbonyl-β-(6-quinotinyl)-D, L-alanine-L-leucine boronic acid [MG-292] A. -(6-Quinolinyl)-D,L-alanine N-Acetyl β-(6-quinolinyl)-D,L-alanine ethyl ester (728 mg, 2.55 mmol) was heated at reflux in 6N HCl (20 mL). After 20 h, the reaction mixture was concentrated to dryness and the residue was dried in vacuo to provide the title compound, which was used directly in the next reaction. B. N-Boc- -(6-Quinolinyl)-D,L-alanine To the crude product of Example 6A in a stirred mixture of 1 ,4-dioxane ( 10 mL), water ( 10 mL), and 2N ΝaOH (5 mL) at 0 °C was added di-tert-butyl pyrocarbonate (556 mg, 2.55 mmol). The reaction mixture was allowed to warm to room temperature. After 23 h, the reaction mixture was acidified to pH 4 and extracted with EtOAc (3 x 50 mL) and n-BuOH (3 x 50 mL). The combined extracts were concentrated to provide the title compound, which was used directly in the next reaction. (lS,2S,3R,5S)-Pinanediol N-Boc-β-(6-quinolinyl)-D, L-alanine-L-leucine boronate By a procedure analogous to that described in Example 2A, the product of Example 6B was coupled with (lS,2S,37?,55)-pinanediol leucine boronate trifluoroacetate salt (943 mg, 2.5 mmol) in the presence of BOP reagent (1.33 g, 3 mmol) and triethylamine (0.37 mL, 2.62 mmol) to provide the title compound (343 mg). D. (lS,2S,3R,5S)-Pinanediol -(6-quinolinyl)-D, L-alanine-L-leucine boronate The product of Example 6C (343 mg, 0.61 mmol) was treated with trifluoroacetic acid (7 mL) and thioanisole (1 mL) in CH 2 C1 2 (15 mL) at 0°C, as described in Example IB, to provide the title compound. E. (lS,2S,3R,5S)-Pinanediol N-(4-morpholine) carbonyl-$-(6- quinolinyl)-D, L-alanine-L-leucine boronate The product of Example 6D was coupled with 4-moφholinecarbonyl chloride (0.14 mL, 1.22 mmol) by a procedure analogous to that described in Example 1 C to produce the title compound (112 mg). N-(4-Morph oline) car bony l-β-(6-quinolinyl)-D, L-alanine-L-leucine boronate Deprotection of the product of Example 6E (153 mg, 0.27 mmol) was effected according to the procedure described in Example 3B. Purification by silica gel chromatography (elution with 50:50:10 hexanes/acetone/methanol) afforded the title compound (87 mg). The product was further purified by reverse phase HPLC; 5 mg of the title compound was recovered. Example 7: N-(4-Morpholine)carbonyl- -(l-naphthyl)-L-alanine-L-leucine methylboronic acid [MG-317J; and N-(4-Morpholine) carbonyl- -(l-naphthyl)-L-alanine-L-leucine dimethylborane [MG-318] To a suspension of MG-273 (prepared as described in Example 1, 101.5 mg, 0.23 mmol) in 3 mL of a 2: 1 mixture of Et 2 O/CH 2 Cl 2 was added 1 ,3-propanedioI (20.0 mL, 0.28 mmol). The resultant clear solution was stirred for 30 min at room temperature, and then anhydrous MgSO 4 was added. Stirring was continued for an additional 30 min. and then the mixture was filtered through a cotton plug and then through a 0.2 mm PTFE filter. The solution was concentrated, toluene (2 mL) was added, and the mixture was again concentrated to produce a white solid. Anhydrous THF (3 mL) was added, and the resultant solution was cooled to 0°C. MeLi (0.8 mL, 1.12 mmol) was added. After 10 min, the mixture was warmed to room temperature. After 20 min, the light red solution was cooled to 0°C, quenched with a few drops of water, and then diluted with 10 mL of IN HCl. The colorless solution was extracted with CH 2 C1 2 (2 x 10 mL), and the combined extract was concentrated to afford a white solid. Purification by flash chromatography (elution with 2-4% MeOH/CHCl 3 , followed by 10% MeOH/CHCl 3 ) afforded MG-317 ( 17.7 mg) and MG-318 (72.1 mg). Example 8: N-Benzyl-(3R)-3-dioxyboryl-5-methylhexanamide [MG-342J tert-Butyl-(3R)-3-[(lS,2S.3R,5S)-(pinanediyldioxy)boryl]-5- methylhexanoate A 200-mL round-bottomed flask was charged with anhydrous THF (50 mL) and tert-butyl acetate (0.48 mL, 3.56 mmol). The solution was cooled to -78 °C under nitrogen, and LDA (1.5 M solution in cyclohexane, 2.2 mL, 3.3 mmol) was added by syringe over 8 min. The resultant solution was stirred for 10 min, and then a solution of (l S,2S,3R,5S)-pinanediol l-bromo-3-methylbutylboronate (Organometallics 9:317 \ (1990)) (1.04 g, 3.15 mmol) in anhydrous THF (15 mL) was added by cannula over 8 min. The reaction mixture was allowed to warm to room temperature and stir overnight. The pale pink solution was concentrated, and the residue was dissolved in 200 mL of ether. The solution was washed with saturated aqueous ΝH 4 C1 and saturated aqueous NaCl. Concentration gave a clear orange oil, which was purified by flash chromatography (elution with 2-3% EtOAc/hexanes) to afford the title compound (584 mg). B. (3R)-3-[(lS,2S,3R,5S)-(pinanediyldioxy)borylJ-5- methylhexanoic acid To a solution of the product of Example 8A (323 mg, 0.89 mmol) in CH 2 C1 2 (8 mL) was added trifluoroacetic acid (2.0 mL, 26 mmol). The resultant mixture was stirred at room temperature for 2 h. The reaction mixture was concentrated and dried overnight under high vacuum to produce a dark brown oil (309.3 mg). N-Benzyl-(3R)-3-[(lS,2S,3R,5S)-pinanediyldioxy)borylJ-5- methylhexanamide To a solution of the product of Example 8B (300 mg, 0.9 mmol) and TBTU (410 mg, 1.08 mmol) in anhydrous acetonitrile (5 mL) was added benzylamine (0.12 mL, 1.10 mmol), followed by diisopropylethylamine (0.50 mL, 2.9 mmol). The reaction mixture was stirred overnight at room temperature, and then was poured into water and extracted with EtOAc. The organic layer was washed with saturated aqueous NaHCO 3 and saturated aqueous NaCl. Concentration gave a dark brown oil, which was purified by flash chromatography (elution with 20% EtOAc hexanes) to afford the title compound (232 mg) as a clear, colorless oil. D. N-Benzyl-(3R)-3-dioxyboryl-5-methylhexanamide The product of Example 8C (223 mg, 0.56 mmol) was deprotected according to the procedure described in Example 3B. Purification by flash chromatography (elution with 5% MeOH/CHCl 3 ) provided a pale yellow oil, which was dissolved in acetonitrile/MeOH. Water was added and the mixture was lyophilized overnight to produce the title compound (108 mg) as a fluffy white solid. Example 9: N-Acetyl-l,2,3,4-tetrahydro-3-isoquinolinecarbonyl-L-leucine boronic acid [MG-310] A. N-Boc-l,2,3,4-Tetrahydro-3-boquinolinecarboxylic acid A solution of 1,2,3 ,4-tetrahydro-3-isoquinolinecarboxylic acid (855 mg, 4.83 mmol), (Boc) 2 O (1.37 g, 6.28 mmol), and INNaOH (6 mL) in a mixture of t-BuOH (12 mL) and water (12 mL) was stirred overnight at room temperature. The reaction mixture was diluted with water (30 mL) and washed with ether-hexanes (1 : 1 , 2 x 25 mL). The organic layer was back-extracted with 10% NaHCO 3 . The combined aqueous layers were carefully acidified to pH 2-3 and extracted with EtOAc (3 x 30 mL). The combined organic extracts were washed with water and saturated aqueous NaCl, dried (MgSO 4 ), and concentrated to provide the title compound (1.27 g) as a white solid. (lS,2S,3R,SS)-Pinanediol N-Boc-l,2,3,4-tetrahydro-3- isoq uinolinecar bony l-L-leucine boronate To a mixture of (lS,2S,37?,55)-pinanediol-L-leucine boronate trifluoroacetate salt (1.14 g, 3.03 mmol), N-Boc- 1,2,3 ,4-tetrahydro- 3-isoquinolinecarboxylic acid (762 mg, 2.75 mmol), and BOP reagent (1.34 g, 3.03 mmol) in DMF (20 mL) was added, over a period of 2 h, DIEA (1.44 mL, 8.25 mmol). The resultant solution was stirred for 1 h after addition was complete. The reaction mixture was poured into water (300 mL) and extracted with EtOAc (3 x 75 mL). The combined organic extracts were washed with dilute aqueous HCl, half-saturated aqueous ΝaHCO 3 , water, and saturated aqueous NaCl, dried (MgSO 4 ), and concentrated. The residue was purified by flash chromatography (elution with 20% EtOAc-hexanes) to provide the title compound (1.04 g) as a white foamy solid. (lS,2S,3R,5S)-Pinanediol l,2,3,4-tetrahydro-3- isoquinolinecarbonyl-L-leucine boronate hydrochloride salt The product of Example 9B (755 mg) was dissolved in CH 2 C1 2 (10 mL) and cooled to 0°C. A solution of 4NHC1 in dioxane (8 mL, 0.03 mol) was added and the reaction mixture was stirred at room temperature. Concentration and trituration with ether-hexanes afforded the title compound (565 mg) as an off-white solid. D. (lS,2S,3R,5S)-Pinanediol N-acetyl-l,2,3,4-tetrahydro-3- isoquinolinecarbonyl-L-leucine boronate The product of Example 9C (262 mg, 0.59 mmol) was treated at room temperature with Ac^O (0.085 mL, 0.89 mmol) and DIEA (0.18 mL, 1.36 mmol) in CH 2 C1 2 (5 mL). After 24 h, the reaction mixture was diluted with CH 2 C1 2 (20 mL), washed with IN HCl, half-saturated ΝaHCO 3 , and water, dried (Na 2 SO 4 ), and concentrated. Purification by flash chromatography (elution with EtOAc-hexanes) afforded the title compound (271 mg) as a white foamy solid. N-Acetyl-l,2,3,4-tetrahydro-3-isoquinolinecarbonyl-L-leucine boronic acid By a procedure analogous to that described in Example 3B, the product of Example 9D (226 mg, 0.49 mmol) was deprotected to provide the title compound (131 mg) as a foamy, oily solid. Example 10: N-(4-Morpholine)carbonyl- -(2-quinolyl)-L-alanine-L-leucine boronic acid [MG-315] A. Diethyl (2-quinolylmethyl)acetamidomalonate To a solution of 2(chloromethyl)quinoline monohydrochloride (5.0 g, 23.4 mmol) and diethyl acetamidomalonate (10.1 g, 46.7 mmol) in EtOH (60 mL) was added sodium methoxide (3.78 g, 70 mmol). The reaction mixture was heated at reflux for 6 h. The reaction mixture was cooled, filtered, and concentrated. The residue was dissolved in EtOAc (400 mL) and extracted with cold 4N HCl (3 x 150 mL). The aqueous layer was neutralized with lONΝaOH and extracted with EtOAc (3 x 200 mL). The combined organic extract was washed with water, dried (anhydrous MgSO 4 ), filtered, and concentrated to give the title compound (8.3 g). B. N-Acetyl- l-(2-quinolyl)-D,L-alanine ethyl ester To a solution of the product of Example 10A (8 g, 22.3 mmol) in EtOH (180 mL) was added 6.1NΝaOH (6.5 mL, 40 mmol). After 2 h, 1 1. IN HCl (3.6 mL, 40 mmol) was added, and the reaction mixture was concentrated to dryness. The residue was suspended in 1,4-dioxane (200 mL) and the mixture was heated at reflux for 90 min. The reaction mixture was concentrated and the residue was purified by silica gel chromatography (elution with 30-50% acetone-hexanes) to provide to title compound (4.3 g). C. N-Acetyl-β-(2-quinolyl)-L-alanine The product of Example 10B (4.3 g, 15 mmol) was treated with Subtilisin Carlsberg (Sigma, 1 1.9 units/mg, 30 mg, 357 units) at room temperature in aqueous ΝaHCO 3 (0.2 , 120 mL). After 2 h. the reaction mixture was extracted with CHC1 3 (6 x 100 mL). The aqueous layer was concentrated to dryness to provide the title compound (3.5 g), which contained salts. D. N-Boc- -(2-Quinolyl)-L-alanine A solution of the product of Example IOC (3.5 g, ca. 7.4 mmol) in 6NHC1 (40 mL) was heated at reflux for 16 h. The solvent was removed and the residue was dried in vacuo. To this residue was added 1 ,4-dioxane (20 mL), water (20 mL), and 2N ΝaOH (10 mL, 20 mmol). The solution was cooled to 0°C and di-t-butyl pyrocarbonate (1.6 g, 7.5 mmol) was added. After 1 h at 0°C, the reaction mixture was warmed to room temperature and stirring was continued for 17 h. The reaction mixture was extracted with CH 2 C1 2 (100 mL) and π-BuOH (4 x 100 mL). The aqueous layer was acidified and again extracted with w-BuOH. The organic extracts were combined and concentrated to provide the title