Abstract:
Novel Compounds are disclosed as potent inhibitors of angiotensin converting enzyme and as orally effective antihypertensive agents. The compounds have the general formula: ##STR1##

Description:
BACKGROUND OF THE INVENTION 
     Angiotensin converting enzyme (peptidyldipeptide hydrolase, hereinafter referred to as ACE) occupies a central role in the physiology of hypertension. The enzyme is capable cf converting the decapeptide angiotensin I, having the sequence 
     AspArgValTyrIleHisProPheHisLeu 
     to an octapeptide, angiotensin II by removal of the carboxyterminal HisLeu. The symbols for various chemical entities are explained in the following table: 
     Ala=L-alanine 
     Arg=L-arginine 
     Asp=L-aspartic acid 
     &lt;Glu=pyro-L-glutamic acid 
     Gly=glycine 
     Hip=Hippuric acid (Benzoyl glycine) 
     His=L-histidine 
     Ile=L-isoleucine 
     Leu=L-leucine 
     Phe=L-phenylalanine 
     Pro=L-proline 
     ΔPro=L-3,4-dehydroproline 
     Ser=L-serine 
     Trp=L-tryptophan 
     Tyr=L-tyrosine 
     Val=L-valine 
     ACE=Aniotensin converting enzyme 
     Hepes=N-2-hydroxyethylpiperazine-N&#39;-2-ethanesulfonic acid 
     Angiotensin I is formed by the action of the enzyme renin, an endopeptidase found in kidney, other tissues and plasma, acting on a serum α-2 globulin. 
     Blood pressure is affected by certain peptides found in the blood. One of these, angiotensin II, is a powerful pressor (blood pressure elevating) agent. Another, brady-kinin, a nonapeptide with the sequence ArgProProGlyPheSerProPheArg is a powerful depressor (blood pressure lowering) agent. In addition to a direct pressor effect, angiotensin II stimulates release of aldosterone which tends to elevate blood pressure by causing retention of extracellular salt and fluids. Angiotensin II is found in measurable amount in the blood of normal humans. However, it is found at elevated concentrations in the blood of patients with renal hypertension. 
     The level of ACE activity is ordinarily in excess, in both normal and hypertensive humans, of the amount needed to maintain observed levels of angiotensin II. However, it has been found that significant blood pressure lowering is achieved in hypertensive patients by treatment with ACE inhibitors. [Gavras, I., et al., New Engl. J. Med. 291, 817 (1974)]. 
     ACE is a peptidyldipeptide hydrolase. It catalyzes the hydrolysis of the penultimate peptide bond at the C-terminal end of a variety of acylated tripeptides and larger polypeptides having an unblocked α-carboxyl group. The action of ACE results in hydrolytic cleavage of the penultimate peptide bond from the carboxyl-terminal end yielding as reaction products a dipeptide and a renant. 
     The reactivity of the enzyme varies markedly depending on the substrate. At least one type of peptide bond, having the nitrogen supplied by proline, is not hydrolyzed at all. The apparent Michaelis constant (Km) varies from substrate to substrate over several orders of magnitude. For general discussion of the kinetic parameters of enzyme catalyzed reactions, see Lehninger, A., Biochemistry, Worth Publishers, Inc., New York, 1970, pp. 153-157. Many peptides which are called inhibitors of the enzymatic conversion of angiotensin I to angiotensin II are in fact substrates having a lower Km than angiotensin I. Such peptides are more properly termed competitive substrates. Examples of competitive substrates include bradykinin, and the peptide BPP 5a  (also called SQ20475) from snake venom, whose sequence is &lt;GluLysTrpAlaPro. 
     Numerous synthetic peptide derivatives have been shown to be ACE inhibitors by Ondetti, et al. in U.S. Pat. No. 3,832,337 issued Aug. 27, 1974. 
