Abstract:
A method for the generation of oligomers or a mixture of oligomers to form a chemical library by amide-forming oligomerization comprises the steps of 1) reacting a mixture of at initiator (I) with monomer (M) to form a dimer of the initiator (I) and the monomer (M) or a pre-oligomer with an initiator (I) attached to a chain of more than one monomer (M) or a mixture thereof by amide-bond formation; 2) optionally adding at least one terminator (T) for the formation of a linear oligomer or a mixture of linear oligomers by amide-bond formation; or, for the formation of a cyclic oligomer or a mixture of cyclic oligomers by amide-bond formation, changing the reaction conditions relative to step 1) so as to form a linking covalent bond between the at least one initiator (I).

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a method for the generation of an oligomer or a mixture of oligomers, in particular for providing chemical libraries for activity screening. It furthermore relates to a method for identifying active compounds from such a library using a tailored screening process. 
       PRIOR ART 
       [0002]    The current practice of modern medicinal chemistry operates on a well-established strategy of hit identification, hit-to-lead, and lead optimization. These processes, in turn, rely on the preparation and high-throughput screening of vast libraries of organic compounds. For hit identification, it is not uncommon to screen one million or more compounds in a high-throughput biological assay. The screening of such large numbers of compounds requires a complex infrastructure of robotics, compound purification, and compound storage. Typically, each compound is prepared, purified, stored, and screened as a single molecular entity. This simplifies the identification of active hits and reduces false positives, but effectively prevents the expansion of chemical libraries beyond a few million compounds. It is now widely recognized that it is not possible for a single facility to store, handle and screen more than a few million organic compounds. 
         [0003]    To circumvent this physical limitation, mixtures of compounds can be screened. Early attempts at the synthesis of screening of unpurified combinatorial mixtures of compounds were stymied by problems with reproducibility and biologically active impurities. Methods for screening large mixtures of tagged compounds, such as phage display, ribosome display, and DNA-encoded libraries provide a powerful and highly successful alternative. The major limitation of these methods is the types of compounds that can be produced through biological methods are largely limited to oligopeptides comprised of naturally occurring L-amino acids. A method to rapidly synthesize, screen, and deconvolute molecules comprised of unnatural units is in high demand. 
         [0004]    The dissertation of Ying-Ling Chiang entitled SEQUENCE-CONTROLLED SYNTHESIS OF β3-PEPTIDES: SOLID-PHASE SYNTHESIS AND DNA-TEMPLATED SYNTHESIS (University of Pennsylvania, 2013) inter alia mentions a living homo-polymerization of α-ketoacid and a single isoxazolidine monomer, but does this hypothetically and not in a specific context of using the reaction results, and further points out the intrinsic solubility and reactivity issues as well as serious decomposition and fragmentation problems. Based on these considerations in the document a different synthetic approach is taken. Termination is not an issue. 
         [0005]    Hiroshi Ishida et al (Tetrahedron Letters 50 (2009) 3258-3260) propose a new synthesis of an enantiopure isoxazolidine monomer for β 3 -aspartic acid in chemoselective β-oligopeptide synthesis via chemoselective α-ketoacid-hydroxylamine amide formation. The proposed route involves nitrone cycloaddition of 3-thiophenylpropanal and circumvents limitations of other potential starting materials. 
         [0006]    Ying-Ling Chiang et al (Helvetica Chimica Acta—Vol. 95 (2012), 2481) propose a new method for the synthesis of enantiomerically pure isoxazolidine monomers for the synthesis of β 3 -oligopeptides via α-keto acid hydroxylamine (KAHA) ligation. The one-pot synthetic method utilizes in situ generated nitrones bearing gulose-derived chiral auxiliaries for the asymmetric 1,3-dipolar cycloaddition with methyl 2-methoxyacrylate. The resulting enantiomerically pure isoxazolidine monomers bearing diverse side chains (proteinogenic and non-proteinogenic) can be synthesized in either configuration (like- and unlike-configured). The scalable and enantioselective synthesis of the isoxazolidine monomers enables the use of the synthesis of b3-oligopeptides via iterative α-keto acidhydroxylamine (KAHA) ligation. 
         [0007]    Nancy Carrillo et al (JACS 2006, 128, 1452-1453) propose an iterative, aqueous synthesis of α 3 -oligopeptides without coupling reagents with isoxazolidine acetals. 
       SUMMARY OF THE INVENTION 
       [0008]    The preparation of thousands or millions of organic compounds in conjunction with high-throughput screening against a biological target is the prevailing method for developing new drugs. Methods that quickly generate chemical libraries embedded with complex functionalities through simple operations, minimal waste and easy purification are thus highly desirable. To meet these criteria, a new approach for the “on-demand” synthesis of chemical libraries has been developed by combining monomeric building blocks in aqueous buffer. This newly proposed method, which, inter alia, utilizes the chemoselective α-ketoacid-hydroxylamine (KAHA) amide-forming ligation as a core technology, allows chemical libraries to be prepared, screened, and deconvoluted using nothing more than standard pipetting or liquid handling techniques. The modular nature of the compounds enables easy tailoring for structure-activity-relationship (SAR) studies. As a proof-of-concept study, hepatitis C virus (HCV) protease inhibitor libraries were synthesized by mixing building blocks directly in multi-well plates. The reactions require no reagents and produce only CO 2  and cyclohexanone as byproducts. Each reaction well contained β-peptide α-ketoamide products and was diluted and subjected to HCV protease assay without workup or purification. Wells containing active HCV protease inhibitors were deconvoluted by preparing focused libraries and the active compounds were synthesized and confirmed with simple operations all under aqueous conditions. The final proposed inhibitor was resynthesized using the same approach, isolated, fully characterized and the activity against HCV protease confirmed to have an IC 50  of 1.0 μM. 
         [0009]    The synthesis of chemical libraries for drug development is often associated with toxic reagents, organic solvents, tedious purifications, and complex handling of large numbers of isolated, individual compounds. Here a method to prepare large chemical libraries “on-demand”, simply by combining building blocks in aqueous solution is proposed. The resulting libraries are subjected to high-throughput screenings in an enzymatic assay without purification or further handling. From the active mixtures, the active compounds can be identified by deconvolution, re-synthesis and isolation. This method offers simple operations, minimal waste, reproducible on-demand library preparation, and avoid the need to store thousands of discrete compounds. No reagents or protecting groups are needed and minimal amounts of innocuous byproducts are formed. This chemistry allows combinatorial mixtures of β-peptide α-ketoamides to be quickly prepared, screened and deconvoluted. Once the building blocks are obtained, the only technique needed for the library preparation is liquid handling with a microliter pipette. The resulting libraries can be screened directly on multi-well plates using an enzymatic assay. The libraries themselves do not need to be stored and can be discarded and quickly reconstituted when needed. We demonstrate the successful preparation of a library of over 6,000 compounds prepared from only 23 building blocks, the screening of this library directly from the reaction mixtures, and its deconvolution to identify a 1.0 μM inhibitor of hepatitis C virus (HCV) protease. 
         [0010]    It is possible to synthesize β-peptides in solution using isoxazolidine monomers ( FIG. 1A ). These monomers react chemoselectively with α-ketoacids to form amide bonds. Through consecutive repetition of monomer couplings and methyl ketoester hydrolysis, β-peptides can be obtained. This method provides a stepwise manner to form functionalized oligomers of controlled sequence; however, for syntheses of compound libraries, methods that can generate a large amount of various compounds in a one-pot fashion are needed. Therefore another type of isoxazolidine monomer (M) was designed that forms a new α-ketoacid upon ligation ( FIG. 1B ). The α-ketoacid can also couple with M and form oligomers of varying length and sequence; the addition of a terminator (T) ceases this oligomerization. To prepare libraries of protease inhibitors, a terminator was designed that forms a C-terminal α-ketoamide, as it is known to be an excellent pharmacophore for several classes of proteases such as serine proteases and cysteine proteases. 
         [0011]    The unpurified mixtures, which contain only peptide oligomers and small amounts of the terminator (T), which is used in excess, can be subjected to biological assays directly without further purification. Importantly, the number of products expands exponentially in the reaction mixture with increasing types of α-ketoacid initiators (I), monomers (M), and terminators (T) ( FIG. 1C ). Simply by increasing the number of each component used in the library can dramatically expand the number of potential molecules formed. For reference, the use of just 50 different initiators (I), monomers (M), and terminators (T) can form well over 300 million molecules, considering only I-T, I-M-T, and I-M-M-T! If I-M-M-M-T and higher oligomers are considered, the numbers quickly reach into the hundreds of millions. For demonstrating this concept, in the following a much smaller number of components and possible products is analyzed (6 initiators, 8 monomers, 9 terminators; around 30,000 compounds). Larger numbers of compounds are of course readily available by biological methods, but there is currently no synthetic method to prepare such large libraries from relatively few starting materials by simply mixing components in aqueous solution followed by direct biological assay. The proof of concept of this approach is given in the detailed description. 
         [0012]    Generally speaking, the present invention thus relates to and proposes a method as claimed in claim  1 , namely a method for the generation of oligomers or a mixture of oligomers to form a chemical library by amide-forming oligomerization. The proposed method comprises the following the steps, in the given order:
   1) reacting a mixture of at least one initiator (I) with at least one monomer (M) to form a dimer (I-M) of the initiator (I) and the monomer (M) or to form a pre-oligomer (I-M-M, I-M-M-M; . . . ) with an initiator (I) attached to a chain of more than one monomer (M) or a mixture thereof by amide-bond formation;   2) adding at least one terminator (T) for the formation of a linear oligomer (I-M-T, I-M-M-T, I-M-M-M-T, . . . ) or a mixture of linear oligomers by amide-bond formation; or, for the formation of a cyclic oligomer or a mixture of cyclic oligomers by amide-bond formation, changing the reaction conditions relative to step 1) so as to form a linking covalent bond between the at least one initiator (I) and a monomer (M), preferably the respective terminal monomer, of the dimer or pre-oligomer formed in step 1).   
 