compound (1-6 g). E. (lS,2S,3R,5S)-Pinanediol N-Boc-$-(2-quinolyl)-L-alanine-L- leucine boronate By a procedure analogous to that described in Example 2A, the product of Example 10D (0.6 g, 1.9 mmol) was coupled with (lS,2S,3_R,55)-pinanediol leucine boronate trifluoroacetate salt (716 mg, 1.9 mmol) in the presence of BOP reagent (0.84 g, 1.9 mmol) and triethylamine (0.27 mL, 1.9 mmol). Purification by silica gel chromatography (elution with 10-30% acetone-hexanes) afforded the title compound (194 mg). (lS,2S,3R,5S)-Pinanediol N-(4-morpholine)carbonyl- - (2-quinolyl)-L-alanine-L-leucine boronate The product of Example 10E (194 mg) was treated with trifluoroacetic acid (7 mL) and thioanisole (1 mL) as described in Example IB. The resultant product was condensed with 4-moφholinecarbonyl chloride (568 mg, 3.8 mmol) as described in Example 2C. Purification by silica gel chromatography (elution with 20-50% acetone-hexanes) afforded the title compound (367 mg). N-(4-Morpholine)carbonyl-β-(2-quinolyl)-L-alanine-L-leucine boronic acid The product of Example 10F (367 mg, 0.64 mmol) was deprotected according to the procedure described in Example 3B to provide the title compound (222 mg). Example 11: N-Boc-l,2,3,4-tetrahydro-l-isoquinolinecarboxylic acid precursor for the synthesis ofMG-310] A. 1,2,3,4-Tetrahydro-l-isoquinolinecarboxylic acid A solution of 1 -isoquinolinecarboxylic acid (1.67 g) in glacial acetic acid (25 mL) was hydrogenated at 60 p.s.i. over PtO 2 (270 mg). When the reaction was complete, the mixture was filtered through diatomaceous earth (Celite), washing the solid pad with MeOH, and the filtrate was concentrated to dryness. The resultant white solid was triturated with cold water and filtered to provide the title compound (775 mg). B. N-Boc-l,2,3,4-tetrahydro-l-isoquinolinecarboxylic acid The product of Example 1 1 B (762 mg, 4.3 mmol) was treated with di-tert-butyl pyrocarbonate (1.13 g, 5.17 mmol) according to the procedure described in Example 6B to afford the title compound (886 mg), as a foamy white solid. Example 12: Diethanolamine N-(4-morpholine)carbonyl- -(l-naphthyl)-L- alanine-L-leucine boronate [MG-286] To a solution of N-(4-moφholine)carbonyl-β-(l-naphthyl)-L-alanine-L- leucine boronic acid (prepared as described in Example 1, 97.4 mg, 0.22 mmol) in CH 2 C1 2 (4 mL) was added a solution of diethanolamine (25.5 mg, 0.24 mmol) in EtOAc (1 mL). The resultant solution was stirred at room temperature for 0.5 h. Anhydrous Νa 2 SO 4 (1.5 g) was added and stirring was continued for an additional 0.5 h. The reaction mixture was filtered and concentrated, and the crude product was purified by stirring in hot EtOAc (2 mL) and precipitation with hexanes (1 mL). The solid was collected, washed with hexanes, and dried to provide the title compound (106 mg). Example 13: N-[3-(4-morpholine)carbonyl-2(R)-(l-naphthyl)methyl] propionyl-L-leucine boronic acid [MG-324] A. 1-Naphthalenecarboxaldehyde To a cold (-78 °C) solution of oxalyl chloride (6.9 mL, 0.079 mol) in dry CH 2 C1 2 (200 mL) was added dropwise dry DMSO (11.2 mL, 0.158 mol). The mixture was stirred for 10 min, and then a solution of 1 -naphthalenemethanol (10.0 g, 0.063 mol) in dry CH 2 C1 2 (40 mL) was added over 15 min. The mixture was stirred for 10 min, and then Et 3 N (44 mL, 0.316 mol) was added slowly. The reaction mixture was allowed to warm to room temperature. After 3.5 h, to the pale yellow heterogeneous mixture was added 10% aqueous citric acid (30 mL) and water (100 mL). The organic phase was washed with water (100 mL) and saturated aqueous NaCl (100 mL), dried (anhydrous MgSO 4 ), filtered, and concentrated. Ether-hexane (1:1) was added, and the mixture was filtered. Concentration provided a pale orange oil (9.7 g). B. Ethyl 3-(l-naphthyl)propenoate To a solution of the product of Example 12A (9.7 g, 62 mmol) in CH 2 C1 2 (150 mL) was added at room temperature (carbethoxymethylene) triphenylphosphorane (25 g, 71 mmol). The resultant mixture was stirred for 1.5 h, and the homogeneous yellow solution was then concentrated to dryness. Ether-hexane (1 :1) was added, the mixture was filtered, and the filtrate was concentrated to dryness to provide a pale orange oil (15.3 g). C. Ethyl 3-(l-naphthyl)propionate The product of Example 12B (15.3 g, 68 mmol) was dissolved in a mixture of EtOAc ( 100 mL) and MeOH ( 10 mL) and hydrogenated at 1 atm. over 10% Pd/C (0.5 g). The reaction was continued for 4 days, replacing the catalyst with fresh catalyst several times. The reaction mixture was filtered and concentrated to provide 13 g of a crude oil. D. 3-(l-Naphthyl)propionic acid To a solution of the product of Example 12C ( 13 g) in a mixture of THF (100 mL) and water (25 mL) was added IN ΝaOH (75 mL, 75 mmol). The brown reaction mixture was stirred at room temperature overnight. The THF was removed, and the aqueous layer was washed with ether (2 x 50 mL). The aqueous layer was acidified to pH 2 with 6N HCl and the precipitated solid was collected, washed with water (100 mL), and lyophilized to give 9.3 g of a pale yellow solid. E. 3-(l-Naphthyl)propionyl chloride To a suspension of the product of Example 12D (4.0 g, 20 mmol) in CH 2 C1 2 (25 mL) at 0°C was added oxalyl chloride (1.9 mL, 22 mmol) and DMF (0.1 mL). The reaction mixture was warmed to room temperature and then heated with a heat gun. Additional oxalyl chloride (0.5 mL) was added and heating was continued to produce a dark homogeneous mixture. The reaction mixture was concentrated, the residue was redissolved in CH 2 Cl 2 -hexane, and the resultant solution was filtered. Concentration afforded 4.9 g of a green liquid. F. 4 (S)-Isopropyl-3-f3-(l -naphthyl)-! -oxopropylJ-2-oxazolidinone To a solution of (45)-(-)-4-isopropyl-2-oxazolidinone (2.32 g, 18 mmol) in dry THF (50 mL) at -78 °C was added dropwise n-BuLi (2.5 in hexanes, 8 mL, 20 mmol). The heterogeneous white mixture was stirred at -78 °C for 30 min, and then a solution of the product of Example 12E (4.9 g, 20 mmol) in dry THF (25 mL) was added dropwise over 15-20 min. After 1.5 h, the reaction was quenched by the addition of IN HCl (25 mL) and saturated aqueous ΝaCl (25 mL). The mixture was stirred at room temperature for 30 min, and then the THF was removed by rotary evaporation. The aqueous layer was extracted with EtOAc, and the combined organic extract was dried (anhydrous MgSO 4 ), filtered, and concentrated. The residue was filtered through a pad of silica gel (elution with 20% EtOAc-hexanes) to provide 2.8 g of a pale pink solid. 3-[3-Benzyloxycarbonyl-2(R)-[(l-naphthyl)methyl]-l- oxopropyl]-4(S)-isopropyl-2-oxazolidinone To a solution of 1,1, 1,3,3 ,3-hexamethyldisilazane (0.75 mL, 3.5 mmol) in dry THF (10 mL) at 0°C was added n-BuLi (2.5Λ in hexanes, 1.45 mL, 3.6 mmol). After 10 min, the mixture was cooled to -78 °C and a solution of the product of Example 12F (1.0 g, 3.2 mmol) in dry THF (8 mL) was added dropwise. After 30-40 min, benzyl bromoacetate (0.75 mL, 4.8 mmol) was added. The mixture was stirred at -78 °C for 1 h, and at 0°C for 5-10 min. The reaction was quenched by the addition of IN HCl (10 mL), and the solution was extracted with ether. The combined organic extract was washed with saturated aqueous ΝaHCO 3 and saturated aqueous NaCl, dried anhydrous MgSO ), filtered and concentrated. The wet solid was triturated with hexane-ether (1 :1), filtered, and dried to give the title compound (0.6 g) as a white solid. H. 3-/2(R)-(l-naphthyl)methyl/-3-/4(S)-isopropyl-2-oxazolidinoylJ propanoic acid To the product of Example 12G (600 mg, 1.3 mmol) was added MeOH (15 mL), EtOH (15 mL), EtOAc (5 mL), and CH 2 C1 2 (5 mL), followed by 10% Pd/C (100 mg). The reaction mixture was hydrogenated under 1 atm. H 2 . The reaction mixture was filtered and concentrated. The residue was triturated with ether-hexanes, the solvents were removed, and the resultant white solid was dried in vacuo to give 480 mg of the title compound. 4(S)-Isopropyl-3-/4-morpholino-2(R)-(l-naphthyl)methyl-l,4- dioxobutylJ-2-oxazolidinone To a solution of the product of Example 12H (473 mg, 1.28 mmol) in dry THF (25 mL) at 0°C was added dropwise under nitrogen moφholine (130 mL, 1.47 mmol). diethyl pyrocarbonate (240 mL, 1.47 mmol), and triethylamine (220 mL, 1.6 mmol). After 2 h, the solvent was removed in vacuo, and the residue was washed with water and extracted with ether-EtOAc (1 :1). The combined organic extract was dried (anhydrous MgSO 4 ), filtered, and concentrated. The residue was triturated with EtOAc-hexanes to provide the title compound (410 mg). 3-(4-morpholine)carbonyl-2(R)-(l-naphthyl)methyl propionic acid To a solution of the product of Example 121 (400 mg, 0.913 mmol) in a mixture of THF (8 mL) and water (2 mL) at 0°C was added LiOH (80 mg, 1.9 mmol). The reaction mixture was stored at 0°C overnight. The reaction mixture was concentrated to remove THF, IN ΝaOH (20 mL) was added, and the mixture was washed with CH 2 C1 2 (15 mL). The aqueous layer was acidified to pH 2 with IN HCl and extracted with CH 2 C1 2 . The combined organic extract was dried (anhydrous MgSO 4 ), filtered, and concentrated. The residue was triturated with ether-hexanes, and the solvents were removed in vacuo to provide the crude product (240 mg) as a white foam. K. (lS,2S ,5S)-PinanediolN-[3-(4-morpholme)carbonyl-2(R)-(l- naphthyl)methyl]propionyl-L-leucine boronate To a solution of the product of Example 12J (230 mg, 0.7 mmol) in DMF (8 mL) at 0°C was added (lS,2S,37?,55 -pinanediol leucine boronate trifluoroacetate salt (293 mg, 0.77 mmol) and TBTU (293 mg, (0.77 mmol). To the resultant mixture was added slowly over 1.5 h diisopropylethylamine (365 mL, 2.1 mmol). After addition was complete, the reaction mixture was stirred for 30 min. Water (100 mL) was added, and the precipitated solid was collected, washed with water (50 mL), and lyophilized to provide the title compound (300 mg). N-[3-(4-morpholine)carbonyl-2(R)-(l-naphthyl)methyl] propionyl-L-leucine boronic acid By a procedure analogous to that described in Example 3B, the product of Example 12K (300 mg, 0.522 mmol) was deprotected to provide the title compound (150 mg). Example 14: trans-4-Phenoxy-L-proline-L-leucine boronic acid [MG-349] A. N-Carbobenzyloxy-trans-4-hydroxy-L-proline According to the literature procedure (J. Am. Chem. Soc. 189 (1957)), trα/w-4-hydroxy-L-proline (5.12 g, 0.039 mol) was treated with benzyl chloroformate (8.5 mL, 0.06 mol) to provide the title compound (6.0 g) as a white solid. B. N-Carbobenzyloxy-trans-4-hydroxy-L-proline methyl ester To a solution of the product of Example 13A (1.08 g, 3.75 mmol) in acetonitrile (4 mL) at 0°C was added dropwise DBU (0.62 mL, 4.12 mmol). After 5 min, Mel (0.28 mL, 4.5 mmol) was added. The reaction mixture was allowed to warm to room temperature and stir overnight. The solvent was removed, the residue was dissolved in ether-EtOAc (1:1, 30 mL), and the resultant solution was washed with IN HCl, dilute aqueous ΝaHCO 3 , water, and saturated aqueous NaCl. The organic layer was dried (anhydrous MgSO 4 ) and concentrated to provide the title compound (822 mg) as a light yellow oil. C. N-Carbobenzyloxy-trans-4-phenoxy-L-proline methyl ester To a mixture of the product of Example 13B (495 mg, 1.71 mmol), phenol (193 mg, 2.05 mmol), and triphenylphosphine (537 mg, 2.05 mmol) in THF (7 mL) at 0°C was added over 1 h diethyl azodicarboxylate (0.32 mL, 2.05 mmol). The reaction mixture was allowed to