     The role of ACE in the pathogenesis of hypertension has prompted a search for inhibitors of the enzyme that could act as antihypertensive drugs. See for example U.S. Pat. Nos. 3,891,616, 3,947,575, 4,052,511 and 4,053,651. A highly effective inhibitor, with high biological activity when orally administered, is D-3-mercapto-2-methylpropanoyl-L-proline, designated SQ14225, disclosed in U.S. Pat. No. 4,046,889 to Ondetti et al., issued Sept. 6, 1977, and in scientific articles by Cushman, D. W. et al., Biochemistry 16, 5484 (1977), and by Ondetti, M. et al., Science 196, 441 (1977). The inhibitor SQ14225 reportedly has an I 50  value of 2.3×10 -8  M. The I 50  value reported by Cushman, et al, supra is the concentration of inhibitor required to produce 50% inhibition of the enzyme under a standard assay system containing substrate at a level substantially above K m . It will be understood that I 50  values are directly comparable when all potential factors affecting the reaction are kept constant. These factors include the source of enzyme, its purity, the substrate used and its concentration, and the composition of the assay buffer. All I 50  data reported herein have been performed with the same assay system and same enzyme (human urinary ACE) and with an approximately 1/2K m  level of substrate and are therefore internally consistent. Discrepancies with data obtained by other workers may be observed. Indeed such discrepancies do exist in the literature, for unknown reasons. See, for example, the I 50  values for BPP 9a  reported by Cushman, D. W., et al., Experientia 29, 1032 (1973) and by Dorer, F. E., et al., Biochim.Biophys.Acta 429, 220 (1976). 
     The mode of action of SQ 14,225 has been based upon a model of the active site of ACE developed by analogy with the better known related enzyme, carboxypeptidase A. The active site was postulated to have a cationic site for binding the carboxyl end group of the substrate and a pocket or cleft capable of binding the side chain of the C-terminal amino acid and providing especially tight binding for the heterocyclic ring of a terminal proline residue. A similar pocket for the penultimate amino acid residue was postulated, and the published data suggested a rather stringent steric requirement, since the D-form of the inhibitor was substantially more potent than its stereoisomer or the 3-methyl and unsubstituted analogs. The sulfhydryl group on the inhibitor, postulated to be bound at the active site near the catalytic center, was believed to play a central role in inactivation of the enzyme by combining with the zinc moiety known to be essential for catalytic activity. Substituents on the sulfhydryl, such as a methyl group, and an S-acetyl derivative, substantially reduced potency of the inhibitor. See Cushman, D. W., et al., Biochemistry, supra. 
     In vitro study of the mechanism by which SQ 14,225 and its analogs act to inhibit ACE has been somewhat hampered by the instability of these molecules under ambient conditions. For example, it has been observed that a fresh aqueous solution of concentration, e.g., 1 mg per ml of SQ 14,225 at a pH of about 8 becomes substantially less active upon standing for as little as 30 minutes, and that activity continues to decrease as the solution stands for longer periods. It is believed that this loss in activity is mainly the result of dimerization of SQ 14,225 occurring at the sulfhydryl end groups, whereby a disulfide is formed which is largely inactive as an inhibitor. Since the free sulfhydryl group is highly reactive and may be readily oxidized to polar acidic moieties such as sulfone and sulfoxide groups, it may also be that the observed in vitro loss of activity of aqueous solutions of SQ 14,225 on standing is in some part a consequence of one or more such oxidation reactions, with formation of a sulfone or sulfoxide which does not function effectively as an inhibitor for ACE. 
     Such reports of SQ 14,225 clinical testing as are currently available, some of which refer to the compound under the name &#34;Captopril&#34;, suggest that the product is sufficiently stable in the normal gastric and intestinal environments of most patients to be an effective inhibitor for ACE when administered orally. It is not yet clear, however, whether there may be a group of patients for which SQ 14,225 is substantially ineffective. Because of the high reactivity of the free sulfhydryl group, SQ 14,225 could readily form mixed disulfides with serum, cellular proteins, peptides or other free sulfhydryl group-containing substances in the gastric or intestinal environments, in addition to the possibility for dimer formation or oxidative degradation reactions. A mixed disulfide with protein may be antigenic and, indeed, occasional allergic reactions have been clinically observed. See Gavras, et al., New England J.Med. 298, 991 (1978). Disulfides and oxidative degradation products of SQ 14,225, if formed, may at best be expected to be largely ineffective as inhibitors. It may be postulated accordingly that dose response to SQ 14,225 may vary with conditions of administration and among individual patients. Moreover, in at least some patients, unwanted side effects may occur and maintenance of an effective concentration of the inhibitor in the body may be difficult to control. 
     Thiolester compounds generally are thought to be highly reactive in that the thiolester linkage is readly hydrolyzable to a sulfhydryl moiety and a carboxylic moiety. Thiolesters are accordingly often used as active ester intermediates for acylation under mild conditions. Such groups as, e.g., acetylthio have been used as blocking groups in the above cited Ondetti, et al. patents. Thiolester intermediates are also postulated to occur in the biosynthesis of cyclic peptides such as tyrocidin or gramicidin S. See Lipmann, F. in Accounts Chem.Res. 6, 361 (1973). 