         [0015]    In step 1) either one single type of dimer can be generated, if one single initiator is used, or if one single initiator is used combined with several different monomers, well-controlled set of dimers of the single initiator with in each case one of the different monomers. In case of corresponding reaction conditions, reactant concentrations, it&#39;s also possible to obtain, when using one single initiator, an oligomer with the initiator combined with a chain of the monomers. In case of using several initiators, it is possible to obtain corresponding mixtures of the different initiators attached to corresponding chains of monomers. Correspondingly therefore the proposed reaction scheme is essentially free from side reactions, does not necessitate the sequential build-up of the chain, and can be controlled as concerns the distribution of the reaction products. It further allows for a well controllable, non-toxic reaction scheme to obtain either individual systems, or mixtures of systems in the sense of chemical libraries. By tailoring the distribution of the systems in a mixture of the corresponding chemical libraries can very efficiently be used in an incomplete factorial design approach to identify chemically and/or biologically active systems with a reduced separation/identification effort. 
         [0016]    Further it is also possible to omit the reaction with the terminator, so that the final linear oligomer is given by structures of the type (I-M, I-M-M, I-M-M-M, . . . ). Preferably however the final linear oligomers terminated by a terminator structure. 
         [0017]    Preferably essentially enantiopure building blocks (for the initiator and/or the monomer and/or the terminator) are used. 
         [0018]    It should further be noted that it is also possible to produce, concomitantly, linear as well as cycling systems, in that e.g. initiators for the generation of linear systems are used in a mixture with initiators for the generation of cycling systems, and in that the step 2) is carried out once for the determination of the linear systems and once for the ring closure of the cyclic systems. 
         [0019]    The importance of producing a large array of compounds in a fast and cost-effective way is well recognized. The proposed method provides compound libraries efficiently and can be practiced routinely in industry and academia. The normally associated concerns like complicated experimental procedures, expensive stoichiometric reagents, the need for protection of common functional groups, the occurrence of false positives, costly waste handling and time-consuming purifications, the major obstacles for library synthesis, which necessitate the preparation and storage of pure, isolated compounds, can be avoided. Herewith a self-assembly process for a β-peptide library synthesis is proposed that does not require reagents or protecting groups and operates under aqueous conditions. The resulting product mixtures can be screened directly in biological assays and the libraries can be rapidly modulated for optimization or deconvolution simply by changing the composition of the monomers or the addition order. The libraries can be quickly and reproducibly resynthesized as needed, rendering the need to isolate and purify each member obsolete. The requisite building blocks are readily synthesized in enantiomerically pure form and are stable to prolonged storage. By simply mixing these building blocks and heating the reaction solutions in microplates, chemical libraries are synthesized “on-demand”. Libraries of oligomers with specific chemical and physical properties can be easily obtained by employing designed building blocks owing to the modular nature of the self-assembly process. In analogy to the powerful phage display method for peptide and protein library syntheses using natural 20 amino acids, the building blocks used here can be incorporated with various functional groups by straightforward syntheses and are tolerated in the mild self-assembly process conditions. 
         [0020]    As a proof of concept given in the detailed description, HCV protease inhibitor libraries were synthesized. Our processes to find target molecules are divided into four phases ( FIG. 8 ). (1) Discovery phase: various building blocks are engaged in library syntheses. Components leading to inactive oligopeptides are eliminated. (2) Optimization phase: focused libraries are constructed from the selected moieties. Active mixtures are identified. (3) Deconvolution phase: the active wells in the focused libraries are deconvoluted and possible lead structures are proposed. (4) Identification phase: potential lead structures are further confined by HPLC separations. Syntheses, purification, and bioactivity determination of these compounds are carried out. Based on the preliminary lead structures, libraries with further optimized building blocks can be synthesized. With this method, a new HCV protease inhibitor with IC50 of 1.0 μM was isolated and fully characterized. Although this is only a modest inhibitor and the resulting molecule does not have ideal oral administration properties, the fact that it was discovered simply by combining a small number of monomers in aqueous conditions speaks to the promising method of lead identification inherit to this approach. 
         [0021]    The proposed method can be applied in industry with the merit of low-cost operations and avoidance of hazardous organic chemicals. The aqueous-mediated library syntheses without involvement of purification steps minimize the amount of organic solvent waste. The self-assembly reactions happen readily at moderate temperature and produce only innocuous by products without costly reagents and metal catalysts. Any laboratory familiar with biological sample handling can design and compose libraries from the constituent building blocks. The method can be enhanced by employing automated microfluidic devices. With stored building blocks, library syntheses can be programmed by automated systems. This reduces the amount of starting materials, ensures the accuracy of liquid handling and allows a high-throughput synthesis process. The freshly prepared libraries are ready for biological assessments directly without further treatment and also avoid the unforeseen decomposition problem with old stored compound libraries. In conclusion, a new method to rapidly synthesize chemical libraries is proposed. This method provides an alternative conceptual operation in compound library generations for various purposes. A large number of β-peptides with various side chains can be generated in a one-pot fashion by the chemoselective reaction between α-ketoacids and isoxazolidine building blocks. The resulting libraries were subjected to HCV protease assays directly without further purification. Strategies for deconvolution and optimization were presented. A new HCV protease inhibitor was identified by this method. 
         [0022]    Specifically, preferably the initiator (I) is selected from the group consisting of: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    with, for the formation of a linear oligomer or a mixture of linear oligomers by amide-bond formation, R being selected from the group consisting of: substituted or unsubstituted alkyl, heteroalkyl, cycloalkyl, heterocycloalkyl, aryl, heteroaryl, alkylaryl, heteroalkylaryl, alkenyl, heteroalkenyl, alkinyl, heteroalkinyl, —C(NH 2 )R I , —C(NH 2 ) {R I —CO—C(NH 2 )}R I  with n=1, 2 and R I  being H or an amino-acid side chain, fluorescent dye, nucleic acid or derivative thereof, peptide nucleic acid, FLAG octapeptide (DYKDDDDK), biotin or affinity tag. Preferably the residue R comprises in the range of 1-10 carbon atoms, more preferably it is selected from the group consisting of benzyl, NO 2  substituted benzyl, ethyl, carboxyethyl, hexyl, tert. Butyl. 
         [0023]    For the formation of a cyclic oligomer or a mixture of cyclic oligomers by amide-bond formation, preferably R is selected from the group consisting of: —{(CH 2 )} n R c , —{(CHCH 3 )} n R c , —{(CH(1,1-dimethylethyl))} n R c , —{(CH(benzyl))} n R c , in each case with n=1,2 and R c  being a linker structure allowing to form a linking covalent amide bond to the respective terminal monomer of the dimer or oligomer formed in step 1). 
         [0024]    In both cases (linear and cyclic oligomer formation) the following definitions preferably apply:
   X+ is a counterion, selected from the group consisting of: K + , Cs + , Li + , Na + , R 4 N + , R 4 P +  or R 3 S +  with R being an organic substituent or H, preferably selected from the group consisting of: substituted or unsubstituted alkyl, heteroalkyl, cycloalkyl, heterocycloalkyl, aryl, heteroaryl, alkylaryl, heteroalkylaryl, alkenyl, heteroalkenyl, alkinyl, heteroalkinyl, preferably the residue R comprises in the range of 1-10 carbon atoms;   X, Y, Z, are, independently from each other, selected from the group consisting of: F, OR, N + R 3 , N + R 2 OR, N + R 2 SR, and N + R 2 NR 2 , and are optionally forming a cyclic or a bicyclic structure; wherein R is an organic substituent or H, preferably selected from the group consisting of: substituted or unsubstituted alkyl, heteroalkyl, cycloalkyl, heterocycloalkyl, aryl, heteroaryl, alkylaryl, heteroalkylaryl, alkenyl, heteroalkenyl, alkinyl, heteroalkinyl, preferably the residue R comprises in the range of 1-10 carbon atoms.   
 
         [0027]    According to a preferred embodiment, the initiator (I) for the formation of a linear oligomer or a mixture of linear oligomers by amide-bond formation is selected from the group consisting of: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein Me=—CH 3 . 
         [0028]    According to yet another preferred embodiment, the initiator (I) for the formation of a cyclic oligomer or a mixture of cyclic oligomers by amide-bond formation is selected in that the linker structure is selected from the group consisting of: a chain of one or two elements selected from the group of: amino acid, —CO((CH 2 ) 2 NH—, and this chain preferably terminated by a group selected from: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0029]    One of these groups can also be directly, so without an element as outlined in the paragraph before, the linker element, so can be the residue R of the initiator as defined above. 
         [0030]    Further preferably, the initiator (I), also but not necessarily, for the formation of a cyclic oligomer or a mixture of cyclic oligomers by amide-bond formation can be given by covalent dimers or trimmers of the above structures, in which case the residue R is a common linker element. Possible are thus structures comprising at least two initiator moieties selected from the group consisting of: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    with
   R being a linker element   X +  being a counterion, selected from the group consisting of: K + , Cs + , Li + , Na + , R 4 N + , R 4 P +  or R 3 S +  with R being an organic substituent or H;   X, Y, Z, being, independently from each other, selected from the group consisting of: F, OR, N + R 3 , N + R 2 OR, N + R 2 SR, and N + R 2 NR 2 , and are optionally forming a cyclic or a bicyclic structure; wherein R is an organic substituent or H;
 
linked by said linker element.
   
 
         [0034]    Such dimers or trimers for the initiator (I) can be of the following general structure 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein
   R 1  is a linker element;   Y being selected from the group consisting of: —PO 3 H, —COOH, —BF 3   − X + , —BXYZ, wherein X + , X, Y, Z are defined as given above in the context of the initiator (I).   
 
         [0037]    The initiator (I) for the formation of a cyclic oligomer or a mixture of cyclic oligomers by amide-bond formation can be given by a structure comprising at least one such terminator moiety as defined further below and at least one such initiator moiety as given just above. Generally speaking, preferably the initiator (I) for the formation of a cyclic oligomer or a mixture of cyclic oligomers can be given by a structure consisting of a hydroxylamine at one end a ketoacid or acylboronate at the other end. 
         [0038]    Preferably the initiator (I) for the formation of a cyclic oligomer or a mixture of cyclic oligomers by amide-bond formation is selected from the group consisting of: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein
   R 1  is a linker element;   R q  is a structural element complementing to a 4, 5, 6, or 7 membered ring, preferably selected from the group consisting of: —C—, —CH 2 —C—, —CHR T —C—, —(CH 2 ) 2 —C—, —(CHR T ) 2 —C—, —(CH 2 )—C—(CH 2 )—, —(CHR T )—C—(CH 2 )—, —(CHR T )—C—(CHR T )—, —(CH 2 ) 3 —C, wherein R T  is selected from the group consisting of: hydrogen, halogen, substituted or unsubstituted alkyl, heteroalkyl, aryl, cycloalkyl, heterocycloalkyl, heteroaryl, alkylaryl, heteroalkylaryl, alkenyl, heteroalkenyl, alkinyl, heteroalkinyl;   R t  is a structural element complementing to a 4, 5, 6, or 7 membered ring, preferably selected from the group consisting of: —CR U -, —CH 2 —CR U -, —CHR T —CR U -, —(CH 2 ) 2 —CR U —, —(CHR T ) 2 —CR U —, —(CH 2 )—CR U —(CH 2 )—, —(CHR T )—CR U —(CH 2 )—, —(CHR T )—CR U -(CHR T )—, —(CH 2 ) 3 —CR U , wherein R T  is selected from the group consisting of: hydrogen, halogen, substituted or unsubstituted alkyl, heteroalkyl, aryl, cycloalkyl, heterocycloalkyl, heteroaryl, alkylaryl, heteroalkylaryl, alkenyl, heteroalkenyl, alkinyl, heteroalkinyl, and wherein R U  is selected from the group consisting of: hydrogen, halogen, substituted or unsubstituted alkyl, heteroalkyl, aryl, cycloalkyl, heterocycloalkyl, heteroaryl, alkylaryl, heteroalkylaryl, alkenyl, heteroalkenyl, alkinyl, heteroalkinyl;   R 7 —R 9  being, independently from each other, selected from the group consisting of: hydrogen, halogen, substituted or unsubstituted alkyl, heteroalkyl, aryl, cycloalkyl, heterocycloalkyl, heteroaryl, alkylaryl, heteroalkylaryl, alkenyl, heteroalkenyl, alkinyl, heteroalkinyl, ester, carbamate, sulfonate, sulfinate, phosphate, silyl, as well as cyclic forms linking these among each other and carbonyl, imidate, thiomidate,   R being selected from the group consisting of: O, S, NR 1 , SiR 1 R 2 , CR 1 R 2 ; wherein R 1  and R 2  are, independently from each other, selected from the group consisting of: hydrogen, halogen, substituted or unsubstituted alkyl, heteroalkyl, aryl, cycloalkyl, heterocycloalkyl, heteroaryl, alkylaryl, heteroalkylaryl, alkenyl, heteroalkenyl, alkinyl, heteroalkinyl
       Y being selected from the group consisting of: —PO 3 H, —COOH, —BF 3   − X + , —BXYZ, wherein X+, X, Y, Z are defined as given above in the context of the initiator (I).   
       
 
         [0045]    According to yet another preferred embodiment, the initiator (I) for the formation of a cyclic oligomer or a mixture of cyclic oligomers by amide-bond formation is selected from the group consisting of: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein boc=tert-butyloxycarbonyl, Ph=phenyl, Bz=benzyl, Fmoc=fluorenylmethyleneoxycarbonyl, Me=—CH 3 . 
         [0046]    The monomer (M) is preferably selected from the group consisting of: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    with preferably the following definitions:
   X is selected from the group consisting of: halogen, —OH, —COOH, —NH 2 , —O-Alkyl (e.g. —OEt, —OiPr, —OBn), —O-Aryl (e.g. —OPh, —OBz), —O—CO-Alkyl, —O—CO-Aryl, —SH, S-Alkyl (e.g. —S-Me), —S-Aryl (e.g. —S-Ph), N-Acyl, —NH-Alkyl, —NH-Aryl, —N(Alkyl) 2 , —N(Aryl) 2 , —N(Alkyl)(Aryl), —CO—NH-Alkyl, —CO—NH-Aryl, —CO—N(Alkyl) 2 , —CO—N(Aryl) 2 , —CO—N(Alkyl)(Aryl), —CN, —NO 2 , —N 3 , —S(O)Aryl, —S(O) 2  Aryl;   Y is selected from the group consisting of: —PO 3 H, —COOH, —BF 3   − X + , —BXYZ, wherein X + , X, Y, Z are defined as given above in the context of the initiator (I),   Z is selected from the group consisting of: —PO 3 H, —COOH, —BF 3   − X + , —BXYZ, wherein X+, X, Y, Z are defined as given above in the context of the initiator (I), as well as derivatives thereof which upon collapse of X and Z upon cleavage of the NO bond lead to Y;   R is selected from the group consisting of: O, S, NR 1 , Si, CHR 1 R 2  (R 1  and R 2  being defined as in the definition of R 1 -R 8  below);   R 1 -R 8  are, independently from each other, selected from the group consisting of: hydrogen, halogen, substituted or unsubstituted alkyl, heteroalkyl, aryl, cycloalkyl, heterocycloalkyl, heteroaryl, alkylaryl, heteroalkylaryl, alkenyl, heteroalkenyl, alkinyl, heteroalkinyl, as well as cyclic forms linking these among each other, wherein preferably the residue comprises in the range of 1-20 carbon atoms;   Q is selected from the group consisting of: O, S, Si, NR 11 , where R 11  is an organic substituent or H, preferably defined as in the definition of R 1 -R 8  above.   
 