warm to room temperature and stir overnight. The reaction mixture was concentrated, and the residue was dissolved in ether (8 mL) and allowed to stand at 0°C overnight. The solution was decanted and the solids were washed with cold ether. The ethereal solution was concentrated, and the residue was purified by flash chromatography (elution with 10-30% EtOAc-hexanes) to provide the title compound (295 mg). D. N-Carbobenzyloxy-trans-4-phenoxy-L-proline The product of Example 13C (285 mg, 0.79 mmol) was dissolved in a mixture of 0.5N aqueous LiOH (20 mL) and MeOH (10 mL), and the resultant solution was stirred at room temperature overnight. The MeOH was removed in vacuo, and the aqueous layer was washed with ether (2 x 20 mL). The aqueous layer was cooled, acidified with 3NHC1, and extracted with EtOAc (3 x 20 mL). The combined organic extract was washed with water and saturated aqueous ΝaCl, dried (anhydrous MgSO 4 ), filtered, and concentrated to provide the title compound (251 mg) as a light yellow solid. E. (lS,2S,3R,5S)-pinanediol N-Carbobenzyloxy-trans-4-phenoxy- L-proline-L-leucine boronate By a procedure analogous to that described in Example 12K, the product of Example 13D (250 mg, 0.72 mmol) was coupled with (lS,2S,37?,5S)-pinanediol leucine boronate trifluoroacetate salt (300 mg, 0.79 -72 mmol) in the presence of TBTU (302 mg, 0.79 mmol) to provide the title compound (355 mg) as a white solid. F. (lS,2S,3R,5S)-pinanediol trans-4-phenoxy-L-proline-L-leucine boronate The product of Example 13E (343 mg) was hydrogenated for 20 h at 1 atm. over 10% Pd/C (45 mg) in EtOH (3 mL). The reaction mixture was filtered through Celite and concentrated to provide the title compound (272 mg). G. trans-4-Phenoxy-L-proline-L-leucine boronic acid By a procedure analogous to that described in Example 3B, the product of Example 13F (270 mg, 0.6 mmol) was deprotected to provide the title compound (130 mg) as a white solid. Example 15: f(3S,5R)-4-/(8-quinolinesulfonyl)aminoJ- 3-hydroxy-5-(l- naphthyl)pentanoyl]-L-leucine boronic acid A. (4S,5S)-l-Boc-4-hydroxy-5-(l-naphthyl)-pyrrolidin-2-one To a solution of N-Boc-β-(l-naphthyl)-L-alanine (1.4 g, 4.44 mmol), 2,2-dimethyl-l,3-dioxane-4,6-dione (704 mg, 4.88 mmol), and 4-DMAP (1.25 g, 10.21 mmol) in CH 2 C1 2 (40 mL) at 0°C was added isopropenyl chloroformate (0.53 mL, 4.8 mmol). The reaction mixture was stirred for 1 h at 0°C and for 2 h at room temperature. The reaction was quenched by the addition of aqueous KHSO 4 . The organic layer was washed with water, dried (anhydrous MgSO 4 ), filtered, and concentrated. The residue was suspended in EtOAc (30 mL) and heated at reflux for 2 h. The solvent was removed in vacuo. The residue was dissolved in CH 2 Cl 2 -HOAc (10:1, 30 mL), and sodium borohydride (310 mg. 8.21 mmol) was added at 0°C. The mixture was stirred for 1 h at 0°C and for 15 h at room temperature. Water was added, and the organic layer was washed with saturated aqueous NaCl, dried (anhydrous MgSO 4 ), filtered, and concentrated. Purification by silica gel chromatography (elution with 20-30% acetone-hexanes) afforded the title compound (1.24 g). B. (3S, 5R) -4- (tert-b utyloxycarbonyl) amino-3-hydroxy-5- (1 - naphthyl)pentanoic acid The product of Example 14B (1.24 g, 3.64 mmol) was dissolved in acetone (15 mL) and aqueous NaOH (\M, 4 mL, 4 mmol) was added. The reaction mixture was stirred at room temperature for 2 h. The mixture was acidified with 10% HCl and extracted with EtOAc (3 x 60 mL). The combined organic extract was washed with water, dried (anhydrous MgSO 4 ), filtered, and concentrated. The residue was purified by silica gel chromatography (elution with 30-50% acetone-hexanes and 70:30: 10 hexane:acetone:methanol) to give the title compound (0.61 g). C. (lS,2S,3R t 5S)-Pinanediol f(3S,5R)-4-(tert-butyloxycarbonyl) amino-3-hydroxy-5-(l-naphthyl)pentanoyl]-L-leucine boronate By a procedure analogous to that described in Example 2, the product of Example 14B (395 mg, 1.1 mmol) was coupled with (lS,2S,3Λ,5S)-pinanediol leucine boronate trifluoroacetate salt (415 mg, 1.1 mol) in the presence of BOP reagent (487 mg, 1.1 mmol) to afford the title compound (261 mg). D. (lS,2S,3R,5S)-Pinanediol [(3S,5R)-4-(8-quinolinesulfonyl) amino-3-hydroxy-5-(l-naphthyl)pentanoyl]-L-leucine boronate The product of Example 14C (261 mg, 0.43 mmol) was dissolved in CH 2 C1 : (10 L) and treated at 0°C with trifluoroacetic acid (5 mL) and thioanisole (1 mL). After 2 h, solvents were evaporated. The residue was dissolved in CH 2 C1 2 (10 mL) and cooled to 0°C. 8-Quinolinesulfonyl chloride (98 mg, 0.43 mmol) and triethylamine (0.12 mL, 0.86 mmol) were added. The reaction mixture was stirred at 0°C for 1 h and at room temperature for 15 h. The solvents were removed, water was added, and the product was extracted with EtOAc (3 x 50 mL). The combined organic extract was washed with saturated aqueous NaHCO 3 and saturated aqueous NaCl, dried (anhydrous MgSO 4 ), and concentrated. The residue was purified by silica gel chromatography (elution with 20-50% EtOAc-hexanes) to provide the title compound (152 mg). E. [(3S,5R)-4-(8-quinolinesulfonyl)amino-3-hydroxy-5- (l-naphthyl)pentanoyl]-L-leucine boronic acid The product of Example 14D (152 mg, 0.22 mmol) was deprotected according to the procedure described in Example 3B to provide the title compound (12.7 mg). Example 16: cis-3-Phenyl-DJ,-proline-L-leucine boronic acid hydrochloride salt/MG-359/ A. Diethyl l-acetyl-4-phenyl-2-pyrrolidinol-5,5-dicarboxylate Sodium spheres (washed 3 x with hexanes and dried in vacuo; 0.13 g, 5.7 mmol) were added to a solution of diethyl acetimidomalonate (12.2 g, 56.1 mmol) in absolute EtOH under nitrogen. After the sodium had dissolved, the solution was cooled in an ice bath and cinnamaldehyde (7.8 mL, 61.7 mmol) was added dropwise. The bath was removed and the reaction mixture was stirred overnight at room temperature. The solution was adjusted to pH 4 with acetic acid (~ 3 mL). Solvents were evaporated and the residue was purified by silica gel chromatography (elution with EtOAc) to give a yellow solid, which was recrystallized (benzene-hexane) to provide the title compound (14.1 g) as a white solid. B. Diethyl l-acetyl-3-phenylpyrrolidine-2,2-dicarboxylate Trifluoroacetic acid (15.4 mL) was added slowly over 15 min to a solution of the product of Example 15A (7.0 g, 20.1 mmol) and triethylsilane (4.9 mL, 30.8 mmol) in CHC1 3 (40 mL). After 3 h, the solvents were evaporated and the residue was dissolved in EtOAc (150 mL), washed with water, 5% aqueous NaHCO 3 , and saturated aqueous NaCl, dried (anhydrous MgSO \ and concentrated to give 5.9 g of a colorless oil. C. N-Acetyl-3-phenylproline ethyl ester The product of Example 15B (5.9 g) was dissolved in 0.5NNaOH (200 mL) and the resultant solution was stirred at room temperature for 21 h. The solution was washed with EtOAc (75 mL) and then acidified to pH 2 with 3N HCl. The precipitated solids were extracted with CHC1 3 . The organic layer was concentrated to give a gummy residue, which was dissolved in toluene (70 mL) and heated at 75 °C for 1 h. The solvent was evaporated to provide the title compound (4.2 g) as a light yellow oil. D. N-Acetyl-trans-3-phenyl-D,L-proline; and N-acetyl-cis-3- phenyl-D,L-pr oline ethyl ester The product of Example 15C (4.2 g, 16 mmol) was dissolved lΛ ΝaOEt in EtOH (100 mL) which contained 2 mL of ethyl trifluoroacetate as a water scavenger, and the resultant solution was heated at reflux for 2 h. The reaction mixture was cooled to room temperature, water (65 mL) was added, and the solution was stirred for 2.5 h. Most of the EtOH was removed by rotary evaporation and the aqueous solution was extracted with CH 2 C1 2 . The aqueous layer was acidified with 3NHC1 and extracted with EtOAc. The organic extract was washed with water and saturated aqueous ΝaCl, dried (anhydrous MgSO 4 ), and concentrated. The orange gummy solid was triturated with ether to provide a yellow solid, which was recrystallized (EtOAc-MeOH) to provide the acid (1.91 g) as light yellow crystals. Concentration of the CH 2 C1 2 extracts afforded the ester (396 mg) as an orange oil. E. cis-3-Phenyl-D,L-pr oline hydrochloride salt The ester obtained in Example 15D (375 mg) was hydrolyzed by heating at reflux in 6N HCl (5 mL) for 17 h. The cooled reaction mixture was washed with EtOAc and the aqueous layer was concentrated to dryness. Recrystallization (MeOH-ether) afforded the title compound (201 mg). F. N-Boc-cis-3-Phenyl-D,L-proline The product of Example 15E (189 mg, 0.84 mmol) was dissolved in a mixture of 2NΝaOH (3 mL) and 1,4-dioxane (3 mL). tert-Butyl pyrocarbonate (218 mg, 1.0 mmol) was added and the reaction mixture was stirred overnight at room temperature. Dioxane was removed by rotary evaporation, water (30 mL) was added, and the mixture was washed with EtOAc. The aqueous phase was cooled to 0°C, acidified with 3NHC1, and extracted with EtOAc. The organic layer was washed with water and saturated aqueous ΝaCl, dried (anhydrous MgSO 4 ), and concentrated to give the title compound (199 mg). G. (lS,2S,3R,5S)-Pinanediol N-Boc-cis-3-phenyl-D,L-proline-L- leucine boronate By a procedure analogous to that described in Example 4B, the product of Example 15F (192 mg, 0.66 mmol) was coupled with (lS,2S,37?,5_S)pinanediol leucine boronate trifluoroacetate salt (274 mg, 0.73 mmol) in the presence of TBTU (277 mg, 0.73 mmol) to provide the title compound (286 mg). H. cis-3-Phenyl-D,L-proline-L-leucine boronic acid hydrochloride salt The product of Example 15G (262 mg) was dissolved in CH 2 C1 2 (5 mL) and treated at 0°C with 4N HCl-dioxane (4 mL). After 2 h, the reaction mixture was concentrated to dryness, and the residue was treated with isobutylboronic acid (66 mg, 0.64 mmol) according to the procedure described in Example 3B to provide the title compound (71 mg) as a white solid. Example 17: trans-3-Phenyl-D,L-proline-L-leucine boronic acid hydrochloride salt [MG-363] A. N-Boc-trans-3-Phenyl-L-proline By a procedure analogous to that described in Example 1A, N-acetyl- trαns-3-phenyl-D,L-proline (prepared as described in Example 15D; 1.5 g, 6.44 mmol) was coupled with (_S)-a-methylbenzylamine (0.92 mL, 7.08 mmol) in the presence of EDC (1.26 g, 7.08 mmol) and HOBT 9956 mg, 7.08 mmol). The diastereomeric products were separated by flash chromatography (elution with 1.5-2.5% HOAc-EtOAc). Fractions corresponding to the slower eluting band were concentrated to provide a clear, colorless oil (913 mg). The oil (900 mg, 2.68 mmol) was dissolved in a mixture of HOAc (7 mL) and 8N HCl and the mixture was heated at reflux for 18 h. The mixture was concentrated to dryness. The residue was dissolved in water (30 mL), washed with EtOAc, and again concentrated to dryness. The residue was redissolved in 1 : 1 water- 1 ,4-dioxane ( 15 mL) and treated with tert-butyl pyrocarbonate (1.13 g, 5.20 mmol) by a procedure analogous to that described in Example 15F to provide the title compound (574 mg) as a white solid. B. trans-3-Phenyl-L-proline-L-leucine boronic acid hydrochloride salt By procedures analogous to those described in Examples 15G-H, the product of Example 16A (332 mg, 1.14 mmol) was coupled with (lS,2S,37?,5S)-pinanediol leucine boronate trifluoroacetate salt (452 mg, 1.20 mmol) and deprotected to provide the title compound (101 mg) as a white solid. Example 18: Kinetic experiments Table II summarizes results from