     Thiolester compounds having potent ACE inhibitory activity and oral effectiveness as anti-hypertensive agents have been disclosed in copending application Ser. No. 941,289, filed Sept. 11, 1978, (now abandoned in favor of its pending continuation Ser. No. 116,950, filed Jan. 30, 1980) and Ser. No. 958,180, filed Nov. 6, 1978 (abandoned in favor of its continuation Ser. No. 116,951, filed Jan. 30, 1980 which was abandoned in favor of its continuation, Ser. No. 295,589, filed Aug. 24, 1981 which was abandoned in favor of its continuation, Ser. No. 524,204, filed Aug. 18, 1983, which was abandoned in favor of its continuation, Ser. No. 680,541, filed Dec. 11, 1984, which was abandoned in favor of its pending continuation Ser. No. 850,055, filed Apr. 10, 1986), both incorporated herein by reference The previously disclosed compounds are: N-[3-(benzoylphenyl-alanylthio-2-D-methylpropanoyl]-L-proline (I 50  ≈1-4×10 -8  M), N-(2-benzoylphenylalanylthiopropanoyl)-L-proline (I 50  ≈4-7× -8  M for racemic compound), N-(3 -benzoylphenyl-analanylthiopropanoyl)-L-proline (I 50  ≈7×10 -7  M), N-[3-benzoylphenylalanylthio)-2-D-methylpropanoyl]-L-3, 4-dehydroproline, N-(2-benzoylphenylalanylthiopropanoyl)-L-3,4-dehydroproline (I 50  =3×10 -9  M for racemic compound), and dehydroproline (I50 N-(3-benzoylphenylalanylthiopropanoyl)-L-3,4-dehydroproline. Unless noted otherwise, all amino acids are in their L-forms. 
     Compounds related to SQ 14,225 have been disclosed by Ondetti, et al., U.S. Pat. Nos. 4,046,889, 4,052,511, 4,053,651, 4,113,715 and 4,154,840. Of interest are disclosed analogs of SQ 14,225 having the five-membered heterocyclic ring of proline replaced by a four or a six-membered ring. The inhibitory potencies of such analogs relative to SQ 14,225 are not disclosed. Substitution of D-proline for L-proline is reported to drastically reduce inhibitory potency of 3-mercaptopropanoyl amino acids (Cushman, D. W., et al., supra). 
     SUMMARY OF THE INVENTION 
     Novel inhibitors of ACE are disclosed which have the general formula ##STR2## wherein 
     R is cyclopentylcarbonyl-L-lysyl or pyro-glutamyl-L-lysyl; 
     A is phenylalanyl, glycyl, alanyl, tryptophyl; tyrosyl, isoleucyl, leucyl, histidyl, or valyl whose α-amino group is in amide linkage with R: 
     R 1  is hydrogen or methyl: 
     R 2  is proline, 3,4-dehydroproline, D,L-3,4-dehydroproline, 3-hydroxyproline, 4-hydroxyproline or L-thiazolidine-4-carboxylic acid whose imino group is in imide linkage with the ##STR3## and n is 0 or 1 such that when n=0, R 1  is methyl. All amino acids are in the L-configuration unless otherwise noted. The disclosed compounds are inhibitors of ACE and are useful as orally effective anti-hypertensive agents. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The discovery of ACE inhibitory potency in the compounds of the present invention provides a unique approach to the design of inhibitory compounds. Although many prior art inhibitors are proline derivatives, substitution of other amino acids for proline has also yielded potent inhibitors. Arginine, phenyl-alanine and alanine are all effective substitutes for proline, so that a trend is not discernible. 
     The substitution of L-3,4-dehydroproline for proline has been studied in several systems. Substitution of L-3,4-ΔPro in the 7 position of bradykinin yields a bradykinin derivative which has significantly reduced physiological activity. See Fisher, G. H. et al., Arch.Biochem.Biophys. 189, 81 (1978). On the other hand, substitution of L-3,4-ΔPro at the 3, 5 or 9 position in ACE inhibitor BPP 9a  enhances its inhibitory activity. In copending application Ser. No. 958,180, applicants found that the compounds having Pro, which are disclosed in said application, have high inhibitory potency and anti-hypertensive effectiveness. However, at present, no rationale can be advanced to explain the diversity of observed results following substitution of ΔPro for proline. Similarly, no clear picture has emerged of the effects of other proline derivatives or analogs substituted at various loci on ACE inhibitors. 