         [0053]    Preferably, the monomer is selected from the following group, with the definitions as given just above: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0054]    As for the monomer of the type: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0055]    The methods of how to synthesize them are outlined in detail in the experimental section. As for the other monomers of the above group, represented with numbering as follows: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    the following synthetic routes are readily available to the skilled person: 
         [0056]    A monomer of formula (V) can be prepared in an analogous manner to that described in Org. Process. Res. Dev., 2012, 16, 687-696, as shown in the following Scheme: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0057]    A monomer of formula (VII) can be prepared in an analogous manner to that described in Helv. Chim. Acta, 2012, 95, 2481, as shown in the following Scheme: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0058]    A monomer of formula (VIII) can be prepared, as shown in the following scheme, by treating a compound of formula (VII) with a suitable base, such as NaOMe, in a suitable solvent, such as methanol: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0059]    A monomer of formulae (XIII) can be prepared as shown in the following Scheme. By treating a compound of formula (IX) with a reducing agent, such as SmI 2 , in a suitable solvent such as methanol/tetrahydrofuran, to give a compound of formula (X). Treatment of a compound of formula (X) with a carboxylic acid activating agent, such as Ac 2 O/AcONa or DCC, in a suitable solvent such as dichloromethane, will give a compound of formula (XI). Conversion of a compound of formula (XI) to a compound of formula (XII) can be carried out either using a triflating agent, such as Tf 2 O or PhNTf 2 , and a suitable base, such as Et 3 N or KHMDS, in a suitable solvent, such as tetrahydrofuran and DMPU. Treatment of a compound of formula (XII) with CO in the presence of a suitable catalyst, such as Pd(PPh 3 ) 4 , and a suitable base, such as Et 3 N, in a suitable solvent such as DMF, could give a compound of formula (XV). 
         [0000]    
       
                 
         
             
             
         
       
     
         [0060]    A monomer of formula (XIV) can be prepared by treating a compound of formula (XII) with a suitable catalyst, such as PdCl 2 dppf, and a suitable boron reagent, such as B 2 (pin) 2 , in the presence of a suitable base, such as AcOK, and in a suitable solvent, such as 1,4-dioxan, at elevated temperature (see Scheme below). Further treatment of XIV with a suitable fluorinating agent, such as KHF 2 , in a suitable solvent, such acetone/water, will give rise to a monomer of formula (XV): 
         [0000]    
       
                 
         
             
             
         
       
     
         [0061]    Monomers of formulae (XIX) and (XX) can be prepared in an analogous manner to monomers (XV) and (XIII) respectively, as described above. 
         [0062]    A monomer of formula (XVIII), where R is e.g. CR 1 R 2 , can be prepared by a cycloaddition reaction between a compound of formula (XVI) and a compound of formula (XVII), in a suitable solvent, such as xylene (see Scheme below). Interconversion of the Y functionality can be carried out using a method known to those skilled in the art. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0063]    According to a preferred embodiment, the monomer (I) is selected from the group consisting of: 
         [0000]    
       
                 
         
             
             
         
       
       
                 
         
             
             
         
       
       
                 
         
             
             
         
       
     
         [0064]    Wherein Me=—CH 3 , tBu=1,1-dimethylethyl, Cbz=benzyloxycarbonyl, iPr=isopropyl, Ph=phenyl. 
         [0065]    The terminator (T), if used, so in case of linear products, is selected from the group consisting of: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    with preferably the following definitions:
   R is selected from the group consisting of: CH 2 , (CH 2 ) 2 , CHR T , CH 2 CHR T , (CH 2 ) 3 , (CH 2 ) 2 CHR T , CH 2 CHR T CH 2 , (CH 2 ) 4 , wherein R T  is selected from the group consisting of: hydrogen, halogen, substituted or unsubstituted alkyl, heteroalkyl, aryl, cycloalkyl, heterocycloalkyl, heteroaryl, alkylaryl, heteroalkylaryl, alkenyl, heteroalkenyl, alkinyl, heteroalkinyl, wherein preferably the residue comprises in the range of 1-20 carbon atoms, more preferably in the range of 1-10 carbon atoms;   R 1  is selected from the group consisting of: substituted or unsubstituted alkyl, heteroalkyl, aryl, cycloalkyl, heterocycloalkyl, heteroaryl, alkylaryl, heteroalkylaryl, alkenyl, heteroalkenyl, alkinyl, heteroalkinyl, ester, carbamate, sulfonate, sulfinate, phosphate, silyl, wherein preferably the residue comprises in the range of 1-20 carbon atoms, more preferably in the range of 1-10 carbon atoms;   R 7 -R 9  are, independently from each other, selected from the group consisting of: hydrogen, halogen, substituted or unsubstituted alkyl, heteroalkyl, aryl, cycloalkyl, heterocycloalkyl, heteroaryl, alkylaryl, heteroalkylaryl, alkenyl, heteroalkenyl, alkinyl, heteroalkinyl, ester, carbamate, sulfonate, sulfinate, phosphate, silyl, as well as cyclic forms linking these among each other and carbonyl, imidate, thiomidate.   
 
         [0069]    According to a preferred embodiment, the terminator (T) is used in excess, and furthermore preferably it is selected from the group consisting of: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    Possible are in particular also structures for the terminator (T) of the type 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    with Me=—CH 3 , tBu=1,1-dimethylethyl. 
         [0070]    Alternatively, the terminator (T), also but not necessarily for the formation of a cyclic oligomer or a mixture of cyclic oligomers by amide-bond formation, can be given by covalent dimers or trimers of the above structures, in which case then R 1  and R 7  or R 8  are common linker elements. So the terminators can also be given by a structure comprising at least two terminator moieties selected from the group consisting of: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    with
   R being selected from the group consisting of: CH 2 , (CH 2 ) 2 , CHR T , CH 2 CHR T , (CH 2 ) 3 , (CH 2 ) 2 CHR T , CH 2 CHR T CH 2 , (CH 2 ) 4 , wherein R T  is selected from the group consisting of: hydrogen, halogen, substituted or unsubstituted alkyl, heteroalkyl, aryl, cycloalkyl, heterocycloalkyl, heteroaryl, alkylaryl, heteroalkylaryl, alkenyl, heteroalkenyl, alkinyl, heteroalkinyl;   R 1  being a common linker element;   R 7 -R 9  being, independently from each other, selected from the group consisting of: hydrogen, halogen, substituted or unsubstituted alkyl, heteroalkyl, aryl, cycloalkyl, heterocycloalkyl, heteroaryl, alkylaryl, heteroalkylaryl, alkenyl, heteroalkenyl, alkinyl, heteroalkinyl, ester, carbamate, sulfonate, sulfinate, phosphate, silyl, as well as cyclic forms linking these among each other and carbonyl, imidate, thiomidate, with the proviso that at least one of R 7  or R 8  is a common linker element.   
 
         [0074]    So dimeric or trimeric structures of the following type are possible for the terminator (T): 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein
   R 1  is a linker element;   R q  is a structural element complementing to a 4, 5, 6, or 7 membered ring, preferably selected from the group consisting of: —C—, —CH 2 —C—, —CHR T —C—, —(CH 2 ) 2 —C—, —(CHR T ) 2 —C—, —(CH 2 )—C—(CH 2 )—, —(CHR T )—C—(CH 2 )—, —(CHR T )—C—(CHR T )—, —(CH 2 ) 3 —C, wherein R T  is selected from the group consisting of: hydrogen, halogen, substituted or unsubstituted alkyl, heteroalkyl, aryl, cycloalkyl, heterocycloalkyl, heteroaryl, alkylaryl, heteroalkylaryl, alkenyl, heteroalkenyl, alkinyl, heteroalkinyl;   R t  is a structural element complementing to a 4, 5, 6, or 7 membered ring, preferably selected from the group consisting of: —CR U -, —CH 2 —CR U -, —CHR T —CR U -, —(CH 2 ) 2 —CR U —, —(CHR T ) 2 —CR U —, —(CH 2 )—CR U —(CH 2 )—, —(CHR T )—CR U —(CH 2 )—, —(CHR T )—CR U —(CHR T )—, —(CH 2 ) 3 —CR U , wherein R T  is selected from the group consisting of: hydrogen, halogen, substituted or unsubstituted alkyl, heteroalkyl, aryl, cycloalkyl, heterocycloalkyl, heteroaryl, alkylaryl, heteroalkylaryl, alkenyl, heteroalkenyl, alkinyl, heteroalkinyl, and wherein R U  is selected from the group consisting of: hydrogen, halogen, substituted or unsubstituted alkyl, heteroalkyl, aryl, cycloalkyl, heterocycloalkyl, heteroaryl, alkylaryl, heteroalkylaryl, alkenyl, heteroalkenyl, alkinyl, heteroalkinyl;   R 7 -R 9  being, independently from each other, selected from the group consisting of: hydrogen, halogen, substituted or unsubstituted alkyl, heteroalkyl, aryl, cycloalkyl, heterocycloalkyl, heteroaryl, alkylaryl, heteroalkylaryl, alkenyl, heteroalkenyl, alkinyl, heteroalkinyl, ester, carbamate, sulfonate, sulfinate, phosphate, silyl, as well as cyclic forms linking these among each other and carbonyl, imidate, thiomidate,
       R being selected from the group consisting of: O, S, NR 1 , SiR 1 R 2 , CR 1 R 2 ; wherein R 1  and R 2  are, independently from each other, selected from the group consisting of: hydrogen, halogen, substituted or unsubstituted alkyl, heteroalkyl, aryl, cycloalkyl, heterocycloalkyl, heteroaryl, alkylaryl, heteroalkylaryl, alkenyl, heteroalkenyl, alkinyl, heteroalkinyl;   
       (A-D) or (D-A), respectively, indicating one of the possible structures illustrated in the dashed box, According to a preferred embodiment, one single initiator (I) can be used in step 1) and, in case of the formation of a linear oligomer or a mixture of linear oligomers by amide-bond formation, one single terminator (T) can be used in step 2).   Step 1) and/or 2) can be carried out, apart from solvent(s), without any added further chemical reagents or catalysts, which simplifies the procedure and reduces toxicity issues. In step 1) and/or 2) organic solvents, water, aqueous buffer or combinations thereof can be used.   
 