kinetic experiments that measured the inhibition of the 20S proteasome by compounds having the formula of compound (1) or (2). P, AA 1 , AA 2 , AA 3 , and Z 1 and Z 2 represent the structures present on formula (1) or (2). The protocol for the kinetic assay described in Tables II- V is as described in Rock et al, Cell 75:761-771 (1994). In these tables, K t values are reported, which are dissociation constants for the equilibrium that is established when enzyme and inhibitor interact to form the enzymeήnhibitor complex. The reactions were performed using SDS-activated 20S proteasome from rabbit muscle. The substrate used was Suc-LLVY-AMC. C r O O In Table III, P, AA 1 , AA 2 , AA 3 , and X are substituents of the general formula: P - AA 1 - AA 2 - AAX X Table III demonstrates that dipeptide boronic acids have lower K, values than the corresponding dipeptide aldehydes. Table III Comparison of Dipeptide Boronic Acids to Dipeptide Aldehydes In Table IV, P, AA 1 , AA 2 , AA 3 , and X are substituents of the general formula: P - AA' - AA 2 - AA 3 - X. Table IV demonstrates the markedly superior selectivity for the 20S proteasome over other proteases, e.g. Cathepsin B, exhibited by the boronic esters/acids as compared to the peptide aldehydes. Table IV Inhibition of the 20S Proteasome by Boronic Ester and Acid Compounds The selectivity of boronic acid inhibitors of the proteasome is further demonstrated in Table V. Table V Selectivity of Boronic Ester and Acid Inhibitors of the 20S Proteasome Example 19: Inhibition of Protein Degradation in C2C12 Cells C2C12 cells (a mouse myoblast line) were labelled for 48 hrs with 35 S-methionine. The cells were then washed and preincubated for 2 hrs in the same media supplemented with 2mM unlabelled methionine. The media was removed and replaced with a fresh aliquot of the preincubation media containing 50% serum, and a concentration of the compound to be tested. The media was then removed and made up to 10% TCA and centrifuged. The TCA soluble radioactivity was counted. Inhibition of proteolysis was calculated as the percent decrease in TCA soluble radioactivity. From this data, an EC 50 for each compound was calculated. Data for compounds of formula (1) or (2) are presented in Table VI. Table VI Inhibition of Protein Degradation in C2C12 Cells by Boronic Ester and Acid Compounds t C O Example 20: MG-273 Inhibits Corticosterone-Induced Cachexia in Rats Rats were stabilized on a diet free from 3-methylhistidine and then placed in metabolic cages for collection of 24-hour urine samples. After two days of urine collections to determine basal 3-methylhistidine output, the rats were treated with daily subcutaneous injections of corticosterone (100 mg/kg). Starting on the second day of corticosterone treatment, some of the rats were also treated with MG-273, administered via a subcutaneous osmotic pump at a dose rate of approximately 120 μg/kg body weight/day. Control rats received vehicle only (25% DMSO/75% PEG (200)), administered in a similar fashion. Figure 1 shows that treatment with MG-273 reduced the urinary output of 3-methylhistidine, which was induced in response to corticosterone treatment. Example 21: MG-273 Inhibits the Activation ofNF-κB This assay was performed as previously described (Palombella, et al. Cell, 78:773-785 (1994)). MG63 osteosarcoma cells were stimulated by treatment with TNF-α for the designated times. Whole cell extracts were prepared and analyzed by electrophoretic mobility shift assay using the PRDII probe from the human IFN-β gene promoter. Figure 2 shows that NF-κB binding activity was inhibited by pretreatment for 1 hour with MG 273. An aldehyde inhibitor of the proteasome, MG-132 (Cbz-L-Leu-L-Leu-L-Leu-H), also inhibited NF-κB binding activity, whereas MG- 102 (Ac-L-Leu-L-Leu-L-Met-H), which is inactive against the 20S proteasome, did not inhibit NF-κB binding activity. Example 22: MG-273 Inhibits Expression of Cell Adhesion Molecules on HUVE Cells HUVECs in microtiter plates were exposed to the indicated concentrations of inhibitor for 1 hour, prior to the addition of 100 U/mL TNF-α. Cell surface binding assays were performed at 4°C, using saturating concentrations of monoclonal antibodies specific for the cell adhesion molecules (Becton Dickenson) and fluorescent-conjugated F(ab') 2 goat anti-murine IgG (Caltag Labs, San Francisco, CA). Fluorescent immunoassays for E-selectin and I-CAM were performed at 4 hours, those for V-CAM at 16 hours. Figure 3 shows that cell -surface expression I-CAM, V-CAM, and E-selectin on TNF-α stimulated HUVECs is significantly inhibited by MG-273 at concentrations of 0.5 μM or above. Example 23: Boronic Acid Compounds Block the DTH Response in Mice Naive mice were sensitized by the application of 20 μL of a 0.5% (v/v) solution of 2,4-dinitrofluorobenzene in 4:1 acetone/olive oil to both of the rear limb footpads. This procedure is performed on two consecutive days, which are referred to as days 0 and 1. The efferent phase of the contact sensitivity response was elicited on day 5 by the application of 10 μL of a 0.2% (v/v) solution of 2,4-dinitrofluorobenzene in 4: 1 acetone/olive oil to both sides of the left ear. The contralateral control ear was treated on both sides with 10 μL of vehicle only. The mice were lightly anaesthetized for this procedure by the intraperitoneal (i.p.) injection of a mixture of ketamine (80 mg/kg, Henry Schein) and xylazine (16 mg/kg, Henry Schein). Test compounds were administered orally as a suspension in 0.5% methylcellulose (4000 centipoises Fisher Scientific) 30 minutes prior to the application of the challenge dose of 2,4-dinitrofluorobenzene to the ears. The dose was delivered in a final volume of 0.5 mL using a 24 gauge 1 inch malleable feeding needle with a 1.25 mm ball tip (Roboz Surgical). Approximately 18 hours after the challenge, ear swelling was determined by measuring both the control and the experimental ear using a Mitutoyo Digital micrometer. The absolute difference in thickness of the experimental (left) ears vs. the control (right) ears was determined for each treatment group. Efficacy was determined by comparing this difference in thickness to the difference calculated for the vehicle control group. Test results are provided in Table VII. Table VII Inhibition of the DTH Response in Mice All publications and U.S. patent applications mentioned hereinabove are hereby incoφorated in their entirety by reference. While the foregoing invention has been described in some detail for purposes of clarity and understanding, it will be appreciated by one skilled in the art from a reading of this disclosure that various changes in form and detail can be made without departing from the true scope of the invention and appended claims.

Claims: What Is Claimed Is: A compound having the formula: and pharmaceutically acceptable salts thereof; wherein P is R 7 -C(θχ or R 7 -SO 2 -, where R 7 is one of aryl, aralkyl, heteroaryl or heteroarylalkyl, the ring portion of any of which can be optionally substituted, or when P is R 7 -C(O)-, R 7 can also be N-moφholinyl; B', at each occurrence, is independently one of N or CH; X 1 , at each occurrence, is independently one of-C(O)-NH-, -CH 2 -NH-, -CH(OH)-CH 2 -, -CH(OH)-CH(OH)-, -CH(OH)-CH 2 -NH-, -CH=CH-, -C(O)-CH 2 -, -SO 2 -NH-, -SO 2 -CH 2 - or -CH(OH)-CH 2 -C(O)-NH-, provided that when B 1 is N, then the X 1 attached to said B 1 is -C(O)-NH-; X 2 is one of -C(O)-NH-, -CH(OH)-CH 2 -, -CH(OH)-CH(OH)-, _C(0)-CH 2 -, -SO 2 -NH-, -SO 2 -CH 2 - or -CH(OH)-CH 2 -C(O)-NH-; R is hydrogen or alkyl, or R forms together with the adjacent R 1 , or when A is zero, forms together with the adjacent R 2 , a nitrogen-containing mono-, bi- or tri-cyclic, saturated or partially saturated ring system having 4-14 ring members, that can be optionally substituted by one or two of keto, hydroxy, alkyl, aryl, aralkyl, alkoxy or aryloxy; R\ at each occurrence, is independently one of hydrogen, alkyl, cycloalkyl, aryl. a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CHr-R 5 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; R 2 is one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 5 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; R 3 is one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R\ where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; R 5 , in each instance, is one of aryl, aralkyl, alkaryl, cycloalkyl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -W-R 6 , where W is a chalcogen and R 6 is alkyl, where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; Z 1 and Z 2 are independently one of alkyl, hydroxy, alkoxy, or aryloxy, or together Z 1 and Z 2 form a moiety derived from a dihydroxy compound having at least two hydroxy groups separated by at least two connecting atoms in a chain or ring, said chain or ring comprising carbon atoms, and optionally, a heteroatom or heteroatoms which can be N, S, or O; and A is 0, 1 , or 2. 2. The compound of claim 1 , wherein: A is zero; X is -C(O)-NH-; R is hydrogen or C,.,alkyl; and R 3 is C,. 6 alkyl. 3. The compound of claim 2, wherein R 3 is C 4 alkyl. 4. The compound of claim 1, wherein: P is R 7 -C(OX or R 7 -SO 2 -, where R 7 is one of quinolinyl, quinoxalinyl, pyridyl, pyrazinyl, furanyl or pyrrolyl. or when P is R 7 -C(O)-, R 7 can also be N-moφholinyl. 5. The compound of claim 1 , wherein P is one of quinolinecarbonyl, pyridinecarbonyl, quinolinesulfonyl, quinoxalinecarbonyl, quinoxalinesulfonyl, pyrazinecarbonyl, pyrazinesulfonyl, furancarbonyl, furansulfonyl or N-moφholinylcarbonyl . 6. The compound of claim 5, wherein P is one of 8-quinolinecarbonyl, 8-quinolinesulfonyl, 2-quinoxalinecarbonyl, 2-quinoxalinesulfonyl, 2-pyrazinecarbonyl, 2-pyrazinesulfonyl, 3-furancarbonyl, 3 -furansulfonyl or N-moφholinecarbonyl. 7. The compound of claim 1 , wherein A is 0. 8. The compound of claim 1 , wherein B 1 , at each occurrence, is CH. 9. The compound of claim 8, wherein X 1 , at each occurrence, is -C(O)-NH-. 10. The compound of claim 9, wherein X 2 is -C(O)-NH-. 11. The compound of claim 1 , wherein R is hydrogen or C,. 8 alkyl. 12. The compound of claim 1, wherein: R', at each occurrence, and R 2 and R 3 are each independently one of hydrogen, C,. g alkyl, C 3 . I0 cycloalkyl, .,,, aryl, a 5-, 6-, 9- or 10- membered heteroaryl group, or -CH 2 -R 5 ; R 5 , in each instance, C 6 . 10 ar(C,. 6 )alkyl,C w alk(C 6 .,o)aιyl, C 3 ., 0 cycloalkyl, C,. g alkoxy, C,. g alkylthio or a 5-, 6-, 9- or 10- membered heteroaryl group; where the ring portion of any of said aryl, aralkyl, alkaryl or 5-, 6-, 9- or 10- membered heteroaryl groups of R 1 . R 2 , R 3 and R 5 can be optionally substituted by one or two substituents independently selected from the group consisting of C 1-6 alkyl, C 3 . g cycloalkyl, C 1-6 alkyl(C 3 . g )cycloalkyl, C 2.g alkenyl, C 2-g alkynyl, cyano, amino, C,. 6 alkylamino, di(C )alkylamino, benzylamino, dibenzylamino, nitro, carboxy, carbo(C,^)alkoxy, trifluoromethyl, halogen, C,. 6 alkoxy, hydroxy, C^alkylthio, C^alkylsulfmyl, C,. 6 alkylsulfonyl, C^^arylthio, C^^arylsulfinyl, C 6 . 10 aryl, C,. 6 alkyl(C 6 .,o)aryl, and halo(C 6 . 10 )aryl. 13. The compound of claim 1, wherein R 3 is C M2 alkyl. 14. The compound of claim 1, wherein R 3 is C|. 6 alkyl. 15. The compound of claim 1, wherein R 3 is C 4 alkyl. 16. The compound of claim 1, wherein R 3 is isobutyl. 