     To date, the effect of the amino acid to the left of the sulfur in the above-shown formula, has not been determined. It is thought that this amino acid functions as an additional recognition site for the enzyme. If this is true, it would be expected that a compound with an amino acid here would be a better inhibitor. It was not known which, if any, amino acids would be effective in this position and which if any would enhance the inhibitory activity of a given compound. Applicants have found that various amino acids are effective and that the hydroxyprolines, proline, L-, and D,L-,3,4-dehydroproline, and thiazolidine -4-carboxylic acid derivatives are all effective anti-hypertensive agents and have high inhibitory potency for ACE. 
     The present invention will be further described by the following examples. In these examples, the thin-layer chromatography (TLC) was performed using silica gel plates. The numerical solvent systems for use in the TLC procedures are as follows. 
     (1) is methanol:chloroform, 1:1 (parts by volume). (2) is benzene:water:acetic acid, 9:1:9 (parts by volume. (3) is acetic acid:water:n-butanol 26:24:150 (parts by volume). (4) is n-butanol:pyridine:acetic acid:water, 15:10:3:12 (parts by volume). (5) is chloroform:methanol:ammonium hydroxide, 60:45:20 (parts by volume). The buffers for paper electrophoresis were: pH 1.9 -formic acid:acetic acid:water, 3:2:25 (parts by volume); pH 5.0 -diethylene glycol:acetic acid:pyridine:water, 100:6:8,5:885 (parts by volume). The tert-butyloxycarbonyl derivatives of the amino acids are commercially available. 
    
    
     EXAMPLE 1 
     ACE activity assay. For most experiments described herein, the enzyme was assayed in 0.05M Hepes buffer, pH 8.0 containing 0.1M NaCl and 0.75M Na 2  SO 4 . The substrate employed was Benzoyl-GlyHisLeu at a final concentration of 1×10 -4  M (K m  ≈2×10 -4  M), together with about 130,000 cpm of [ 3  H]-Benzoyl-GlyHisLeu (25 Ci/mmole). Enzyme was diluted in the above buffer such that 40 μ1 buffered enzyme was capable of hydrolyzing 13% of substrate in a 15-minute incubation at 37° C. To initiate the assay, 40 μl of enzyme and 10 μl of water or inhibitor dissolved in water were preincubated for five minutes at 37° C. Substrate, 50 μl, was then added to initiate reaction and the solution was incubated for 15 minutes at 37° C. To terminate the reaction, 1 ml of 0.1M HCl was added, following which 1 ml of ethyl acetate was added. The mixture was agitated on a rotary mixer and centrifuged briefly to separate the phases. 
     An aliquot, 500 μl, of the ethyl acetate layer was transferred to a liquid scintillation vial containing 10 ml of Riafluor, trademark New England Nuclear Corporation, Boston, Massachusetts. For determination of I 50  values, enzyme activity in the presence of inhibitor at a series of different concentrations was compared to activity in the absence of inhibitor. A plot of inhibitor concentration versus percent inhibition yielded the I 50  value. 
     EXAMPLE 2 
     Synthesis of 3-acetylthiopropanoyl-L-proline-t-butyl ester. 
     3-acetylthiopropanoic acid, 0.865 g, was dissolved in 2 ml redistilled tetrahydrofuran (THF) and cooled to 0° C. A cooled solution of dicyclohexylcarbodiimide, 1.2031 g in 2 ml of THF was added, following which a cooled solution of L-proline-t-butyl ester, 1 g, was added. The reaction mixture was stirred at 0° C. for one hour, then at 4° C. overnight. The reaction mixture was then filtered and the precipitate was washed with ethyl acetate. Solvents of the filtrates were removed under reduced pressure in a rotary evaporator. The residue was dissolved in ethyl acetate which was then washed three times with cold 1N citric acid, twice with saturated NaCl, twice with cold 1N NaHCO 3  and three times with saturated NaCl. The solution was dried over anhydrous MgSO 4  and filtered. The solvent was removed under reduced pressure in a rotary evaporator at 30° C. yielding a clear colorless oily product in approximately 87% yield. The product migrated as a single spot in thin layer chromatography in five solvent systems. 