         [0082]    According to yet another preferred embodiment, in step 1) more than 1, preferably 2-6, more preferably 2-4 different monomers can be used. 
         [0083]    In step 1) the reaction can be carried over to lead to oligomers with at least 2 interlinked monomers, preferably in the range of 2-10, more preferably in the range of 2-6 interlinked monomers. 
         [0084]    In step 1) the reaction conditions, preferably temperature and/or pressure, and/or reactant concentrations and/or reactant addition order and/or reactant addition time can be selected so as to lead, between different batches, to targeted different distributions of different oligomers in the mixture. 
         [0085]    In step 1) one single initiator (I), one single monomer (M) and, in case of the generation of a linear oligomers, in step 2) one single terminator (T) can be used, and the reaction conditions in step 1) and/or step 2) can be adapted such as to form a specific trimer structure. 
         [0086]    Furthermore the present invention relates to a method of identification of biologically and/or chemically active systems from a chemical library preferably based on at least one mixture of oligomers made using a method as outlined above, wherein preferably the mixtures of oligomers are screened for activity prior to purification or separation of the compounds from the mixture. 
         [0087]    According to a preferred embodiment of this method, a number of specifically differing mixtures made using a method as outlined above can be used, checking these mixtures for biological and/or chemical activity, inferring from activity patterns initiators and/or monomers and/or terminators inducing activity, optionally preparing further mixtures using a method as outlined above based on the identified active initiators and/or monomers and/or terminators only, thereby successively reducing the number of possible active oligomers. Like this the method can be very efficiently used in an incomplete factorial design screening process. 
         [0088]    Further embodiments of the invention are laid down in the dependent claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0089]    Preferred embodiments of the invention are described in the following with reference to the drawings, which are for the purpose of illustrating the present preferred embodiments of the invention and not for the purpose of limiting the same. In the drawings, 
           [0090]      FIG. 1  shows the chemoselective couplings of α-ketoacid initiators (I) and isoxazolidine monomers (M); (A) iterative synthesis of β-peptides; (B) one-pot chemical library synthesis; (C) possible products expand dramatically with increasing numbers of building blocks; 
           [0091]      FIG. 2  shows HPLC traces of product distributions with building blocks, I 1  (1.0 equiv), M 1  (0.50 equiv), M 2  (0.50 equiv) and T 1  (2.0 equiv), while M 1  and M 2  were added at different order; (A) M 1  was added first; (B) M 2  was added first; 
           [0092]      FIG. 3  shows preliminary libraries, library 1 (A) and library 2 (B); each well contained one initiator (1.0 equiv), three monomers (total 2.0 equiv) and one terminator (2.0 equiv); row and column had its assigned specific initiator/terminator; for example in library 1 (A), A1-A8 wells contained initiator I 1  and A3-H3 wells had terminator T 3 ; each preliminary library was divided into four sectors and each sector differed only in monomer mixtures: three out of four types of monomers; for example in library 1 (A), well A1 had initiator I 1 , monomer M 1  M 3  M 4  and terminator T 1  while well E5 had initiator I 1 , monomer M 1  M 2  M 4  and terminator T 1 ; active wells are presented dark (for selection standard, see description of the preferred embodiments); 
           [0093]      FIG. 4  shows a focused library, library 3; in sector 1, each well contained the assigned initiator (1.0 equiv), monomer M 2  M 4  M 5 (total 2.0 equiv) and the assigned terminator (2.0 equiv); after the oligomerization and termination were complete, sector 1 was 10-fold diluted to give sector 2; active wells are presented dark (for selection standard, see description of the preferred embodiments); 
           [0094]      FIG. 5  shows deconvolution library 1 and 2, library 4 and 5; active wells are presented dark (Among the active wells in rows B-E, wells with the best results are highlighted (for selection standard, see description of the preferred embodiments); 
           [0095]      FIG. 6  shows HPLC traces and the fluorescence read outs from the HCV assay of well A5, C5 and E4 in library 5; the fraction at 27.5 min was active in the HCV assay; 
           [0096]      FIG. 7  shows the synthesis of the lead compound I 3 -M 5 -M 5 -M 5 -T 4  in a) and in b) HPLC trace of this compound and its dose-response curve to HCV protease; the compound inhibits HCV protease with an IC50 of 1.0 μM; 
           [0097]      FIG. 8  shows a possible standard operating procedure for “on-demand” chemical library synthesis; 
           [0098]      FIG. 9  shows HPLC traces of product distributions with building blocks, I 1  (1.0 equiv), M1 (1.0, 2.0, 3.0 equiv respectively) and T 1  (2.0 equiv); and 
           [0099]      FIG. 10  illustrates the experimental procedures of library synthesis. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0100]    As a proof of concept, the “on-demand” library synthesis was used to prepare and screen inhibitors of HCV protease. The library syntheses were performed directly in 96-well plates. In contrast to the conventional “one well-one-compound” technique, each well, containing a mixture of products, was diluted and screened directly. 
         [0101]    Active compounds were identified by simple deconvolution strategies. This method provides a rapid and simple way to generate protease inhibitor libraries and to identify the potent leads. Two initial experiments to study the reactivity of monomers (M) and terminators (T) were performed. First, we determined if the product distributions reflected the quantities of monomers used in the self-assembly process. One equivalent of monomer M 1  produced I 1 -M 1 -T 1  as the major product. The product distributions shifted to higher oligomers when more M 1  was used (see further below). By simply varying the quantities of monomers, the distribution of products could be altered. The reproducibility of the results was confirmed by repetitions, as well as reactions involving different monomers. Second, we demonstrated that the product sequences could be biased by addition order. An experiment with M 1  and M 2  added in different order was performed. In  FIG. 2A , M 1  was added to react with initiator I 1  for 2 h, followed by addition of M 2  and finally termination with T 1 . Among I 1 -M-M-T 1  products, I 1 -M 1 -M 2 -T was found to be the main product while I 1 -M 2 -M -T 1  was not detected. Higher order products I 1 -M 1 -M 2 -M 2 -T 1  and I 1 -M 1 -M-M 2 -T 1  were also formed. Products were isolated and characterized by MS/MS. These results showed that when a product was constituted of both M 1  and M 2 , M 1  was incorporated before M 2  as it was added first. The same trend was observed in the experiment when M 2  was added first ( FIG. 2B ) with product distributions considerably changed when M 1  and M 2  were added in a different order. 
         [0102]    Our finding that product distributions could be systematically controlled by changing monomer quantities and the addition order allowed us to predict the major products in the reaction mixture when more types of monomers were involved. This self-assembly method was applied to protease inhibitor library syntheses and a commercially available HCV protease assay was chosen for screening. HCV protease lacks a well-defined pocket in its active site. The major interaction between the reported inhibitors and the shallow enzyme pocket is a reversible covalent bond formation from nucleophilic attack of the protease serine of the catalytic triad to the α-ketoamide on the inhibitors and hydrophobic interactions with the rest of the molecules. Two preliminary libraries (library 1 and 2) of a total size ˜6,000 compounds (˜40 compounds/well) were synthesized and screened. Based on the reported inhibition mechanism, both monomers and terminators used had hydrophobic side chains and the crucial α-ketoamide functional group was produced in the termination step. 
         [0103]    The arrangement of initiators, monomers and terminators in preliminary libraries is shown in  FIG. 3 : (1) Row/column had its assigned specific initiator/terminator. Wells containing active initiators and terminators were recognized based on the highest occurrence from the active wells. For example, in library 1, initiator I 1  I 3  and terminator T 2  T 4  were identified. (2) Each library was divided into four sectors and wells in the same sector had the assigned three out of four monomers. The same approach can be used with a larger number of monomers, initiators or terminators, thereby increasing the number of possible products that can be formed in each well. This setting was designed to facilitate the monomer selection processes. The three monomers in the sector with the least numbers of active wells were considered inactive and eliminated. For example, in library 1 sector containing M 1  M 2  M 3  but not M 4  had the least active wells. This benzyl monomer M 4  was selected for further studies. In library 2, initiator I 2  I 3  I 5 , monomer M 5  and terminator T 5  T 8  T 9  were selected according to the same principle. By this evaluation standard, we were able to quickly eliminate the poorly performing moieties and select building blocks for the following focused library. 
         [0104]    Selected initiator I 1  I 2  I 3  I 5 , monomer M 2  M 4  M 5  and terminator T 2  T 4  T 5  T 8  T 9  were included in a 5×4 well focused library (library 3, sector 1). (Both M 1  and M 2  were the second best monomers in library 1, but M 2  had an ester functionality, which could provide some varieties to the monomer selection, and was therefore preferred over M 1 ). The selection was made on the basis of a biochemical assay, in this case inhibitor of HCV protease as determined by a commercially available kit using a fluorescent substrate. The size of this focused library was limited to ˜800 compounds. The library arrangement is shown in  FIG. 4 . After the library was synthesized, it was diluted to make sector 2 (10-fold dilution). Well C2 showed positive results both in the original and in 10-fold diluted concentration while C5 showed reactivity only at the original concentration; well C 2  was therefore chosen for deconvolution. To simplify the deconvolution of well C2 in library 3, which contained initiator I 3 , monomer M 2  M 4  M 5 , and terminator T 4 , we assumed that if the active compound was constituted with the inclusion of three molecules of the monomer, reaction wells containing only one or two equivalents of monomers were less active. 
         [0105]    Based on this assertion, library 4 was synthesized. The library setting is shown in  FIG. 5 : (1) Well A1, A2, A3, A4, B1 and B 2 : each well contained only one type of monomers. Well A1 and A4 both had monomer M 2  but in different quantity to cover the different range of product distribution. Well A2/B1 and A3/B2 shared the same principle. (2) Well B3, B4, C1: wells consisted of two types of indicated monomers added at different order. (3) Well C2, C3 and C4: as in (2), but monomers were added at the same time. Comparing the overlapping/non-overlapping products of wells in (2) and (3), possible lead structures could be further limited. From the HCV assay result, well B2 and C4 showed activity. The active compound in B2 could be I 3 -M 5 -M 5 -T 4  or I 3 -M 5 -M 5 -M 5 -T 4 . Because there was no inhibition shown in C1, the active molecule in C4 is most likely to be I 3 -M 5 -M 5 -T 4 , I 3 -M 5 -M 2 -T 4 , I 3 -M 5 -M 5 -M 5 -T 4 , I 3 -M 5 -M 2 -M 2 -T 4  or I 3 -M 5 -M 5 -M 2 -T 4 . Further deconvolution of these confined lead candidates was performed. Different quantities of monomer M 5  were used to deconvolute the active compounds in well B2 in library 4. In library 5 row A ( FIG. 5 ), positive results were observed with wells containing more than one equivalent of M 5 . Between I 3 -M 5 -M 5 -T 4  and I 3 -M 5 -M 5 -M 5 -T 4 , I 3 -M 5 -M 5 -M 5 -T 4  was more likely to be the inhibitor. Rows B-E were designed to deconvolute C4 well in library 4. M 5  and M 2  were added in different ratios and quantities as indicated at the same time. A positive trend was seen when the ratio of M 5 /M 2  was greater than 1. This result implied that the lead compounds incorporated more M 5  than M 2 . Possible active structures were thus limited to I 3 -M 5 -M 5 -M 5 -T 4  and I 3 -M 5 -M 5 -M 2 -T 4 . At this stage, possible lead structures were already confined, but before the organic syntheses and purifications of each proposed compound was performed, HPLC analyses were carried out. HPLC traces of well A5, C5 and E4 in library 5 are shown in  FIG. 6 . The fractions were collected, lyophilized, redissolved in DMSO, and subjected to the HCV protease assay. The result showed that the fraction at 27.5 min was responsible for the HCV protease inhibition in all these three wells and corresponded to I 3 -M 5 -M 5 -M 5 -T 4 . Compound I 3 -M 5 -M 5 -M 5 -T 4  was synthesized under aqueous conditions by combining 1.0 equiv I3 and 3.6 equiv M 5  for 2 h, followed by the addition of 2.0 equiv T 4 . The product was isolated by HPLC and fully characterized. The pure material was subjected to the HCV protease assay and an IC50 of 1.0 μM was measured ( FIG. 7 ). This confirmed its identity as the most active compound in the focused library, library 3. 
       Experimental Details: 
     Synthesis of Libraries, Initial Studies: 
     (a) Product Distribution with Various Amount of M 1    
       [0106]    To a 5:1  t BuOH/50 mM Tris-HCl buffer, pH 7.0 (abbreviated as buffer in the following context) (v/v) (80 μL) solution, initiator I 1  (1.3 mg, 8.1 μmol, 1.0 equiv) and monomer M 1  (2.5 mg, 8.1 μmol, 1.0 equiv) were added. The mixture was allowed to stir at 45° C. for 2 h. The solution was cooled to RT and terminator T 1  (3.3 mg, 16 μmol, 2.0 equiv) in 16 μL 5:1  t BuOH/buffer was added and the mixture was heated at 45° C. for 2 h. Two other experiments with monomer M 1  (5.0 mg, 2.0 equiv and 7.5 mg, 3.0 equiv) were performed separately according to the same procedure. The reaction mixtures were analyzed by HPLC (gradient 10 to 90% CH 3 CN with 0.1% TFA in 20 min). For the results see  FIG. 9 . 
       (b) Product Distribution with Different Addition Order of M 1  and M 2    
       [0107]    To a 5:1  t BuOH/buffer (60 μL) solution, initiator I 1  (1.0 mg, 6.1 μmol, 1.0 equiv) and monomer M 1  (0.93 mg, 3.1 μmol, 0.50 equiv) were added. The mixture was allowed to stir at 45° C. for 2 h. The solution was cooled to RT and monomer M 2  (1.2 mg, 3.1 μmol, 0.50 equiv) was added. The mixture was allowed to stir at 45° C. for 2 h. The solution was cooled to RT and terminator T 1  was added and the mixture was heated at 45° C. for 2 h. The experiment with the reversed order of M 1  and M 2  addition was performed according to the same procedure. The reaction mixtures were analyzed by HPLC (gradient 10 to 90% CH 3 CN with 0.1% TFA in 20 min). For the results see  FIG. 2 . 
         [0000]    General Procedures for “on-Demand” Synthesis of Libraries: 
       (a) Liquid Handling 
       [0108]    The syntheses of libraries were carried out in 96-well plates (Thermofast AB-1100). Required amounts of initiators and monomers were dissolved in 5:1  t BuOH/buffer solution and added to their corresponding wells by a micropipettor (Eppendorf Research). 
       (Details are Described in the Synthesis of Each Library) 
     (b) Oligomerization and Termination Conditions 
       [0109]    After complete addition of initiators and monomers, the 96-well plate was capped, centrifuged for 2 min at 2000 rpm, and heated in the PCR machine (equipped with a heated lid at 110° C. to prevent solvent condensation on the cap) at 45° C. for 2 h. The plate was cooled to RT and corresponding terminators from stock solutions were added to each well by a micropipettor. The plate was capped, centrifuged and heated at 45° C. for 2 h. 
       (c) Dilutions 
       [0110]    The resulting crude mixtures were serially diluted with 5:1  t BuOH/buffer to reach the optimal concentration for biological assays. (Details are described in the synthesis of each library.) 
       (d) HCV Protease FRET Assay 
       [0111]    The HCV protease assay kits were purchased from ProteinOne and experiments were performed according to the manual provided. Without inhibitors, HCV protease cleaves the FRET substrate and results in a fluorescence signal at 530 nm (excitation wavelength at 490 nm). 
         [0112]    With the optimal concentration, 1.0 μL diluted crude mixture from each well was transferred to an assay plate (Sigma NUNC Maxisorp). Assay solution (100 μL) was added to each well by a multichannel pipette (Eppendorf Research) and the fluorescence signals were recorded immediately. Signals were recorded every 5 min for 2 h and active inhibitors were identified by reduced fluorescence. In the control experiments, all initiators, monomers and terminators were proved inactive at 10 μM. Likewise, solvents, 5:1  t BuOH/buffer solution and DMSO, did not interfere the results. 
       Libraries 
     (a) Library 1 and 2: Preliminary Library 1 and 2 
       [0113]    For library 1 and 2, the arrangement of the initiators, monomers, and terminators is indicated in  FIG. 3A   FIG. 3B . The wells in the same row had the same assigned initiator and the wells in the same column had the same assigned terminator. In each preliminary library, a total of four types of monomers were used. The plate was divided into four sectors and each of them contained three out of four types of monomers. During the oligomerization process, each well contained one initiator (0.5 μmol, 1.0 equiv) and three assigned monomers (total 1.0 μmol, 2.0 equiv) in a total of 5 μL 5:1  t BuOH/buffer solution. The three monomers were in a 1:1:1 ratio (i.e. 0.67 μmol of each monomer). For example, well A1 in library 1 contained initiator I 1  (0.50 μmol, 1.0 equiv), monomer M 1  (0.33 μmol, 0.67 equiv), monomer M 3  (0.33 μmol, 0.67 equiv) and monomer M 4  (0.33 μmol, 0.67 equiv). When the oligomerization process was finished, terminator T 1  (1.0 μmol, 1.0 μL, 2.0 equiv) was added. 
         [0114]    After the reactions were complete, each well was diluted to 35 μL with 5:1  t BuOH/buffer and 1.0 μL of this crude solution was used for the HCV assay. The relative fluorescence units (RFU) values from assay results were normalized, with the highest value as 100, and active wells were identified with normalized RFU value &lt;65 (Table 1). 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 RFU raw values (above) and normalized values (below) at 90 min in (a) library 1 (b) library 2. 
               