17. The compound of claim 1, wherein R 2 is one of isobutyl, 1 -naphthylmethyl, 2-naphthylmethyl, 3 -pyridylmethyl, 2-pyridylmethyl 6- quinolinylmethyl, 3-indolylmethyl, benzyl, 4-fluorobenzyl, 4-hydroxybenzyl, 4-(2'-pyridylmethoxy)benzyl, 4-(benzyloxy)benzyl, benzylnaphthylmethyl or phenethyl. 18. The compound of claim 1, wherein Z 1 and Z 2 are independently one of C,. 6 alkyl, hydroxy, C^alkoxy, or C^^aryloxy. 19. The compound of claim 18, wherein Z 1 and Z 2 are both hydroxy. 20. The compound of claim 1, wherein together Z 1 and Z 2 form a moiety derived from a dihydroxy compound selected from the group consisting of pinacol, perfluoropinacol, pinanediol, ethylene glycol, diethylene glycol, 1,2-cyclohexanediol, 1,3-propanediol, 2,3-butanediol, glycerol or diethanolamine. 21. The compound of claim 1 , wherein : P is one of quinolinecarbonyl, pyridinecarbonyl, quinolinesulfonyl, quinoxalinecarbonyl, quinoxalinesulfonyl, pyrazinecarbonyl, pyrazinesulfonyl, furancarbonyl, furansulfonyl or N-moφholinylcarbonyl; A is zero; X 2 is -C(O)-NH-; R is hydrogen or C,. g alkyl; R 2 and R 3 are each independently one of hydrogen, C,. g alkyl, C 6 . 10 ar(C,. 6 )alkyl, pyridylmethyl, or quinolinylmethyl; and Z 1 and Z 2 are both hydroxy, C, ^alkoxy, or C 6 ., 0 aryloxy, or together Z 1 and Z 2 form a moiety derived from a dihydroxy compound selected from the group consisting of pinacol, perfluoropinacol, pinanediol, ethylene glycol, diethylene glycol, 1 ,2-cyclohexanediol, 1,3-propanediol, 2,3-butanediol, glycerol or diethanolamine. 22. The compound of claim 1 , wherein: P is one of 8-quinolinecarbonyl, 8-quinolinesulfonyl, 2-quinoxalinecarbonyl, 2-quinoxalinesulfonyl, 2-pyrazinecarbonyl, 2-pyrazinesulfonyl, 3-pyridinecarbonyl, 3-pyridinesulfonyl, 3-furancarbonyl, 3-furansulfonyl or N-moφholinecarbonyl; A is zero; X 2 is-C(O)-NH-; R is hydrogen or C,. g alkyl; R 3 is isobutyl; R- is one of isobutyl, 1 -naphthylmethyl, 2-naphthylmethyl, -pyridylmethyl, 2-pyridylmethyl 6-quinolinylmethyl, 3-indolylmethyl, benzyl, 4-fluorobenzyl, 4-hydroxybenzyl, 4-(2'-pyridylmethoxy)benzyl, 4-(benzyloxy)benzyl, benzylnaphthylmethyl or phenethyl; and Z 1 and Z 2 are independently one of hydroxy, C, or together Z 1 and Z 2 form a moiety derived from a dihydroxy compound selected from the group consisting of pinacol, perfluoropinacol, pinanediol, ethylene glycol, diethylene glycol, 1,2-cyclohexanediol, 1,3-propanediol, 2,3-butanediol, glycerol or diethanolamine. 23. The compound of claim 1, wherein said compound is one of: N-(2-pyrazine)carbonyl-L-phenylalanine-L-leucine boronic acid, N-(2-quinoline)sulfonyl-L-homophenylalanine-L-leucine boronic acid, N-(3-pyridine)carbonyl-L-phenylalanine-L-leucine boronic acid, N-(4-mθφholine)carbonyl-L-phenylalanine-L-leucine boronic acid, N-(4-moφholine)carbonyl-β-( 1 -naphthyl)-L-alanine-L-leucine boronic acid, N-(8-quinoline)sulfonyl-β-(l-naphthyl)-L-alanine-L-leucine boronic acid, N-(4-moφholine)carbonyl-(0-benzyl)-L-tyrosine-L-leucine boronic acid, N-(4-moφhoIine)carbonyl-L-tyrosine-L-leucine boronic acid, N-(4-moφholine)carbonyl-[0-(2-pyridylmethyl)]-L-tyrosine-L-leucine boronic acid; or isosteres, pharmaceutically acceptable salts or boronate esters thereof. 24. The compound of claim 23, wherein said compound is N-(2-pyrazine)carbonyl-L-phenylalanine-L-leucine boronic acid, or an isostere, pharmaceutically acceptable salt or boronate ester thereof. 25. A compound having the formula: P— Ν- B 1 — X 1 {— CH— X 2 — CH— B(Z 1 )(Z 2 ) I I I R R 1 R 2 R 3 (la) wherein P is hydrogen or an amino-group-protecting moiety; B 1 , at each occurrence, is independently one of N or CH; X 1 , at each occurrence, is independently one of-C(O)-NH-, -CH 2 -NH-, -CH(OH)-CH 2 -, -CH(OH)-CH(OH)-, -CH(OH)-CH 2 -NH-, -CH=CH- -C(O)-CH 2 -, -SO 2 -NH-, -SO 2 -CH 2 - or -CH(OHH H 2 -C(O)-NH-, provided that when B 1 is N, then the X 1 attached to said B' is -C(O)-NH-; X 2 is one of -C(O)-NH-, -CH(OH)-CH 2 -, -CH(OH)-CH(OH)-, -C(O)-CH 2 -, -SO 2 -NH- -SO 2 -CH 2 - or -CH(OH)-CH 2 -C(O)-NH-; R is hydrogen or alkyl, or R forms together with the adjacent R 1 , or when A is zero, forms together with the adjacent R 2 , a nitrogen-containing mono-, bi- or tri-cyclic, saturated or partially saturated ring system having 4-14 ring members, that can be optionally substituted by one or two of keto, hydroxy, aryl, alkoxy or aryloxy; R 1 at each occurrence, R 2 and R 3 are each independently one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 5 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; R\ in each instance, is one of aryl, aralkyl, alkaryl, cycloalkyl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -W-R 6 , where W is a chalcogen and R 6 is alkyl, where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted, provided that at least one R 1 , R 2 or R 3 is naphthylmethyl, pyridylmethyl or quinolinylmethyl; Z 1 and Z 2 are independently one of alkyl, hydroxy, alkoxy, or aryloxy, or together Z 1 and Z 2 form a moiety derived from a dihydroxy compound having at least two hydroxy groups separated by at least two connecting atoms in a chain or ring, said chain or ring comprising carbon atoms, and optionally, a heteroatom or heteroatoms which can be N. S, or O; and or ring, said chain or ring comprising carbon atoms, and optionally, a heteroatom or heteroatoms which can be N, S, or O; and A is O, l, or 2; provided that the compound is other than isovaleryl-phenylalanine- norvaline-[(naphthylmethyl), (4,4,5,5-tetramethyl- 1 ,3,2-dioxaborolane-2- yl)]methylamide or (3-t-butylsulfonyl)propionyl-norvaline-( 1 -naphthyl, dihydroxyboryl)methylamide. 26. The compound of claim 25, wherein P is R 7 -C(O)-, R 7 -SO 2 -, R 7 -NH-C(OX or R 7 -O-C(O)-, and R 7 is one of alkyl, aryl, alkaryl, aralkyl, heteroaryl or heteroarylalkyl, any of which can be optionally substituted, or when P is R 7 -C(O)-, then R 7 can also be saturated or partially saturated heterocycle. 27. The compound of claim 25, wherein P is R 7 -C(O>- or R 7 -SO 2 -; and R 7 is one of .,,, aryl, ., 0 ar(C,^)alkyl, 5- to 10-membered heteroaryl or 5- to 10-membered any of which can be optionally substituted, or when P is R 7 -C(O)-, R 7 can also be N-moφholinyl. 28. The compound of claim 25, wherein B 1 is CH, and X 1 and X 2 are each -C(O>-NH-. 29. The compound of claim 25, wherein R 1 and R 2 are independently selected from the group consisting of alkyl and — CH 2 — R 5 , where R 5 is one of C 6- ιo ryl, C M0 alk(C 6 ., 0 )aryl, C 3-I0 cycloalkyl, or a 5-, 6-, 9- or 10-membered heterocycle. 30. The compound of claim 25, wherein A is zero. 31. The compound of claim 25, wherein R 2 is quinolinylmethyl. 32. The compound of claim 25, wherein said compound is one of: N-(4-moφholine)carbonyl-β-(l-naphthyl)-L-alanine-L-leucine boronic acid, or N-(8-quinoline)sulfonyl-β-(l -naphthyl)-L-alanine-L-leucine boronic acid; or isosteres, pharmaceutically acceptable salts or boronate esters thereof. 33. A compound having the formula: and pharmaceutically acceptable salts thereof; wherein P is hydrogen or an amino-group-protecting moiety; B', at each occurrence, is independently one of Ν or CH; X 1 , at each occurrence, is independently one of-C(O)-ΝH-, -CH 2 -NH-, -CH(OH)-CH 2 -, -CH(OH)-CH(OH)-, -CH(OH)-CH 2 -NH-, -CH=CH-, -C(O>-CH 2 -, -SO 2 -NH- -SO 2 -CH 2 - or -CH(OH)-CH 2 -C(O)-NH-, provided that when B' is N, then the X 1 attached to said B 1 is -C(O)-NH-; X 2 is one of -C(O)-NH-, -CH(OH)-CH 2 -, -CH(OH)-CH(OH)-, -C(OXCH 2 -, -SO 2 -NH-, -SO 2 -CH 2 - or -CH(OH)-CH 2 -C(O)-NH-; R forms together with the adjacent R 1 , or when A is zero, forms together with the adjacent R 2 , a nitrogen-containing mono-, bi- or tri-cyclic, saturated or partially saturated ring system having 4-14 ring members, and one or two optional substituents selected from the group consisting of keto, hydroxy, alkyl, aryl, aralkyl, alkoxy and aryloxy; when A is 2, the R 1 that is not adjacent to N-R is one of hydrogen, alkyl, cycloalkyl. aryl, a 5- to 10-membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 5 ; when A is 1 or 2, R 2 is one of hydrogen, alkyl, cycloalkyl, aryl, a 5- to 10- membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 5 ; R 3 is one of hydrogen, alkyl, cycloalkyl, aryl, a 5- to 10-membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 5 ; R 5 , in each instance, is independently one of aryl, aralkyl, alkaryl, cycloalkyl, a 5- to 10-membered saturated, partially unsaturated or aromatic heterocycle or -W-R 6 , where W is a chalcogen and R 6 is alkyl; Z 1 and Z 2 are independently one of alkyl, hydroxy, alkoxy, or aryloxy, or together Z 1 and Z 2 form a moiety derived from a dihydroxy compound having at least two hydroxy groups separated by at least two connecting atoms in a chain or ring, said chain or ring comprising carbon atoms, and optionally, a heteroatom or heteroatoms which can be N, S, or O; and A is O, l, or 2. 34. The compound of claim 33, wherein the nitrogen-containing ring system is selected from the group consisting of: 35. The compound of claim 33, wherein P is R 7 -C(O)-, R 7 -SO 2 -, R 7 -NH-C(O>- or R 7 -O-G(O)-, and R 7 is one of alkyl, aryl, alkaryl, aralkyl, heteroaryl or heteroarylalkyl, any of which can be optionally substituted, or when P is R 7 -C(O)-, then R 7 can also be saturated or partially saturated heterocycle. 36. The compound of claim 35, wherein P is R 7 -C(O)- or R 7 -SO 2 -; and R 7 is one of . 10 aryl, .,,, ar(C,^)alkyl, 5- to 10-membered heteroaryl or 5- to 10-membered heteroaryl(C,^)alkyl, any of which can be optionally substituted, or when P is R 7 -C(O)-, R 7 can also be N-moφholinyl. 37. The compound of claim 33, wherein B 1 is CH, and X 1 and X 2 are each -C(O)-NH-. 38. The compound of claim 33, wherein R 1 and R 2 are independently selected from the group consisting of alkyl and — CH 2 — R 5 , where R 5 , in each instance, is one of ., 0 aryl, ., 0 ar(C,. 6 )alkyl,C alk(C 6-10 )aryl, C 3 ., 0 cycloalkyl, C,. g alkoxy, C,. g alkylthio or a 5-, 6-, 9- or 10- membered heteroaryl group, where the ring portion of any of said -ioaryl, ar(C,. 6 )alkyl,C 1-6 al^C^mJaryl, or 5-, 6-, 9- or 10- membered heteroaryl can be optionally substituted by one or two substituents independently selected from the group consisting of C,^ alkyl, C 3 . g cycloalkyl, C ,. 6 alkyl(C 3 .Jcycloalkyl, C 2 _ g alkenyl, C 2 . g alkynyl, cyano, amino, C,^ alkylamino, di(C,^)alkylamino, benzylamino, dibenzylamino, nitro, carboxy, carbo(C,. 6 )alkoxy, trifluoromethyl, halogen, C,. 6 alkoxy, .,,, aryl, ., 0 aryl(C,. 6 )alkyl, C(,, 0 aryl(C,. 6 )alkoxy, hydroxy, C 1-6 alkylthio, C 6 alkylsulfinyl, C,^ alkylsulfonyl, C 6 . 10 arylthio, ., 0 arylsulfinyl, C 6. , 0 arylsulfonyl, C 6 ., 0 aryl, C|. 6 alkyl(C 6 . l0 )aryl and halo(C 6 ., 0 )aryl. 39. The compound of claim 33, wherein A is zero. 40. The compound of claim 33, wherein P is hydrogen. 41. The compound of claim 33,wherein: A is zero; P is hydrogen; X is -C(O)-NH-; R forms together with the adjacent R 2 , a nitrogen-containing ring system selected from the group consisting of: R 3 is C,. 6 alkyl; and Z 1 and Z 2 are both hydroxy, C 1-6 alkoxy, or C^oaryloxy, or together Z 1 and Z 2 form a moiety derived from a dihydroxy compound selected from the group consisting of pinacol, perfluoropinacol, pinanediol, ethylene glycol, diethylene glycol, 1,2-cyclohexanediol, 1,3-propanediol, 2,3-butanediol, glycerol or diethanolamine. 42. The compound of claim 33, wherein said compound is L-proline-L-leucine boronic acid, or isosteres, pharmaceutically acceptable salts or boronate esters thereof. 43. A compound having the formula: and pharmaceutically acceptable salts thereof; wherein P is hydrogen or an amino-group-protecting moiety; B 1 , at each occurrence, is independently one of N or CH; X 1 , at each occurrence, is independently one of-C(O)-NH-, -CH 2 -NH-, -CH(OH>-CH 2 -, -CH(OH)-CH(OH)-, -CH(OH)-CH 2 -NH-, -CH=CH-, -C(O)-CH 2 -, -SO 2 -NH- -SO 2 - H 2 - or -CH(OH)-CH 2 -C(O)-NH-, provided that when B ' is N, then the X 1 attached to said B ' is -C(O)-NH-; X 2 is one of -C(O)-NH-, -CH(OH)-CH 2 - -CH(OH)-CH(OH)-, -C(O)-CH 2 - -SO 2 -NH-, -SO 2 -CH 2 - or -CH(OH)-CH 2 -C(O)-NH-; R is hydrogen or alkyl, or R forms together with the adjacent R 1 , or when A is zero, forms together with the adjacent R 2 , a nitrogen-containing mono-, bi- or tri-cyclic, saturated or partially saturated ring system having 4-14 ring members, and one or two optional substituents selected from the group consisting of keto, hydroxy, aryl, alkoxy and aryloxy; R 1 at each occurrence, R 2 and R 3 are each independently one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 5 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; R 5 , in each instance, is one of aryl, aralkyl, alkaryl, cycloalkyl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -W-R 6 , where W is a chalcogen and R 6 is alkyl, where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted, provided that at least one R 1 , R 2 or R 3 is where R 9 is one of hydrogen, alkyl, cycloalkyl, aryl, aralkyl, heteroaryl or heteroarylalkyl; wherein the alkyl is optionally substituted with one of C,. 6 alkyl, halogen monohalo (C,^) alkyl, and trifluoromethyl; and wherein said cycloalkyl, aryl, aralkyl, heteroaryl and heteroarylalkyl groups can be optionally substituted with one or two of C,. 6 alkyl, C 3 . g cycloalkyl, C,. 6 alkyl(C 3 . g )cycloalkyl, C 2 . 8 alkenyl, C 2 . g alkynyl, cyano, amino, C,. 6 alkylamino, di(C,^)alkylamino, benzylamino, dibenzylamino, nitro, carboxy, carbo(C,. 6 )alkoxy, trifluoromethyl, halogen, C,. 6 alkoxy, ., 0 aryl, .,,, aryl(C, alkyl, ., 0 aryl(C,. 6 )alkoxy, hydroxy, C alkylthio, C w alkylsulfinyl, C,. 6 alkylsulfonyl, C wo arylthio, C 6 . 10 arylsulfinyl, C^ I0 arylsulfonyl, C frl0 aryl, C,^alkyl(C 6 . l0 )aryl, and halo(C 6 . 10 )aryl; A 1 and A 2 are independently one of hydrogen, halogen, C,. 6 alkyl, monohalo(C,. 6 )alkyl, or trifluoromethyl; Z 1 and Z 2 are independently one of alkyl, hydroxy, alkoxy, or aryloxy, or together Z 1 and Z 2 form a moiety derived from a dihydroxy compound having at least two hydroxy groups separated by at least two connecting atoms in a chain or ring, said chain or ring comprising carbon atoms, and optionally, a heteroatom or heteroatoms which can be N, S, or O; and A is 0, 1 , or 2. 44. The compound of claim 43, wherein P is R 7 -C(O)-, R 7 -SO 2 -, R 7 -NH-C(O>- or R 7 -O-C(O)-, and R 7 is one of alkyl, aryl, alkaryl, aralkyl, heteroaryl or heteroarylalkyl, any of which can be optionally substituted, or when P is R 7 -C(O)-, then R 7 can also be saturated or partially saturated heterocycle. 45. The compound of claim 43, wherein P is R 7 -C(O)- or R 7 -SO 2 -; and R 7 is one of .,,, aryl, .,,, ar(C|^)alkyl, 5- to 10-membered heteroaryl or 5- to 10-membered heteroaryl(C,. 6 )alkyl, any of which can be optionally substituted, or when P is R 7 -C(O)-, R 7 can also be N-moφholinyl. 46. The compound of claim 43, wherein X 1 and X 2 are each _C(θ)-NH-. 47. The compound of claim 43, wherein one of R 1 , R 2 or R 3 is where A 1 and A 2 are independently one of hydrogen, C,. 6 alkyl, halogen, monohalo (C, alkyl or trifluoromethyl; R 9 is one of C,. g alkyl, C 3 . l0 cycloalkyl, .,0 aryl, C^ 10 ar(C,. 6 )alkyl, a 5- to 10-membered heteroaryl or a 5- to 10-membered heteroaryl(C,. 6 )alkyl; and the remaining R 1 , R 2 and R 3 are independently selected from the group consisting of alkyl and — CH 2 — R 5 , where R 5 , in each instance, is one of C 6 ., 0 aryl, C 6 ., 0 ar(C ) . 6 )alkyl,C,. 6 alk(C 6- ,o)aryl, C w cycloalkyl, C,. g alkoxy, C,.„ alkylthio or a 5-, 6-, 9- or 10- membered heteroaryl group, where the ring portion of any of said C(_ ]0 aryl, ., 0 ar(C,. 6 )alkyl,C,. 6 al^C^^ary!, or 5-, 6-, 9- or 10- membered heteroaryl can be optionally substituted by one or two substituents independently selected from the group consisting of C,. 6 alkyl, C 3 . g cycloalkyl, C,. 6 alkyl(C 3 . g )cycloalkyl, C 2 . g alkenyl, C 2 . g alkynyl, cyano, amino. C,. 6 alkylamino, di(C,. 6 )alkylamino, benzylamino. dibenzylamino, nitro, carboxy, carbo(C,. 6 )alkoxy, trifluoromethyl, halogen, C,. 6 alkoxy, ., 0 aryl, C 6 . 10 aryl(C,. 6 )alkyl, C^. Q aryl(C,. 6 )alkoxy, hydroxy, C,^ alkylthio, C,^ alkylsulfinyl, C,. 6 alkylsulfonyl, C 6 ., 0 arylthio, . ,,, arylsulfinyl, ., 0 arylsulfonyl, Q| 0 aryl, C,. 6 alkyl(Q, 0 )aryl and halo(Q, 0 )a_ryl. 48. The compound of claim 43, wherein A is zero. 49. The compound of claim 43, wherein: A is zero; P is one of R 7 -C(O)-, R 7 -SO 2 - R 7 -NH-C(O)- or R 7 -O-C(O)-; R 7 is one of quinolinyl, quinoxalinyl, pyridyl, pyrazinyl, furanyl or pyrrolyl, or when P is R 7 -C(O)-, R 7 can also be N-moφholinyl; X 2 is -C(O)-NH-; R 2 is: where A' and A 2 are independently one of hydrogen, C,. 6 alkyl, halogen, monohalo (C,. 6 ) alkyl or trifluoromethyl; R 9 is one of hydrogen, C,. g alkyl, phenyl, benzyl, phenethyl or pyridylmethyl; R 3 is C,. 6 alkyl; and Z 1 and Z 2 are both hydroxy, ^alkoxy, or Q 10 aryloxy, or together Z 1 and Z 2 form a moiety derived from a dihydroxy compound selected from the group consisting of pinacol, perfluoropinacol, pinanediol, ethylene glycol, diethylene glycol, 1,2-cyclohexanediol, 1 ,3-propanediol, 2,3-butanediol, glycerol or diethanolamine. 50. The compound of claim 43, wherein said compound is one of: N-(4-moφholine)carbonyl-(O-benzyl)-L-tyrosine-L-leucine boronic acid, N-(4-moφholine)carbonyl-L-tyrosine-L-leucine boronic acid, or N-(4-moφholine)carbonyl-[O-(2-pyridylmethyl)]-L-tyrosine-L-leucine boronic acid; or isosteres, pharmaceutically acceptable salts or boronate esters thereof. 51. A compound having the formula: P— Ν- -B 1 — X 1 -CH- X 2 — CH— B(Z 1 )(Z 2 ) I I I R R 1 R 2 R 3 (la) and pharmaceutically acceptable salts thereof; wherein A is zero; P is hydrogen or an amino-group-protecting moiety; X 2 is one of -C(O}-ΝH-, -CH 2 -NH-, -CH(OH)-CH 2 -, -CH(OHXCH(OHK -CH(OH)-CH 2 -NH-, -CH=CH-, -C(O)-CH 2 -, -SO 2 -NH- -SO 2 -CH 2 - or -CH(OH)-CH 2 -C(O)-NH-; R is hydrogen or alkyl, or R forms together with the adjacent R 2 , a nitrogen-containing mono-, bi- or tri-cyclic, saturated or partially saturated ring system having 4-14 ring members, where said ring system can be optionally substituted by one or two of keto, hydroxy, aryl, alkoxy or aryloxy; R 2 and R 3 are each independently one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R S , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; R 5 , in each instance, is one of aryl, aralkyl, alkaryl, cycloalkyl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -W-R 6 , where W is a chalcogen and R 6 is alkyl, where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; and Z 1 and Z 2 are independently one of alkyl, hydroxy, alkoxy, or aryloxy, or together Z 1 and Z 2 form a moiety derived from a dihydroxy compound having at least two hydroxy groups separated by at least two connecting atoms in a chain or ring, said chain or ring comprising carbon atoms, and optionally, a heteroatom or heteroatoms which can be N, S, or O; provided that P is not C,. 6 alkoxycarbonyl, C M alkylcarbonyl or phenyl(C,. 3 )alkyl. 52. The compound of claim 51, wherein P is R 7 -C(O)-, R 7 -SO 2 -, R 7 -NH-C(O)- or R 7 -O-C(O)-, and R 7 is one of alkyl, aryl, alkaryl, aralkyl, heteroaryl or heteroarylalkyl, where the ring portion of any of said aryl, alkaryl, aralkyl, heteroaryl or heteroarylalkyl can be optionally substituted, or when P is R 7 -C(O)-, then R 7 can also be a saturated or partially unsaturated heterocycle. 53. The compound of claim 51 , wherein P is R 7 -C(O)- or R 7 -SO 2 -; and R 7 is one of Q 10 aryl, Q, 0 ar(C w )alkyl, a 5- to 10-membered heteroaryl or a 5- to 10-membered heteroaryl(C,^)alkyl, any of which can be optionally substituted, or when P is R 7 -C(O)-, R 7 can also be N-moφholinyl. 54. The compound of claim 51 , wherein B' is CH, and X 1 and X 2 are each -C(O)-NH-. 55. The compound of claim 51 , wherein R 2 and R 3 are independently selected from the group consisting of C,. g alkyl and — CH 2 — R 5 , where R 5 is one of Q I0 aryl, C,^ alk(Q, 0 )aryl, Q, 0 ar(C,. 6 )alkyl, C 3 . g cycloalkyl, or a 5-, 6-, 9- or 10-membered heterocycle. 56. The compound of claim 51, which is N-(3-phenylpropionyl)-L- phenylalanine-L-leucine boronic acid, or isosteres, pharmaceutically acceptable salts or boronate esters thereof. 57. The compound of claim 51 , wherein said compound is one of: N-(2-pyrazine)carbonyl-L-phenylalanine-L-leucine boronic acid, N-(2-quinoline)sulfonyl-L-homophenylalanine-L-leucine boronic acid, N-(3-pyridine)carbonyl-L-phenylalanine-L-leucine boronic acid, N-(4-moφholine)carbonyl-L-phenylalanine-L-leucine boronic acid, N-(4-moφholine)carbonyl-β-( 1 -naphthyl)-L-alanine-L-leucine boronic acid, N-(8-quinoline)sulfonyl-β-( 1 -naphthyl)-L-alanine-L-leucine boronic acid, N-(4-moφholine)carbonyl-(O-benzyl)-L-tyrosine-L-leucine boronic acid, N-(4-moφholine)carbonyl-L-tyrosine-L-leucine boronic acid, or N-(4-moφholine)carbonyl-[O-(2-pyridylmethyl)]-L-tyrosine-L-leucine boronic acid; or isosteres, pharmaceutically acceptable salts or boronate esters thereof. 58. A compound having the formula: and pharmaceutically acceptable salts thereof; wherein Y is one of R 8 -C(O)-, R 8 -SO 2 -, R 8 -NH-C(O)- or R 8 -O-C(O)-, where R 8 is one of alkyl, aryl, alkaryl, aralkyl, any of which can be optionally substituted, or when Y is R 8 -C(O)~ or R 8 -SO 2 -, then R 8 can also be an optionally substituted 5-10 membered, saturated, partially unsaturated or aromatic heterocycle; X 3 is a covalent bond or -C(O)-CH 2 -; R 3 is one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 5 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; R 5 , in each instance, is one of aryl, aralkyl, alkaryl, cycloalkyl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -W-R 6 , where W is a chalcogen and R 6 is alkyl, where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; and Z 1 and Z 2 are independently alkyl, hydroxy, alkoxy, aryloxy, or together form a moiety derived from dihydroxy compound having at least two hydroxy groups separated by at least two connecting atoms in a chain or ring, said chain or ring comprising carbon atoms, and optionally, a heteroatom or heteroatoms which can be N, S, or O; provided that when Y is R 8 -C(O)-, R 8 is other than phenyl, benzyl or C,. 3 alkyl. 59. The compound of claim 58, wherein P is R 8 -C(O)- or R 8 -SO 2 -; and R 8 is one of Q, 0 aryl, Q, 0 ar(C,^)alkyl, or a 5-10 membered heteroaryl, any of which can be optionally substituted, or when P is R 8 -C(O)-, R 8 can also be N-moφholinyl. 60. The compound according to claim 58, wherein Y is one of where R 4 is Q., 2 alkyl. 