               
                   
               
             
             
               
                 A 
               
             
          
           
               
                   
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
               
               
                   
               
               
                 A 
                 784 
                 339 
                 768 
                 668 
                 759 
                 680 
                 714 
                 597 
               
               
                 B 
                 721 
                 407 
                 568 
                 634 
                 670 
                 787 
                 774 
                 739 
               
               
                 C 
                 724 
                 375 
                 647 
                 316 
                 708 
                 667 
                 693 
                 439 
               
               
                 D 
                 720 
                 450 
                 695 
                 658 
                 654 
                 715 
                 714 
                 625 
               
               
                 E 
                 690 
                 210 
                 335 
                 603 
                 396 
                 534 
                 513 
                 479 
               
               
                 F 
                 656 
                 188 
                 541 
                 514 
                 452 
                 471 
                 540 
                 540 
               
               
                 G 
                 617 
                 482 
                 516 
                 293 
                 464 
                 555 
                 564 
                 328 
               
               
                 H 
                 689 
                 686 
                 665 
                 707 
                 670 
                 573 
                 571 
                 544 
               
               
                   
               
             
          
           
               
                   
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
               
               
                   
               
               
                 A 
                 100 
                 43 
                 98 
                 85 
                 96 
                 86 
                 91 
                 76 
               
               
                 B 
                 92 
                 52 
                 72 
                 81 
                 85 
                 100 
                 98 
                 94 
               
               
                 C 
                 92 
                 48 
                 82 
                 40 
                 90 
                 85 
                 88 
                 56 
               
               
                 D 
                 91 
                 57 
                 88 
                 84 
                 83 
                 91 
                 91 
                 79 
               
               
                 E 
                 88 
                 27 
                 43 
                 77 
                 50 
                 68 
                 65 
                 61 
               
               
                 F 
                 83 
                 24 
                 69 
                 65 
                 57 
                 60 
                 69 
                 69 
               
               
                 G 
                 78 
                 61 
                 66 
                 37 
                 59 
                 71 
                 72 
                 42 
               
               
                 H 
                 88 
                 87 
                 84 
                 90 
                 85 
                 73 
                 73 
                 69 
               
               
                   
               
             
          
           
               
                 B 
               
             
          
           
               
                   
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
                 9 
                 10 
               
               
                   
               
               
                 A 
                 361 
                 357 
                 253 
                 254 
                 215 
                 308 
                 292 
                 277 
                 270 
                 255 
               
               
                 B 
                 249 
                 372 
                 431 
                 245 
                 226 
                 241 
                 349 
                 382 
                 248 
                 292 
               
               
                 C 
                 419 
                 412 
                 358 
                 353 
                 274 
                 274 
                 404 
                 371 
                 331 
                 286 
               
               
                 D 
                 415 
                 379 
                 345 
                 370 
                 313 
                 357 
                 269 
                 378 
                 360 
                 280 
               
               
                 E 
                 360 
                 277 
                 391 
                 330 
                 277 
                 208 
                 248 
                 230 
                 240 
                 247 
               
               
                 F 
                 297 
                 353 
                 361 
                 239 
                 240 
                 203 
                 396 
                 337 
                 235 
                 259 
               
               
                 G 
                 402 
                 280 
                 255 
                 345 
                 314 
                 275 
                 375 
                 376 
                 337 
                 355 
               
               
                 H 
                 427 
                 309 
                 429 
                 375 
                 386 
                 317 
                 289 
                 317 
                 294 
                 263 
               
               
                   
               
             
          
           
               
                   
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
                 9 
                 10 
               
               
                   
               
               
                 A 
                 84 
                 83 
                 59 
                 59 
                 50 
                 72 
                 68 
                 65 
                 63 
                 59 
               
               
                 B 
                 58 
                 87 
                 100 
                 57 
                 53 
                 56 
                 81 
                 89 
                 58 
                 68 
               
               
                 C 
                 98 
                 96 
                 83 
                 82 
                 64 
                 64 
                 94 
                 86 
                 77 
                 67 
               
               
                 D 
                 97 
                 88 
                 80 
                 86 
                 73 
                 83 
                 63 
                 88 
                 84 
                 65 
               
               
                 E 
                 84 
                 65 
                 91 
                 77 
                 65 
                 48 
                 58 
                 54 
                 56 
                 58 
               
               
                 F 
                 69 
                 82 
                 84 
                 56 
                 56 
                 47 
                 92 
                 79 
                 55 
                 60 
               
               
                 G 
                 94 
                 65 
                 59 
                 80 
                 73 
                 64 
                 87 
                 88 
                 79 
                 83 
               
               
                 H 
                 100 
                 72 
                 100 
                 87 
                 90 
                 74 
                 67 
                 74 
                 69 
                 61 
               
               
                   
               
             
          
         
       
     
       (b) Library 3: Focused Library 
       [0115]    In library 3, the arrangement of the initiators, monomers, and terminators is indicated in  FIG. 4 . In sector 1, the wells in the same row had the same assigned initiator, the wells in the same column had the same assigned terminator and every well contained all three monomers. 
         [0116]    During the oligomerization process, each well in sector 1 contained one initiator (0.5 μmol, 1.0 equiv) and all three monomers (total 1.0 μmol, 2.0 equiv) in a total of 5.0 μL 5:1  t BuOH/buffer solution. The three monomers were in a 1:1:1 ratio (i.e. 0.67 μmol of each monomer). For example, well A1 contained initiator I 1  (0.50 μmol, 1.0 equiv), monomer M 2  (0.33 μmol, 0.67 equiv), monomer M 4  (0.33 μmol, 0.67 equiv), and monomer M 5  (0.33 μmol, 0.67 equiv). When the oligomerization process was finished, terminator T 2  (1.0 μmol, 1.0 μL, 2.0 equiv) was added. After the reactions were complete, each well was diluted to 35 μL with 5:1  t BuOH/buffer. Sector 1 was serially diluted with 5:1  t BuOH/buffer solution to give sector 2, 3, and 4 and 1.0 μL of each well was used for the HCV assay. Based on the assay results (Table 2), well C2 was selected. 

 
         [0117]    In library 4, the arrangement of the initiators, monomers, and terminators is indicated in  FIG. 5A . During the oligomerization process, each well contained initiator I 3  (0.5 μmol, 1.0 equiv) and monomers (as indicated) in a total of 5.0 μL 5:1  t BuOH/buffer solution. For example, well A1 contained initiator I 3  (0.50 μmol, 1.0 equiv), and monomer M 2  (0.5 μmol, 1.0 equiv). Well C3 contained initiator I 3  (0.50 μmol, 1.0 equiv), monomer M 5  (0.50 μmol, 1.0 equiv) and monomer M 4  (0.50 μmol, 1.0 equiv). 
         [0118]    For wells where two monomers were added in sequence (well B3, B4, and C1), the first monomer (0.50 μmol, 1.0 equiv) was allowed to react with the initiator (0.5 μmol, 1.0 equiv) in a total 5.0 μL 5:1  t BuOH/buffer solution for 2 h. The second monomer (0.50 mol, 1.0 equiv) in 2.0 μL 5:1  t BuOH/buffer solution was added to the solution and the reaction mixture was allowed to react for another 2 h. For example, well C1 contained initiator I 3  (0.50 μmol, 1.0 equiv) and monomer M 2  (0.50 μmol, 1.0 equiv) in a total 5.0 μL 5:1  t BuOH/buffer solution. After 2 h reaction time, monomer M 5  (0.50 μmol, 1.0 equiv) in 2.0 μL 5:1  t BuOH/buffer solution was added. 
         [0119]    When the oligomerization process was finished, terminator T 4  (1.0 μmol, 1.0 μL, 2.0 equiv) was added to each well. After the reactions were complete, each well was diluted to 50 μL with 5:1  t BuOH/buffer (first dilution). From this diluted solution, 10 μL were taken and further diluted with 5:1  t BuOH/buffer to 100 μL (second dilution). One microliter of the second dilution solution was used for the HCV assay. Based on the assay results (Table 3), wells B2 and C4, with the lowest RFU, were considered active. 

 
         [0120]    In library 5, the arrangement of the initiators, monomers, and terminators is indicated in  FIG. 5B . Row A was designed for deconvolution of B2 in library 4, row B to E for C4 in library 4. 
         [0121]    During the oligomerization process, each well contained initiator I 3  (0.5 μmol, 1.0 equiv) and monomers (as indicated) in a total of 5.0 μL 5:1  t BuOH/buffer solution. For example, well A4 contained initiator I 3  (0.50 μmol, 1.0 equiv) and monomer M 5  (2.0 μmol, 4.0 equiv) in a total 5.0 μL 5:1  t BuOH/buffer solution. Well C2 contained initiator I 3  (0.50 μmol, 1.0 equiv), monomer M 2  (1.0 μmol, 2.0 equiv) and monomer M 5  (0.50 μmol, 1.0 equiv) in a total 5.0 μL 5:1  t BuOH/buffer solution. 
         [0122]    When the oligomerization process was finished, terminator T 4  (1.0 μmol, 1.0 μL, 2.0 equiv.) was added to each well. After the reactions were complete, each well was diluted to 50 μL with 5:1  t BuOH/buffer (first dilution). From this diluted solution, 10 μL were taken and further diluted with 5:1  t BuOH/buffer to 100 μL (second dilution). One microliter of the second dilution solution was used for the HCV assay. Active wells were identified with normalized RFU value &lt;65. In the library of deconvolution of well C4 in library 4 (rows B-E), highlighted wells were with normalized RFU≦40 (Table 4). 