61. A compound having the formula: Y— ■ IS — X 3 — CH— B(Z , )(Z 2 ) I 1 3 H R 3 (2a) and pharmaceutically acceptable salts thereof; where Y is P is one of R 7 -C(OX, R 7 -SO 2 - R 7 -NH-C(O)- or R 7 -O-C(OX, where R 7 is one of alkyl, aryl, alkaryl, aralkyl, any of which can be optionally substituted, or when Y is R 7 -C(O>- or R 7 -SO 2 -, R 7 can also be an optionally substituted 5-10 membered saturated, partially unsaturated or aromatic heterocycle; X 3 is a covalent bond or -C(O)-CH 2 -; R 1 , at each occurrence, is independently one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 5 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; R 3 is one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 5 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; R 5 , in each instance, is one of aryl, aralkyl, alkaryl, cycloalkyl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -W-R 6 , where W is a chalcogen and R 6 is alkyl, where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; and Z 1 and Z 2 are independently alkyl, hydroxy, alkoxy, aryloxy, or together form a moiety derived from dihydroxy compound having at least two hydroxy groups separated by at least two connecting atoms in a chain or ring, said chain or ring comprising carbon atoms, and optionally, a heteroatom or heteroatoms which can be N, S, or O. 62. The compound of claim 61, wherein Y is: 63. A pharmaceutical composition, comprising a compound of claims 1, 25, 33, 43, 51, 58 or 61, or a pharmaceutically acceptable salt thereof, and a pharmaceutically acceptable carrier or diluent. 64. A pharmaceutical composition, comprising a compound of claims 22, 28, 41, 49, 55, 60 and 62, or a pharmaceutically acceptable salt thereof, and a pharmaceutically acceptable carrier or diluent. 65. A pharmaceutical composition, comprising a compound of claims 23, 32, 42, 50, 56 and 57 or an isostere, pharmaceutically acceptable salt or boronate ester thereof, and a pharmaceutically acceptable carrier or diluent. 66. The pharmaceutical composition of claim 65, wherein said compound is present in an amount effective to inhibit the proteasome function in a mammal. 67. A method of inhibiting the growth of a cancer cell, comprising contacting a cell in need of such inhibiting with an effective growth-inhibiting amount of a compound of claims 1, 25, 33, 43, 51, 58 or 61. 68. A method for reducing the rate of muscle protein degradation in a cell comprising contacting a cell in need of said reducing with an effective amount of a proteasome inhibitor of the formula: or a pharmaceutically acceptable salt thereof; wherein P 10 is hydrogen or an amino-group-protecting moiety; B 11 is independently one of N or CH; X 1 ', at each occurrence, is independently one of-C(O)-NH-, -CH 2 -NH-, -CH(OH)-CH 2 -, -CH(OH)-€H(OHK -CH(OH>-CH 2 -NH-, -CH=CH- -C(Oy-CH 2 - -SO-r-NH-, -SO 2 -CH 2 - or -CH(OH)-CH 2 -C(O>-NH-, provided that when B" is N, then X 11 is -C(O)-NH; X' 2 is one of -C(O)-NH-, -CH(OH)-CH 2 -, -CH(OH)-CH(OH)-, -C(O)-CH 2 -, -SO 2 -NH-, -SO 2 -CH 2 - or -CH(OH)-CH 2 -C(O)-NH-; R 10 is hydrogen or alkyl, or R 10 forms together with the adjacent R", or when A 10 is zero, forms together with the adjacent R 12 , a nitrogen-containing mono-, bi- or tri-cyclic, saturated or partially saturated ring system having 4-14 ring members, that can be optionally substituted by one or two of keto, hydroxy, alkyl, aryl, aralkyl, alkoxy or aryloxy; R", at each occurrence, is independently one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R' 5 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; R' 2 and R 13 are each independently one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH -R 15 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted, where R 15 is aryl, aralkyl, alkaryl, cycloalkyl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle, or -chalcogen-alkyl, where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; Z" and Z 12 are independently alkyl, hydroxy, alkoxy, aryloxy, or Z" and Z 12 together form a dihydroxy compound having at least two hydroxy groups separated by at least two connecting atoms in a chain or ring, said chain or ring comprising carbon atoms, and optionally, a heteroatom or heteroatoms which can be N, S, or O; and A'° is 0, l, or 2 69. A method for reducing the activity of NF-κB in a cell, comprising contacting a cell in need of said reducing with an effective amount of a proteasome inhibitor of the formula: p i o — N-JB 11 — X 11 j— CH— X 12 — CH— B(Z 11 )(2 12 ) R 10 R 11 R 12 R 13 (lb) A 10 or a pharmaceutically acceptable salt thereof; wherein P'° is hydrogen or an amino-group-protecting moiety; B" is independently one of N or CH; X 1 ', at each occurrence, is independently one of-C(O)-NH-, -CH 2 -NH-, -CH(OH)-CH 2 -, -CH(OHXCH(OHX, -CH(OH)-CH 2 -NH- -CH=CH-, -C(O)-CH,-, -SO,-NH-, -SO 2 -CH 2 - or -CH(OH)-CH 2 -C(O)-NH-, provided that when B" is N, then X" is -C(O>-NH; X' 2 is one of -C(O)-NH-, -CH(OH)-CH 2 -, -CH(OH)-CH(OH)-, -C(O)-CH 2 -, -SO 2 -NH-, -SO 2 -CH 2 - or -CH(OH)-CH 2 -C(O)-NH-; R'° is hydrogen or alkyl, or R 10 forms together with the adjacent R", or when A 10 is zero, forms together with the adjacent R 12 , a nitrogen-containing mono-, bi- or tri-cyclic, saturated or partially saturated ring system having 4-14 ring members, that can be optionally substituted by one or two of keto, hydroxy, alkyl, aryl, aralkyl, alkoxy or aryloxy; R", at each occurrence, is independently one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 15 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; R ,: and R 13 are each independently one of hydrogen, alkyl, cycloalkyl, aryl. a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R'\ where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted, where R 15 is aryl, aralkyl, alkaryl, cycloalkyl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle, or -chalcogen-alkyl, where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; Z" and Z 12 are independently alkyl, hydroxy, alkoxy, aryloxy, or Z" and Z 12 together form a dihydroxy compound having at least two hydroxy groups separated by at least two connecting atoms in a chain or ring, said chain or ring comprising carbon atoms, and optionally, a heteroatom or heteroatoms which can be N, S, or O; and A'° is O, l , or 2. 70. A method for reducing the rate of intracellular protein breakdown, comprising contacting cells in need of said reducing with an effective amount of a proteasome inhibitor of the formula: or a pharmaceutically acceptable salt thereof; wherein P 10 is hydrogen or an amino-group-protecting moiety; B" is independently one of N or CH; X 1 ', at each occurrence, is independently one of-C(O)-NH-, -CH 2 -NH-, -CH(OH)-CH 2 -, -CH(OH)-CH(OH)-, -CH(OH)-CH 2 -NH-, -CH=CH-, -C(O)-CH 2 -, -SO 2 -NH- -SO 2 -CH 2 - or -CH(OH)-CH 2 -C(O)-NH-, provided that when B" is N, then X" is -C(O)-NH; X' 2 is one of -C(O)-NH-, -CH(OH)-CH 2 - -CH(OH)-CH(OH)-, -C(OXCH 2 - -SO 2 -NH- -SO 2 -CH 2 - or -CH(OH>-CH 2 -C(O)-NH-; R'° is hydrogen or alkyl, or R'° forms together with the adjacent R", or when A 10 is zero, forms together with the adjacent R 12 , a nitrogen-containing mono-, bi- or tri-cyclic, saturated or partially saturated ring system having 4-14 ring members, that can be optionally substituted by one or two of keto, hydroxy, alkyl, aryl, aralkyl, alkoxy or aryloxy; R", at each occurrence, is independently one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 15 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; R 12 and R 13 are each independently one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 15 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted, where R 15 is aryl, aralkyl, alkaryl, cycloalkyl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle, or -chalcogen-alkyl, where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; Z 1 ' and Z 12 are independently alkyl, hydroxy, alkoxy, aryloxy, or Z 1 ' and Z 12 together form a dihydroxy compound having at least two hydroxy groups separated by at least two connecting atoms in a chain or ring, said chain or ring comprising carbon atoms, and optionally, a heteroatom or heteroatoms which can be N, S, or O; and A ,0 is 0, l, or 2. 71. A method for reducing the rate of degradation of p53 protein in a cell, comprising administering to a cell in need of said reducing an effective amount of a proteasome inhibitor of the formula: P IO_ N _|B 11 — X 11 -CH— X 12 — CH— B(Z 11 )(Z 12 ) R 10 R 1 R 12 R 13 (lb) A 10 or a pharmaceutically acceptable salt thereof; wherein P 10 is hydrogen or an amino-group-protecting moiety; B" is independently one of N or CH; X 1 ', at each occurrence, is independently one of-C(O)-NH-, -CH 2 -NH-, -CH(OH>-CH 2 -, -CH(OH)-CH(OHX, -CH(OH)-CH -NH r , -CH-CH-, -C(O)-CH 2 - -SO 2 -NH-, -SO 2 -CH 2 - or -CH(OH)-CH 2 -C(O)-NH-, provided that when B" is N, then X" is -C(O)-NH; X' 2 is one of -C(O)-NH-, -CH(OH)-CH 2 -, -CH(OH)-CH(OH)-, -C(OXCH 2 - -SO 2 -NH- -SO 2 -CH 2 - or -CH(OH)-CH 2 -C(O>-NH-; R 10 is hydrogen or alkyl, or R 10 forms together with the adjacent R", or when A 10 is zero, forms together with the adjacent R 12 , a nitrogen-containing mono-, bi- or tri-cyclic, saturated or partially saturated ring system having 4-14 ring members, that can be optionally substituted by one or two of keto, hydroxy, alkyl, aryl, aralkyl, alkoxy or aryloxy; R", at each occurrence, is independently one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 15 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; R 12 and R 13 are each independently one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 15 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted, where R 15 is aryl, aralkyl, alkaryl, cycloalkyl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle, or -chalcogen-alkyl, where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; Z" and Z' 2 are independently alkyl, hydroxy, alkoxy, aryloxy, or Z" and Z 12 together form a dihydroxy compound having at least two hydroxy groups separated by at least two connecting atoms in a chain or ring, said chain or ring comprising carbon atoms, and optionally, a heteroatom or heteroatoms which can be N, S, or O; and A ,0 is 0, l, or 2. 72. A method for inhibiting cyclin degradation in a cell, comprising contacting a cell in need of said reducing with an effective amount of a proteasome inhibitor of the formula: or a pharmaceutically acceptable salt thereof; wherein P'° is hydrogen or an amino-group-protecting moiety; B" is independently one of N or CH; X ' ' , at each occurrence, is independently one of -C(O)-NH-, -CH 2 -NH-, -CH(OH)-CH 2 -, -CH(OH)-CH(OH)-, -CH(OH)-CH 2 -NH-, -CH=CH-, -C(O)-CH 2 -, -SO 2 -NH- -SO 2 -CH 2 - or -CH(OH)-CH 2 -C(O)-NH-, provided that when B" is N, then X" is -C(O)-NH; X 12 is one of -C(O)-NH-, -CH(OH)-CH 2 -, -CH(OH)-CH(OH)-, _C(θ)-CH 2 -, -SO 2 -NH- -SO 2 -CH 2 - or -CH(OH)-CH 2 -C(O)-NH-; R'° is hydrogen or alkyl, or R 10 forms together with the adjacent R", or when A 10 is zero, forms together with the adjacent R 12 , a nitrogen-containing mono-, bi- or tri-cyclic, saturated or partially saturated ring system having 4-14 ring members, that can be optionally substituted by one or two of keto, hydroxy, alkyl, aryl, aralkyl, alkoxy or aryloxy; R". at each occurrence, is independently one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 15 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; R ι: and R' 3 are each independently one of hydrogen, alkyl, cycloalkyl, aryl. a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 15 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted, where R ιs is aryl, aralkyl, alkaryl, cycloalkyl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle, or -chalcogen-alkyl, where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; Z" and Z 12 are independently alkyl, hydroxy, alkoxy, aryloxy, or Z" and Z 12 together form a dihydroxy compound having at least two hydroxy groups separated by at least two connecting atoms in a chain or ring, said chain or ring comprising carbon atoms, and optionally, a heteroatom or heteroatoms which can be N, S, or O; and A 10 is 0, 1, or 2. 73. A method of preventing or treating an inflammatory condition in a patient in need thereof, said method comprising administering to said patient a proteasome inhibitor of the formula: or a pharmaceutically acceptable salt thereof; wherein P 10 is hydrogen or an amino-group-protecting moiety; B" is independently one of N or CH; X", at each occurrence, is independently one of-C(O)-NH-, -CH 2 -NH-, -CH(OH)-CH,-, -CH(OH)-CH(OH)-, -CH(OH)-CH 2 -NH-, -CH=CH- -C(O)-CH 2 -, -SO 2 -NH- -SO 2 -CH 2 - or -CH(OH)-CH 2 -C(O)-NH-, provided that when B" is N. then X" is -C(O)-NH; X 12 is one of -C(O)-NH- -CH(OH)-CH 2 -, -CH(OH)-CH(OH)-, -C(O>-CH : -. -SO-.-NH-, -SO-.-CH , - or -CH(OH)-CH 2 -C(O)-NH-; R 10 is hydrogen or alkyl, or R 10 forms together with the adjacent R 11 , or when A 10 is zero, forms together with the adjacent R 12 , a nitrogen-containing mono-, bi- or tri-cyclic, saturated or partially saturated ring system having 4-14 ring members, that can be optionally substituted by one or two of keto, hydroxy, alkyl, aryl, aralkyl, alkoxy or aryloxy; R 11 , at each occurrence, is independently one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 15 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; R 12 and R 13 are each independently one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 15 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted, where R 15 is aryl, aralkyl, alkaryl, cycloalkyl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle, or -chalcogen-alkyl, where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; Z 11 and Z 12 are independently alkyl, hydroxy, alkoxy, aryloxy, or Z u and Z 12 together form a dihydroxy compound having at least two hydroxy groups separated by at least two connecting atoms in a chain or ring, said chain or ring comprising carbon atoms, and optionally, a heteroatom or heteroatoms which can be N, S, or O; and A 10 is 0, 1, or 2. 74. The method of claim 73, wherein said patient has been diagnosed with, or is at risk of developing, a condition selected from the group consisting of tissue rejection, organ rejection, arthritis, an infection, dermatoses, inflammatory bowel disease, and an autoimmune disease. 75. A method for inhibiting antigen presentation in a cell comprising administering to a cell in need thereof an effective amount of a proteasome inhibitor of the formula: p10_ N J B 11_χ11 -CH— X 12 — CH— B(Z 11 )(Z 12 ) R 10 I I R 11 R 12 R 13 (lb) A 10 or a pharmaceutically acceptable salt thereof; wherein P'° is hydrogen or an amino-group-protecting moiety; B" is independently one of N or CH; X 1 ', at each occurrence, is independently one of-C(O)-NH-, -CH 2 -NH-, -CH(OH)-CH 2 -, -CH(OH)-CH(OH)-, -CH(OH)-CH 2 -NH-, -CH=CH-, -C(O)-CH 2 -, -SO 2 -NH- -SO 2 -CH 2 - or -CH(OH)-CH 2 -C(O)-NH-, provided that when B" is N, then X" is -C(O)-NH; X 12 is one of -C(O)-NH-, -CH(OH)-CH 2 -, -CH(OH)-CH(OH)-, -C(O)-CH 2 -, -SO 2 -NH- -SO 2 -CH 2 - or -CH(OH)-CH 2 -C(O)-NH-; R 10 is hydrogen or alkyl, or R 10 forms together with the adjacent R 1 ', or when A 10 is zero, forms together with the adjacent R 12 , a nitrogen-containing mono-, bi- or tri-cyclic, saturated or partially saturated ring system having 4-14 ring members, that can be optionally substituted by one or two of keto, hydroxy, alkyl, aryl, aralkyl, alkoxy or aryloxy; R", at each occurrence, is independently one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 15 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; R 12 and R 13 are each independently one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or --CH 2 -R'\ where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted, where R 15 is aryl, aralkyl, alkaryl, cycloalkyl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle, or -chalcogen-alkyl, where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; Z 1 ' and Z 12 are independently alkyl, hydroxy, alkoxy, aryloxy, or Z" and Z 12 together form a dihydroxy compound having at least two hydroxy groups separated by at least two connecting atoms in a chain or ring, said chain or ring comprising carbon atoms, and optionally, a heteroatom or heteroatoms which can be N, S, or O; and A 10 is 0, 1, or 2. 76. A method for inhibiting inducible NF-κB dependent cell adhesion in an animal in need of said inhibiting, comprising administering to said animal an effective amount of a proteasome inhibitor of the formula: or a pharmaceutically acceptable salt thereof; wherein P 10 is hydrogen or an amino-group-protecting moiety; B" is independently one of N or CH; X", at each occurrence, is independently one of-C(O)-NH-, -CH 2 -NH-, -CH(OHXCH 2 - -CH(OH)-CH(OH}-, -CH(OH}-CH 2 -NH-, -CH=CH-, -C(O)-CH : -. -SO 2 -NH- -SO 2 -CH 2 - or -CH(OH)-CH 2 -C(O)-NH-, provided that when B" is N, then X" is -C(O)-NH; X 12 is one of -C(O)-NH-, -CH(OH)-CH 2 -, -CH(OH)-CH(OH)-, -C(OXCH -. -SO : -NH-, -SO 2 -CH 2 - or -CH(OH)-CH 2 -C(O)-NH-; R 10 is hydrogen or alkyl, or R 10 forms together with the adjacent R", or when A 10 is zero, forms together with the adjacent R ,'τa nitrogen-containing mono-, bi- or tri-cyclic, saturated or partially saturated ring system having 4-14 ring members, that can be optionally substituted by one or two of keto, hydroxy, alkyl, aryl, aralkyl, alkoxy or aryloxy; R", at each occurrence, is independently one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 15 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; R 12 and R l3 are each independently one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 15 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted, where R 15 is aryl, aralkyl, alkaryl, cycloalkyl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle, or -chalcogen-alkyl, where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; Z" and Z 12 are independently alkyl, hydroxy, alkoxy, aryloxy, or Z" and Z 12 together form a dihydroxy compound having at least two hydroxy groups separated by at least two connecting atoms in a chain or ring, said chain or ring comprising carbon atoms, and optionally, a heteroatom or heteroatoms which can be N, S, or O; and A ,0 is 0, l, or 2. 77. A method for inhibiting HIV replication in an animal in need of said inhibiting, comprising administering to said animal an effective amount of a proteasome inhibitor of the formula: pio or a pharmaceutically acceptable salt thereof: wherein P 10 is hydrogen or an amino-group-protecting moiety; B" is independently one of N or CH; X 1 ', at each occurrence, is independently one of-C(O)-NH-, -CH 2 -NH-, -CH(OH)-CH 2 -, -CH(OH)-CH(OHX, -CH(OH)-CH -NH r , -CH=CH- -C(O)-CH 2 -, -SO 2 -NH- -SO 2 -CH 2 - or -CH(OH)-CH 2 -C(O>-NH-, provided that when B" is N, then X" is -C(O)-NH; X 12 is one of -C(O}-NH-, -CH(OH)-CH 2 -, -CH(OH)-CH(OH)-, -C(O)-CH 2 -, -SO 2 -NH- -SO 2 -CH 2 - or -CH(OH)-CH 2 -C(O)-NH-; R'° is hydrogen or alkyl, or R 10 forms together with the adjacent R 1 ', or when A 10 is zero, forms together with the adjacent R 12 , a nitrogen-containing mono-, bi- or tri-cyclic, saturated or partially saturated ring system having 4-14 ring members, that can be optionally substituted by one or two of keto, hydroxy, alkyl, aryl, aralkyl, alkoxy or aryloxy; R", at each occurrence, is independently one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 15 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; R 12 and R 13 are each independently one of hydrogen, alkyl, cycloalkyl, aryl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle or -CH 2 -R 15 , where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted, where R' 5 is aryl, aralkyl, alkaryl, cycloalkyl, a 5-10 membered saturated, partially unsaturated or aromatic heterocycle, or -chalcogen-alkyl, where the ring portion of any of said aryl, aralkyl, alkaryl or heterocycle can be optionally substituted; Z" and Z 12 are independently alkyl, hydroxy, alkoxy, aryloxy, or Z" and Z 12 together form a dihydroxy compound having at least two hydroxy groups separated by at least two connecting atoms in a chain or ring, said chain or ring comprising carbon atoms, and optionally, a heteroatom or heteroatoms which can be N, S, or O; and A 10 is 0, 1, or 2. 78. The method of claims 67, 68, 69, 70, 71, 72, 73, 75, 76 or 77 wherein said protesome inhibitor is one of: N-(2-pyrazine)carbonyl-L-phenylalanine-L-leucine boronic acid, N-(2-quinoline)sulfonyl-L-homophenylalanine-L-leucine boronic acid, N-(3-pyridine)carbonyI-L-phenylalanine-L-leucine boronic acid, N-(4-moφholine)carbonyl-L-phenylalanine-L-leucine boronic acid, N-(4-moφholine)carbonyl-β-( 1 -naphthyl)-L-alanine-L-leucine boronic acid, N-(8-quinoline)sulfonyl-β-( 1 -naphthyl)-L-alanine-L-leucine boronic acid, N-(4-moφholine)carbonyl-(O-benzyI)-L-tyrosine-L-leucine boronic acid, N-(4-moφholine)carbonyl-L-tyrosine-L-leucine boronic acid, or N-(4-moφholine)carbonyl-[O-(2-pyridylmethyl)]-L-tyrosine-L-leucine boronic acid; or isosteres, pharmaceutically acceptable salts or boronate esters thereof. 79. A method for reducing the rate of muscle protein degradation in a cell comprising contacting said cell with a compound of claim 58 or 61. 80. A method for reducing the activity of ΝF-κB in a cell comprising contacting said cell with a compound of claim 58 or 61. 81. A method for reducing the rate of intracellular protein breakdown comprising contacting cells with a compound of claim 58 or 61. 82. A method for reducing the rate of degradation of p53 protein in a cell comprising administering to said cell a compound of claim 58 or 61. 83. A method for inhibiting cyclin degradation in a cell comprising contacting said cell with a compound of claim 58 or 61. 84. A method of preventing or treating an inflammatory condition in a patient in need thereof, said method comprising administering to said patient a compound ofclaim 58 or 61. 85. The method of claim 84, wherein said patient has been diagnosed with, or is at risk of developing, a condition selected from the group consisting of tissue rejection, organ rejection, arthritis, an infection, dermatoses, inflammatory bowel disease, asthma, osteoporosis, osteoarthritis and an autoimmune disease. 86. A method for inhibiting the growth of a cancer cell, comprising contacting said cell with a compound of claim 58 or 61. 87. A method for inhibiting antigen presentation in a cell comprising administering to said cell a compound of claim 58 or 61. 88. A method for inhibiting NF-κB dependent cell adhesion in an animal comprising administering to said animal a compound of claim 58 or 61. 89. A method for inhibiting HIV replication in an animal comprising administering to said animal a compound of claim 58 or 61.

Inventors: ADAMS JULIAN, BAEVSKY MATTHEW, GRENIER LOUIS, MA YU-TING, PLAMONDON LOUIS, STEIN ROSS, ADAMS, JULIAN, BAEVSKY, MATTHEW, GRENIER, LOUIS, MA, YU-TING, PLAMONDON, LOUIS, STEIN, ROSS
Applicants: PROSCRIPT INC, PROSCRIPT, INC.
Application year: 1995
Main classification: A61K31
IPC codes: A61K31, A61K38, A61P11, A61P17, A61P19, A61P29, A61P31, A61P35, A61P37, A61P43, C07D207, C07D213, C07D215, C07D241, C07D295, C07D307, C07D401, C07D471, C07F5, C07K5