 
       (e) HPLC Separations: Deconvolution of Well A5, C5, and E4 in Library 5 
       [0123]    From the selected well A5, C5 and E4, in library 5, 10 μL first dilution solution was taken and subjected to analytical HPLC separation (gradient of 10 to 90% CH 3 CN with 0.1% TFA, 30 min). Major fractions were collected and the solvent was removed by lyophilization. Each fraction was redissolved in 50 μL DMSO. One microliter was used for the HCV protease assay. 
         [0124]    In the case of A5, four fractions (1, 2, 3, 4) were collected and only fraction 3 (retention time=27.5 min) showed inhibition of HCV protease ( FIG. 6 , top). 
         [0125]    In the case of C5, five fractions (1, 2, 3, 4, 5) were collected and only fraction 5 (retention time=27.5 min) showed inhibition of HCV protease ( FIG. 6 , middle). 
         [0126]    In the case of E4, five fractions (1, 2, 3, 4, 5) were collected and only fraction 4 (retention time=27.5 min) showed inhibition of HCV protease ( FIG. 6 , bottom). 
         [0127]    Lead Inhibitor Synthesis, Isolation, and Characterizations: To a 5:1  t BuOH/buffer solution (0.30 mL, 0.1 M) of α-ketoglutaric acid I 3  (4.4 mg, 0.030 mmol, 1.0 equiv), monomer M 5  (40 mg, 0.12 mmol, 3.6 equiv) was added. The mixture was allowed to stir at 45° C. for 2 h. The solution was cooled to RT and terminator T 4  (24 mg, 0.060 mmol, 2.0 equiv) was added and the mixture was allowed to react at 45° C. for another 2 h. The crude reaction mixture was purified by preparative HPLC (gradient of 55 to 75% CH 3 CN with 0.1% TFA, 30 min) at 28 min and the collected product fraction was lyophilized to give as a white solid (6.6 mg, 0.0071 mmol, 24%). [α]D 25  (c=0.055, HFIP)=−5.2; mp&gt;200° C.;  1 H NMR (600 MHz, d 6 -DMSO) δ 12.04 (br s, 1H), 9.07 (t, J=6.4 Hz, 1H), 7.74 (d, J=8.1 Hz, 1H), 7.53 (d, J=9.1 Hz, 1H), 7.48 (d, J=8.7 Hz, 2H), 7.35 (d, J=8.5 Hz, 2H), 7.30 (d, J=8.5 Hz, 2H), 4.29 (d, J=6.4 Hz, 2H), 4.14-4.07 (m, 1H), 4.03-3.95 (m, 3H), 3.00 (dd, J=5.8, 16.7 Hz, 1H), 2.84 (dd, J=7.5, 16.7 Hz, 1H), 2.45-2.00 (m, 12H), 1.73-1.60 (m, 10H), 1.60-1.47 (m, 7H), 1.38 (s, 9H), 1.36-1.27 (m, 3H), 1.16-1.00 (m, 9H), 0.97-0.86 (m, 6H);  13 C NMR (150 MHz, d 6 -DMSO) δ 196.4, 173.9, 171.9, 170.2, 169.8, 169.5, 169.5, 160.9, 137.7, 131.4, 129.2, 128.2, 79.5, 50.3, 50.2, 50.0, 44.2, 42.2, 41.4, 40.9, 40.8, 40.5, 38.6, 38.4, 38.2, 31.3, 30.2, 29.5, 29.4, 29.2, 27.7, 27.1, 27.1, 27.1, 27.0, 26.0, 26.0, 25.9, 25.8, 25.7; IR (thin film) ν 3303, 2925, 2852, 1644, 1539 cm −1 ; HRMS (ESI) calcd for C 49 H 75 ClN 5 O 10  [M+H] +  928.5197. found, 928.5204. 
         [0128]    IC 50 : Ten different concentrations (10, 5.0, 2.5, 1.0, 0.75, 0.50, 0.30, 0.25, 0.080, 0.050 μM in DMSO) of I 3 -M 5 -M 5 -M 5 -T 4  were measured with HCV protease assay. Fluorescence signals were recorded at 90 min. The experiment was repeated in triplicate. IC 50  was calculated via nonlinear regression using the software package GraphPad Prism 5. 
       General Methods: 
       [0129]    Chemicals were purchased from Acros, Sigma-Aldrich, or ABCR and used without further purification. Thin layer chromatography (TLC) was performed on glass backed plates pre-coated with silica gel (Merck, Silica Gel 60 F254) and were visualized by fluorescence quenching under UV light or by staining with ceric sulfate or potassium permanganate. Flash column chromatography was performed on Silicycle Silica Flash F60 (230-400 Mesh) using a forced flow of air at 0.5-1.0 bar. NMR spectra were measured on VARIAN Mercury 300 MHz, 75 MHz, Bruker Avance 400 MHz, 100 MHz or Bruker AV-II 600 MHz, 150 MHz with a cryoprobe. Chemical shifts are expressed in parts per million (ppm) and are referenced to CDCl 3  7.26 ppm, 77.0 ppm; CD 3 OD 3.31 ppm, 49.0 ppm; d 6 -DMSO 2.50 ppm, 39.5 ppm. Coupling constants are reported as Hertz (Hz). Splitting patterns are indicated as follows: br, broad; s, singlet; d, doublet; t, triplet; dd, doublet of doublet; dt, doublet of triplet; m, multiplet. Infrared (IR) spectra were recorded on a JASCO FT/IR-4100 spectrophotometer and are reported as wavenumber (cm −1 ). Optical rotations were measured in a Jasco P-2000 polarimeter with a 100 mm path length cell operating at the sodium D line (589 nm) and reported as [o]D 25  (concentration g/100 mL, solvent), T=temperature (° C.). High-resolution mass spectra and MS/MS spectra were measured on a Bruker Daltonics maXis ESI-QTOF by the mass spectrometry service of the Laboratorium flir Organische Chemie at the ETH Ziirich. Melting points were measured on an Electrothermal Mel-Temp melting point apparatus using open glass capillaries and are uncorrected. HPLC (high performance liquid chromatography) was performed on JASCO analytical and preparative instruments. Columns used for the analytical and preparative HPLC were Shiseido CAPCELL PAK C18 UG120 (4.6 mm I.D.×250 mm) and Shiseido Capcell Pak C18 MG II (10 mm I.D.×250 mm) column with flow rates 1.0 mL/min and 10 mL/min respectively. The mobile phase were MQ-H 2 O with 0.1% TFA (eluent A) and HPLC grade CH 3 CN with 0.1% TFA (eluent B). Signals were monitored at 220, 254 and 301 nm. Library synthesis was performed on Techne PCR machine. The fluorescence was recorded on Molecular Devices and Thermo plate readers. 
       Synthesis of Monomers and Terminators 
     General Procedures (A) Synthesis of Monomers. (B) Synthesis of Terminators: 
       [0130]    
       
                 
         
             
             
         
       
     
       (a) Preparation of 3-Methylene-1,4-dioxaspiro[4.5]decan-2-one (2) 
       [0131]    To a stirred solution of 5-chloromethyl-2,2-pentamethylene-1,3-dioxolan-4-one (1) (1.0 equiv) in CHCl 3  (0.50 M), NEt 3  (2.0 equiv) was added and the solution heated to reflux for 18 h. The solution was cooled to RT and CHCl 3  was removed under reduced pressure to provide acrylate (2), which was used without further purification. 
       (b) Preparation of Methyl 2-Methoxyacrylate (8) 
       [0132]    Methyl 2-methoxy acrylate (8) was prepared according to the literature procedure. 
       (c) Cycloaddition 
       [0133]    A toluene solution (0.20-0.50 M) of 2,3:5,6-O-diisopropylidene- D -gulose oxime (3) 1  (1.0 equiv), aldehyde 4 (1.0 equiv) and acrylate 2 or 8 (1.0-2.0 equiv) was heated with a Dean-Stark apparatus fitted with a reflux condenser for 24 h. The solution was cooled to RT and toluene removed under reduced pressure. The crude cycloadduct  D -gulose-isoxazolidine 5 or 9 was purified by flash chromatography and recrystallization. 
       (d) Cleavage of Chiral Auxiliary 
       [0134]    To a solution of cycloadduct 5 or 9 (1.0 equiv) in CH 3 CN (0.10 M), HClO 4  (70% w/w, 3.0 equiv) was added and the mixture was allowed to stir at RT for 5 h. The reaction mixture was neutralized with saturated NaHCO 3  and extracted with EtOAc (3×). The combined organic layers were washed with brine (2×), dried over Na 2 SO 4 , and filtered. The solvent was removed under reduced pressure, and the crude reaction mixture was purified by flash chromatography to afford unprotected isoxazolidine 6 or 10. 
       (e) Preparation of Monomer HCl Salts 
       [0135]    Unprotected isoxazolidine 6 (1.0 equiv) was dissolved in Et 2 O (0.1 M) and 4 M HCl in dioxane (1.1 equiv) was added. The solution was allowed to stir at RT for 15 min and a white precipitate formed. The precipitate was collected by filtration and dried under vacuum to provide the desired isoxazolidine hydrochloride salt 7 as a white solid. 
       (f) Preparation of Terminators 
       [0136]    The corresponding amine (5.0-10 equiv) was added to isoxazolidine 10 (1.0 equiv) and the mixture was allowed to stir at RT for 12 h. The reaction mixture was diluted with EtOAc and washed with saturated NaHCO 3  (2×). The combined organic layers was washed with brine (2×), dried over Na 2 SO 4 , filtered and concentrated under reduced pressure to give the product. If necessary, further purification was performed by flash chromatography. 
       Experimental Procedures and Characterization Data for the Synthesis of Monomers: 
       D -Gulose-β 3 h-(thiophen 2-yl)-isoxazolidine (12) 
       [0137]    
       
                 
         
             
             
         
       
     
         [0138]    Acrylate 2 was prepared according to General Procedure (a) from 5-chloromethyl-2,2-pentamethylene-1,3-dioxolan-4-one (1) (0.15 g, 0.73 mmol, 1.0 equiv) and NEt 3  (0.19 mL, 1.5 mmol, 2.1 equiv) in CHCl 3  (1.5 mL). The cycloaddition was performed according to General Procedure (c) from the crude acrylate 2 , D -gulose oxime 3 (0.20 g, 0.73 mmol, 1.0 equiv) and thiophene 2-carboxaldehyde (68 μL, 0.73 mmol, 1.0 equiv) in toluene (1.5 mL). The cycloadduct was purified by flash chromatography (9:1 hexanes/EtOAc) and recrystallized from hexanes/EtOAc to afford the product as a white solid (0.19 g, 0.35 mmol, 48%). [α] D   25  (c=0.3, CH 2 Cl 2 )=+15.0; mp=162-164° C.;  1 H NMR (400 MHz, CDCl 3 ) δ 7.24 (dd, J=1.2, 5.1 Hz, 1H), 7.16-7.04 (m, 1H), 6.95 (dd, J=3.5, 5.1 Hz, 1H), 5.13 (dd, J=3.6, 8.1 Hz, 1H), 4.93 (d, J=6.0 Hz, 1H), 4.84 (s, 1H), 4.67 (d, J=4.0, 6.0 Hz, 1H), 4.35 (dt, J=6.8, 8.5 Hz, 1H), 4.18 (dd, J=6.8, 8.5 Hz, 1H), 4.07 (dd, J=4.0, 8.5 Hz, 1H), 3.68 (dd, J=6.8, 8.5 Hz, 1H), 3.19 (dd, J=8.1, 14.0 Hz, 1H), 2.71 (dd, J=3.6, 14.0 Hz, 1H), 1.91-1.58 (m, 8H), 1.50-1.42 (m, 5H), 1.40 (s, 3H), 1.37 (s, 3H), 1.29 (s, 3H);  13 C NMR (100 MHz, CDCl 3 ) δ 168.9, 142.9, 126.8, 125.5, 125.3, 112.9, 112.1, 109.8, 105.5, 97.1, 84.6, 83.7, 80.3, 75.6, 66.0, 60.2, 43.1, 37.4, 36.3, 26.7, 26.1, 25.3, 24.8, 24.3, 22.9, 22.8; IR (thin film) ν 2984, 2938, 1866, 1801, 1372, 1209, 1090 cm −1 ; HRMS (ESI) calcd for C 26 H 36 NO 9 S [M+H] +  538.2105. found, 538.2098. 
       β 3 h-(Thiophen 2-yl)-isoxazolidine hydrochloride (M 8 ) 
       [0139]    
       
                 
         
             
             
         
       
     
         [0140]    Auxiliary Cleavage was performed according to General Procedure (d) from  D -gulose-β 3  h-(thiophen-2-yl)-isoxazolidine (12) (0.40 g, 0.74 mmol, 1.0 equiv) and HClO 4  (0.19 mL, 2.2 mmol, 3.0 equiv) in CH 3 CN (7.4 mL). The crude reaction mixture was purified by flash chromatography (3:1 hexanes/EtOAc) to afford the unprotected β 3 h-(thiophen-2-yl)-isoxazolidine as a colorless liquid (0.21 g, 0.71 mmol, 96%). According to General Procedure (e), the unprotected isoxazolidine was redissolved in Et 2 O (7.0 mL) and treated with 4 M HCl in dioxane (0.20 mL, 0.81 mmol, 1.1 equiv) to give hydrochloride salt M 8  as a white solid (0.20 g, 0.60 mmol, 85%). [α] D   25  (c=0.3, CH 2 Cl 2 )=+30.0; mp=115-116° C.;  1 H NMR (300 MHz, CD 3 OD) δ 7.55 (dd, J=1.2, 5.1 Hz, 1H), 7.43-7.27 (m, 1H), 7.11 (d, J=3.6, 5.1 Hz, 1H), 5.40-5.09 (m, 1H), 3.46-3.24 (m, 1H), 2.90 (dd, J=8.2, 14.2 Hz, 1H), 2.0-1.45 (m, 10H);  3 C NMR (100 MHz, CD 3 OD) δ 168.1, 136.4, 129.3, 128.5, 128.4, 113.9, 108.4, 60.9, 44.5, 38.2, 36.9, 25.2, 24.0, 24.0; IR (thin film) ν 3208, 2940, 2864, 1801, 1449, 1373, 1269, 1177, 931, 703 cm −1 ; HRMS (ESI) calcd for C 14 H 18 NO 4 S [M+H] +  296.0951. found, 296.0945. 
         D -Gulose-β 3 h-(2-fluorophenyl)-isoxazolidine (13) 
       [0141]    
       
                 
         
             
             
         
       
     
         [0142]    Acrylate 2 was prepared according to General Procedure (a) from 5-chloromethyl-2,2-pentamethylene-1,3-dioxolan-4-one (1) (1.5 g, 7.4 mmol, 1.0 equiv) and NEt 3  (2.0 mL, 15 mmol, 2.0 equiv) in CHCl 3  (15 mL). The cycloaddition was performed according to General Procedure (c) from the crude acrylate 2 , D -gulose oxime 3 (2.0 g, 7.4 mmol, 1.0 equiv) and 2-fluorobenzylaldehyde (0.78 mL, 7.4 mmol, 1.0 equiv) in toluene (15 7.9 mL). The cycloadduct was purified by flash chromatography (5:1 hexanes/EtOAc) and recrystallized from hexanes to afford the product as a white solid (2.2 g, 3.9 mmol, 53%). [α]D 25  (c=0.5, CH 3 OH)=+24.1; mp=105-106° C.;  1 H NMR (400 MHz, CDCl 3 ) δ 7.67-7.62 (m, 1H), 7.35-7.20 (m, 1H), 7.15-7.09 (m, 1H), 7.06-6.99 (m, 1H), 5.16 (dd, J=8.2, 3.6 Hz, 1H), 4.97 (d, J=6.0 Hz, 1H), 4.85 (s, 1H), 4.67 (dd, J=6.0, 4.0 Hz, 1H), 4.32 (dt, J=6.6, 8.5 Hz, 1H), 4.14 (dd, J=6.6, 8.5 Hz, 1H), 3.94 (dd, J=4.0, 8.5 Hz, 1H), 3.64 (dd, J=6.6, 8.5 Hz, 1H), 3.22 (dd, J=8.2, 13.9 Hz, 1H), 2.55 (dd, J=3.6, 13.9 Hz, 1H), 1.92-1.80 (m, 2H), 1.80-1.54 (m, 6H), 1.50-1.37 (m, 5H), 1.33 (s, 3H), 1.30 (s, 3H), 1.29 (s, 3H);  13 C NMR (100 MHz, CDCl 3 ) δ 168.9, 160.3 (d, J=247 Hz), 129.1 (d, J=8.2 Hz), 128.6 (d, J=3.6 Hz), 126.7 (d, J=12.7 Hz), 124.0 (d, J=3.6 Hz), 115.2 (d, J=21.4 Hz), 112.9, 111.9, 109.7, 105.2, 97.1, 84.4, 83.7, 80.3, 75.6, 66.0, 57.8, 42.7, 37.4, 36.2, 26.6, 26.1, 25.3, 24.9, 24.2, 22.9, 22.8; IR (thin film) ν 2986, 2866, 1802, 1490, 1454, 1156, 1036, 849 cm −1 ; HRMS (ESI) calcd for C 28 H 37 FNO 9  [M+H] +  550.2447. found, 550.2441. 
       β 3 h-(2-Fluorophenyl)-isoxazolidine hydrochloride (M 6 ) 
       [0143]    
       
                 
         
             
             
         
       
     
         [0144]    Auxiliary Cleavage was performed according to General Procedure (d) from  D -gulose-β 3  h-(2-fluorophenyl)-isoxazolidine (13) (1.0 g, 1.8 mmol, 1.0 equiv) and HClO 4  (0.47 mL, 5.5 mmol, 3.1 equiv) in CH 3 CN (18 mL). The crude reaction mixture was purified by flash chromatography (5:1 hexanes/EtOAc) to afford the unprotected β 3 h-(2-fluorophenyl)-isoxazolidine as a colorless liquid (0.51 g, 1.7 mmol, 94%). According to General Procedure (e), the unprotected isoxazolidine was redissolved in Et 2 O (17 mL) and treated with 4 M HCl in dioxane (0.48 mL, 1.9 mmol, 1.1 equiv) to give hydrochloride salt M 6  as a white solid (0.40 g, 1.2 mmol, 71%). [0]D 25  (c=0.5, CH 3 OH)=+15.2; mp=137-138° C.;  1 H NMR (400 MHz, CD 3 OD) δ 7.70-7.63 (m, 1H), 7.54-7.45 (m, 1H), 7.33-7.20 (m, 2H), 5.31 (m, 1H), 3.40-3.32 (m, 1H), 2.93 (dd, J=7.3, 14.2 Hz, 1H), 1.97-1.80 (m, 4H), 1.79 (m, 4H), 1.60-1.34 (m, 2H);  13 C NMR (100 MHz, CD 3 OD) δ 167.9, 162.3 (d, J=246.5 Hz), 132.7 (d, J=8.5 Hz), 130.1 (d, J=3.1 Hz), 126.0 (d, J=3.6 Hz), 122.0 (d, J=13.7 Hz), 116.8 (d, J=21.6 Hz), 114.0, 108.1, 59.13 (d, J=3.4 Hz), 42.6, 38.2, 36.8, 25.2, 24.0, 24.0; IR (thin film) ν 3398, 2940, 2864, 1798, 1682, 1454 cm −1 ; HRMS (ESI) calcd for C 16 H 19 FNO 4  [M−Cl] +  308.1293. found, 308.1287. 
       Experimental Procedures and Characterization Data for the Synthesis of Terminators 
     (3S,5R)-3-Isobutyl-5-methoxyisoxazolidine-5-carboxamide (T 1 ) 
       [0145]    
       
                 
         
             
             
         
       
     
         [0146]    Terminator T 1  was prepared according to General Procedure (f) from ammonium hydroxide solution (25% w/w, 0.50 mL, 3.3 mmol, 8.3 equiv) and (3S,5R)-methyl 3-isobutyl-5-methoxyisoxazolidine-5-carboxylate (86 mg, 0.40 mmol, 1.0 equiv). The product was isolated as a white solid (73 mg, 0.36 mmol, 90%). [α] D   25  (c=0.4, CH 3 OH)=+118.0; mp=136-138° C.;  1 H NMR (400 MHz, CDCl 3 ) δ 6.68 (br d, 1H), 6.51 (br d, 1H), 5.59 (br d, 1H), 3.50-3.36 (m, 1H), 3.28 (s, 3H), 2.63 (dd, J=8.2, 13.6 Hz, 1H), 1.98 (dd, J=8.4, 13.6 Hz, 1H), 1.70-1.55 (m, 1H), 1.53-1.27 (m, 2H), 0.94-0.83 (m, 6H);  13 C NMR (100 MHz, CDCl 3 ) δ 170.5, 109.0, 59.2, 51.6, 48.4, 40.4, 26.5, 22.6, 22.6; IR (thin film) ν 3399, 3220, 3145, 2952, 2875, 1666, 1227, 1047 cm −1 ; HRMS (ESI) calcd for C 9 H 19 N 2 O 3  [M+H] + 203.1390. found, 203.1381. 
       (3S,5R)—N-Cyclopropyl-3-isobutyl-5-methoxyisoxazolidine-5-carboxamide (T 2 ) 
       [0147]    
       
                 
         
             
             
         
       
     
         [0148]    Terminator T 2  was prepared according to General Procedure (f) from cyclopropylamine (0.20 mL, 2.9 mmol, 4.8 equiv) and (3S,5R)-methyl 3-isobutyl-5-methoxyisoxazolidine-5-carboxylate (0.13 g, 0.60 mmol, 1.0 equiv). The product was isolated as a colorless liquid (92 mg, 0.38 mmol, 63%). [α] D   25  (c=0.8, CH 3 OH)=+67.4;  1 H NMR (400 MHz, CDCl 3 ) δ 6.66 (br d, 1H), 5.55 (br d, 1H), 3.47-3.30 (m, 1H), 3.24 (s, 3H), 2.84-2.72 (m, 1H), 2.58 (dd, J=8.2, 13.6 Hz, 1H), 1.98 (dd, J=8.3, 13.6 Hz, 1H), 1.70-1.55 (m, 1H), 1.50-1.27 (m, 2H), 0.95-0.85 (m, 6H), 0.85-0.70 (m, 2H), 0.55-0.45 (m, 2H);  13 C NMR (100 MHz, CDCl 3 ) δ 168.8, 109.1, 59.3, 51.5, 48.1, 40.5, 26.5, 22.7, 22.6, 22.3, 6.6, 6.3; IR (thin film) ν 3318, 2956, 1678, 1519, 1074, 1035 cm −1 ; HRMS (ESI) calcd for C 12 H 23 N 2 O 3  [M+H] + 243.1703. found, 243.1696. 
       ((3S,5R)-3-Isobutyl-5-methoxyisoxazolidin-5-yl)(morpholino)methanone (T 3 ) 
       [0149]    
       
                 
         
             
             
         
       
     
         [0150]    Morpholine (0.30 mL, 3.4 mmol, 6.2 equiv), 1,2,4-triazole (8.0 mg, 0.12 mmol, 0.22 equiv) and 1,8-diazabicyclo[5.4.0]undec-7-ene (16 μL, 0.11 mmol, 0.20 equiv) were added to (3S,5R)-methyl 3-isobutyl-5-methoxyisoxazolidine-5-carboxylate (0.12 g, 0.55 mmol, 1.0 equiv). The mixture was stirred at 60° C. for 12 h. The crude reaction mixture was purified by flash chromatography (1:3 hexanes/EtOAc) and the product was isolated as a clear liquid (89 mg, 0.33 mmol, 60%). [α] D   25  (c=0.6, CH 2 Cl 2 )=+71.7;  1 H NMR (400 MHz, CDCl 3 ) δ 5.50 (br d, 1H), 3.93-3.42 (m, 9H), 3.28 (s, 3H), 2.88 (dd, J=8.0, 12.9 Hz, 1H), 1.90 (dd, J=8.0, 12.9 Hz, 1H), 1.75-1.56 (m, 1H), 1.47 (dd, J=6.9, 13.8 Hz, 1H), 1.36 (dd, J=7.0, 13.8 Hz, 1H), 0.96-0.58 (m, 6H);  13 C NMR (100 MHz, CDCl 3 ) δ 165.3, 110.2, 67.0, 66.9, 59.3, 51.4, 46.7, 45.8, 43.1, 40.9, 26.5, 22.7, 22.7; IR (thin film) ν 3203, 2956, 2867, 1651, 1434, 1253, 1116, 1068, 1029 cm −1 ; HRMS (ESI) calcd for C 13 H 24 N 2 NaO 4  [M+Na] + 295.1628. found, 295.1613. 
       tert-Butyl 3-((3S,5R)-5-((4-chlorobenzyl)carbamoyl)-5-methoxyisoxazolidin-3-yl)propanoate (T 4 ) 
       [0151]    
       
                 
         
             
             
         
       
     
         [0152]    Terminator T 4  was prepared according to General Procedure (f) from 4-chlorobenzylamine (1.5 mL, 12 mmol, 8.6 equiv) and tert-butyl 3-((3S,5R)-5-((4-chlorobenzyl)carbamoyl)-5-methoxyisoxazolidin-3-yl)propanoate (0.40 g, 1.4 mmol, 1.0 equiv). The crude reaction mixture was purified by flash chromatography (1:1 hexanes/EtOAc) and the product was isolated as a white solid (0.48 g, 1.2 mmol, 86%). [α] D   25  (c=0.5, CH 2 Cl 2 )=+51.0; mp=65-67° C.;  1 H NMR (400 MHz, CDCl 3 ) δ 7.29 (d, J=8.5 Hz, 2H), 7.18 (d, J=8.5 Hz, 2H), 6.95 (br t, 1H), 5.68 (br d, 1H), 4.50-4.35 (m, 2H), 3.42-3.30 (m, 1H), 3.26 (s, 3H), 2.60 (dd, J=8.2, 13.6 Hz, 1H), 2.30 (t, J=7.4 Hz, 2H), 2.05 (dd, J=8.1, 13.6 Hz, 1H), 1.88-1.75 (m, 2H), 1.42 (s, 9H);  13 C NMR (100 MHz, CDCl 3 ) δ 171.9, 167.4, 136.1, 133.5, 129.1, 128.9, 109.1, 80.7, 60.5, 51.5, 47.6, 42.6, 32.9, 28.0, 26.4; IR (thin film) ν 3353, 2978, 2936, 1726, 1681, 1523, 1154, 1089 cm −1 ; HRMS (ESI) calcd for C 19 H 27 ClN 2 NaO 5  [M+Na] + 421.1501. found, 421.1490. 
       (3S,5R)—N-Allyl-3-isobutyl-5-methoxyisoxazolidine-5-carboxamide (T 5 ) 
       [0153]    
       
                 
         
             
             
         
       
     
         [0154]    Terminator T 5  was prepared according to General Procedure (f) from allylamine (0.35 mL, 4.6 mmol, 5.9 equiv) and (3S,5R)-methyl 3-isobutyl-5-methoxyisoxazolidine-5-carboxylate (0.17 g, 0.78 mmol, 1.0 equiv). The product was isolated as a white solid (0.18 g, 0.74 mmol, 95%). [u]D 25  (c=2.0, CH 2 Cl 2 )=+95.3; mp=33-35° C.;  1 H NMR (300 MHz, CDCl 3 ) δ 6.75 (br t, 1H), 5.88-5.73 (m, 1H), 5.59 (br d, 1H), 5.21-5.09 (m, 2H), 4.01-3.79 (m, 2H), 3.50-3.32 (m, 1H), 3.25 (s, 3H), 2.60 (dd, J=8.1, 13.6 Hz, 1H), 1.99 (dd, J=8.4, 13.6 Hz, 1H), 1.72-1.53 (m, 1H), 1.54-1.27 (m, 2H), 0.93-0.85 (m, 6H);  13 C NMR (100 MHz, CDCl 3 ) δ 167.4, 133.4, 116.7, 109.2, 59.3, 51.5, 48.2, 41.5, 40.5, 26.5, 22.7, 22.6; IR (thin film) ν 2957, 2871, 2837, 1672, 1645, 1526, 1261, 1148, 991, 919 cm −1 ; HRMS (ESI) calcd for C 12 H 23 N 2 O 3  [M+H] +  243.1703. found, 243.1697. 
         D -Gulose-/β 3 h-(4-methoxyphenyl)-isoxazolidine (14) 
       [0155]    
       
                 
         
             
             
         
       
     
         [0156]    The cycloaddition was followed General Procedure (c) from methyl 2-methoxy acrylate 8 (1.7 g, 15 mmol, 2.1 equiv),  D -gulose oxime 3 (2.0 g, 7.3 mmol, 1.0 equiv) and 4-methoxybenzaldehyde (1.0 g, 7.3 mmol, 1.0 equiv) in toluene (35 mL). The cycloadduct was purified by flash chromatography (3:1 hexanes/EtOAc) and recrystallized from hexanes to afford the product as a white solid (2.5 g, 4.9 mmol, 67%). [α] D   25  (c=0.3, CH 2 Cl 2 )=+54.0; mp=58-60° C.;  1 H NMR (300 MHz, CDCl 3 ) δ 7.36 (d, J=8.7 Hz, 2H), 6.83 (d, J=8.7 Hz, 2H), 5.08 (d, J=6.0 Hz, 1H), 4.74 (s, 1H), 4.67 (dd, J=4.2, 6.0 Hz, 1H), 4.32-4.18 (m, 2H), 4.18-4.06 (m, 1H), 3.92-3.81 (m, 4H), 3.77 (s, 3H), 3.60 (dd, J=7.0, 8.3 Hz, 1H), 3.41 (s, 3H), 2.95 (dd, J=8.4, 13.6 Hz, 1H), 2.56 (dd, J=8.0, 13.6 Hz, 1H), 1.61 (s, 3H), 1.42 (s, 3H), 1.31 (s, 3H), 1.28 (s, 3H);  13 C NMR (100 MHz, CDCl 3 ) δ 168.8, 159.2, 130.2, 128.8, 114.0, 112.6, 109.5, 104.4, 98.2, 84.4, 82.5, 80.7, 75.8, 66.2, 65.9, 55.2, 52.9, 51.8, 48.7, 26.6, 26.0, 25.3, 24.8; IR (thin film) ν 2986, 2937, 1750, 1515, 1456, 1372, 1251, 1067 cm −1 ; HRMS (ESI) calcd for C 25 H 36 NO 10  [M+H] +  510.2334, found, 510.2332. 
       β 3 h-(4-Methoxyphenyl)-isoxazolidine (15) 
       [0157]    
       
                 
         
             
             
         
       
     
         [0158]    Auxiliary Cleavage was performed according to General Procedure (d) from  D -gulose-β 3  h-(4-methoxyphenyl)-isoxazolidine 14 (2.6 g, 5.1 mmol, 1.0 equiv) and HClO 4  (1.3 mL, 15 mmol, 2.9 equiv) in CH 3 CN (50 mL). The crude reaction mixture was purified by flash chromatography (1:1 hexanes/EtOAc) to afford the product as a white solid (0.96 g, 3.6 mmol, 71%).[α]D 25  (c=0.5, CH 2 Cl 2 )=+42.2; mp=92-93° C.;  1 H NMR (400 MHz, CDCl 3 ) δ 7.35 (d, J=8.7 Hz, 2H), 6.89 (d, J=8.7 Hz, 2H), 5.70 (br d, 1H), 4.48-5.57 (m, 1H), 3.87 (s, 3H), 3.80 (s, 3H), 3.42 (s, 3H), 2.91 (dd, J=9.3, 13.5 Hz, 1H), 2.52 (dd, J=7.4, 13.5 Hz, 1H);  13 C NMR (100 MHz, CDCl 3 ) δ 168.0, 159.7, 129.0, 128.0, 114.2, 108.3, 63.7, 55.2, 52.8, 51.7, 48.4; IR (thin film) ν 2951, 2838, 1749, 1516, 1437, 1059, 811 cm −1 ; HRMS (ESI) calcd for C 13 H 18 NO 5  [M+H] +  268.1179. found, 268.1183. 
       (3R,5R)—N-β-(Diethylamino)propyl)-5-methoxy-3-(4-methoxyphenyl)isoxazolidine-5-carboxamide (T 6 ) 
       [0159]    
       
                 
         
             
             
         
       
     
         [0160]    Terminator T 6  was prepared according to General Procedure (f) from N,N-diethyl-1,3-diaminopropane (0.60 mL, 3.7 mmol, 4.7 equiv) and β 3 h-(4-methoxyphenyl)-isoxazolidine 15 (0.17 g, 0.78 mmol, 1.0 equiv) in DMF (0.60 mL). The product was isolated as a colorless liquid (0.21 g, 0.58 mmol, 74%). [α] D   25  (c=0.5, CH 2 Cl 2 )=+26.1;  1 H NMR (400 MHz, CDCl 3 ) δ 8.11 (br t, 1H), 7.32 (d, J=8.5 Hz, 2H), 6.85 (d, J=8.5 Hz, 2H), 5.71 (br d, 1H), 4.43-4.34 (m, 1H), 3.76 (s, 3H), 3.48-3.30 (m, 2H), 3.30 (s, 3H), 2.78 (dd, J=8.4, 13.6 Hz, 1H), 2.60-2.43 (m, 7H), 1.70-1.62 (m, 2H), 1.03 (t, J=7.1 Hz, 6H);  13 C NMR (100 MHz, CDCl 3 ) δ 167.0, 159.6, 128.9, 128.0, 114.2, 109.3, 63.6, 55.2, 52.0, 51.2, 48.0, 46.7, 39.3, 25.6, 11.4; IR (thin film) ν 2967, 2936, 2811, 1679, 1515, 1251, 1034, 830 cm −1 ; HRMS (ESI) calcd for C 19 H 32 N 3 O 4  [M+H] +  366.2387. found, 366.2397. 
       (3S,5R)-3-Benzyl-5-methoxy-N-(2-(pyrrolidin-1-yl)ethyl)isoxazolidine-5-carboxamide (T 7 ) 
       [0161]    
       
                 
         
             
             
         
       
     
         [0162]    Terminator T 7  was prepared according to General Procedure (f) from 2-cyclopentylethanamine (0.47 mL, 3.7 mmol, 10 equiv) and (3S,5R)-methyl 3-benzyl-5-methoxyisoxazolidine-5-carboxylate (87 mg, 0.35 mmol, 1.0 equiv). The crude reaction mixture was purified by flash chromatography (9:1 CH 2 Cl 2 /CH 3 OH) and the product was isolated as a clear liquid (87 mg, 0.26 mmol, 73%). [α] D   25  (c=1.0, CH 2 Cl 2 )=+58.4;  1 H NMR (400 MHz, CDCl 3 ) δ 7.35-7.13 (m, 5H), 7.06 (br t, 1H), 5.75 (br d, 1H), 3.76-3.57 (m, 1H), 3.54-3.31 (m, 2H), 3.28 (s, 3H), 2.98 (dd, J=6.0, 13.7 Hz, 1H), 2.76 (dd, J=7.9, 13.6 Hz, 1H), 2.66-2.45 (m, 7H), 2.16 (dd, J=7.9, 13.6 Hz, 1H), 1.87-1.61 (m, 4H);  13 C NMR (100 MHz, CDCl 3 ) δ 167.5, 137.4, 128.7, 128.7, 126.7, 109.2, 61.8, 54.5, 53.9, 51.5, 47.2, 38.0, 37.4, 23.5; IR (thin film) ν 3370, 2937, 2800, 1677, 1527, 1455, 1150, 1085, 701 cm −1 ; HRMS (ESI) calcd for C 18 H 28 N 3 O 3  [M+H] +  334.2125. found, 334.2132. 
       (3S,5R)-3-Isobutyl-5-methoxy-N-(pyridin-4-ylmethyl)isoxazolidine-5-carboxamide (T) 
       [0163]    
       
                 
         
             
             
         
       
     
         [0164]    Terminator T 8  was prepared according to General Procedure (f) from 4-(aminomethyl)pyridine (0.40 mL, 4.0 mmol, 5.1 equiv) and (3S,5R)-methyl 3-isobutyl-5-methoxyisoxazolidine-5-carboxylate (0.17 g, 0.78 mmol, 1.0 equiv). The crude reaction mixture was purified by flash chromatography (9:1 CH 2 Cl 2 /CH 3 OH) and the product was isolated as a white solid (0.18 g, 0.61 mmol, 78%). [α] D   25  (c=1.0, CH 2 Cl 2 )=+68.6; mp=82-84° C.;  1 H NMR (400 MHz, CDCl 3 ) δ 8.53 (d, J=4.9 Hz, 2H), 7.13-7.20 (m, 3H), 5.59 (br d, 1H), 4.57-4.37 (m, 2H), 3.47-3.37 (m, 1H), 3.26 (s, 3H), 2.61 (dd, J=8.2, 13.6 Hz, 1H), 2.02 (dd, J=8.3, 13.6 Hz, 1H), 1.69-1.57 (m, 1H), 1.50-1.32 (m, 2H), 0.93-0.86 (m, 6H);  13 C NMR (100 MHz, CDCl 3 ) δ 168.0, 150.1, 146.6, 122.1, 109.1, 59.4, 51.5, 48.0, 42.0, 40.5, 26.5, 22.6, 22.6; IR (thin film) ν 3204, 2955, 1679, 1523, 1220, 1030 cm 1 ; HRMS (ESI) calcd for C 15 H 24 N 3 O 3  [M+H] +  294.1812. found, 294.1812. 
       (3S,5R)-3-Isobutyl-5-methoxy-N-(4-sulfamoylphenethyl)isoxazolidine-5-carboxamide (T 9 ) 
       [0165]    
       
                 
         
             
             
         
       
     
         [0166]    Terminator T 9  was prepared according to General Procedure (f) from 4-(2-aminoethyl)benzenesulfonamide (0.79 g, 4.0 mmol, 5.1 equiv) and (3S,5R)-methyl 3-isobutyl-5-methoxyisoxazolidine-5-carboxylate (0.17 g, 0.78 mmol, 1.0 equiv) and in DMF (2.0 mL). The crude reaction mixture was purified by flash chromatography (19:1 CH 2 Cl 2 /CH 3 OH) and the product was isolated as a white solid (0.26 g, 0.68 mmol, 87%).[α] D   25  (c=0.4, CH 2 Cl 2 )=+55.0; mp=68-70° C.;  1 H NMR (400 MHz, CDCl 3 ) δ 7.85 (d, J=8.4 Hz, 2H), 7.34 (d, J=8.4 Hz, 2H), 6.78 (br t, 1H), 5.58 (br d, 1H), 4.98 (s, 2H), 3.76-3.61 (m, 1H), 3.61-3.50 (m, 1H), 3.41-3.28 (m, 1H), 3.17 (s, 3H), 2.93 (t, J=7.0 Hz, 2H), 2.51 (dd, J=8.1, 13.6 Hz, 1H), 1.97 (dd, J=8.5, 13.6 Hz, 1H), 1.70-1.55 (m, 1H), 1.50-1.31 (m, 2H), 0.94-0.88 (m, 6H);  13 C NMR (100 MHz, CDCl 3 ) δ 167.8, 143.8, 140.6, 129.4, 126.6, 109.1, 59.2, 51.5, 48.3, 40.4, 39.8, 35.3, 26.5, 22.7, 22.6; IR (thin film) ν 2938, 2866, 1803, 1451, 1372, 1230, 1089, 849 cm −1 ; HRMS (ESI) calcd for C 17 H 27 N 3 NaO 5 S [M+Na] + 408.1564. found, 408.1550. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 LIST OF REFERENCE SIGNS 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 I 
                 initiator 
                   t Bu 
                 tert. Butyl 
               
               
                   
                 T 
                 teiminator 
                 Boc 
                 tert-Butyloxycarbonyl 
               
               
                   
                 M 
                 monomer 
                 Bz 
                 benzyl 
               
               
                   
                 Me 
                 methyl 
                 Et 
                 ethyl 
               
               
                   
                 Ph 
                 phenyl 
                   i Pr 
                 iso-propyl