     EXAMPLE 3 
     Synthesis of 3-mercaptopropanoyl-L-proline. 
     The product from Example 2, 3-acetylthiopropanoyl-L-proline-t-butyl ester, 0.5 g, was mixed with 4.5 ml of 5.5N methanolic ammonia at room temperature under nitrogen for one hour to remove the acetyl group. The solvent was then removed at 25° C. with a rotary evaporator. After the product was taken up in methanol and reevaporated twice more in the rotary evaporator, the clear oily residue was dissolved in ethyl ether, washed twice with 5% potassium bisulphate and once with saturated NaCl, dried over MgSO 4  and filtered. Residual solvent was removed in vacuo to yield a clear oily product, migrating as a single spot on thin layer chromatography in three separate solvent systems. The t-butyl ester protecting group was removed by reaction with trifluoroacetic acid in anisole. 
     EXAMPLES 4-6 
     By substituting 2-acetylthiopropanoic acid, 3-acetylthio-2-D-methylpropanoic acid, or 3-acetylthio-2-D,L-methylpropanoic acid for the 3-acetylthiopropanoic acid in Example 2 and substantially following the procedures of Examples 2 and 3, the following compounds are obtained. By removing the t-butyl ester protecting group with trifluoroacetic acid in anisole as a first step, the dicyclohexylamine salt can be formed to assist in the resolution of isomers. The acetyl protecting group can be removed in a second step using methanolic ammonia, as described in Example 3. 
     
         ______________________________________Example  Compound______________________________________4        2-mercaptopropanoyl-L-proline5        3-mercapto-2-D-methylpropanoyl-L-proline6        3-mercapto-2-D,L-methylpropanoyl-L-proline______________________________________ 
    
     EXAMPLE 7 
     Synthesis of 3-mercapto-2-methyl-propanoyl-L-3,4-dehydroproline. 
     L-3,4-dehydroproline (A Pro), 1 mmole, is dissolved in DMF and the solution is cooled to -15° C. The solution is neutralized by adding 1 equivalent of N-ethyl morpholine. In a separate reaction vessel at -10° C., one equivalent of 3-acetylthio-2-methyl-propanoic acid in an equal volume of DMF is mixed with 1.1 equivalent of 1,1&#39;-carbonyldiimidazole, and the solution is stirred for one hour. The first solution containing Δ 3  Pro is mixed with the second, containing 3-acetylthio-2-methyl propanoic acid while maintaining the temperature at -10° C. The combined solution is stirred for 1 hour at -10° C. The solution is then allowed to warm slowly to room temperature. The solvent is removed on a rotary evaporator under reduced pressure at 40° C. Ethyl acetate (25 ml) is added and the solution is cooled to 0° C. Two ml of 1N citric acid is added, the two phases are mixed and then allowed to separate. The phases are separated with a separating funnel, and the organic phase is washed twice more with 2 ml 1N citric acid, two times with saturated NaCl and finally dried over anhydrous MgSO 4 . The MgSO 4  is removed by filtration, and the solvent is removed with a rotary evaporator. The residue is dissolved and recrystallized from a non-polar solvent such as benzene to yield 3-acetylthio-2-D,L-methylpropanoyl -L-3,4-dehydroproline. When the 2-D-methyl isomer is desired, the residue is dissolved in acetonitrile (approximately 3 ml) and the solution is warmed to 40° C. One equivalent of dicyclohexylamine is added, and the solution is allowed to stand at room temperature overnight. The crystals are collected by filtration and are washed three times with acetonitrile. When further purification is required, the material can be recrystallized from isopropanol. The acetyl protecting group can be removed as in Example 3. 
     EXAMPLES 8-11 
     By substituting D,L-3,4-dehydroproline, L-3-hydroxyproline, L-4-hydroxyproline, or L-thiazolidine-4-carboxylic acid for the L-3,4dehydroproline in Example 7 and substantially following the procedures of Example 7 the following compounds are obtained. 
     
         ______________________________________Ex-ample Compound______________________________________ 8    3-mercapto-2-D-methylpropanoyl-D,L-3,4-dehydroproline 9    3-mercapto-2-D-methylpropanoyl-L-3-hydroxyproline10    3-mercapto-2-D-methylpropanoyl-L-4-hydroxyproline11    3-mercapto-2-D-methylpropanoyl-L-thiazolidine______________________________________ 
    
     EXAMPLE 12 
     Similarly, by substituting 3-acetylthiopropanoic acid or 2-acetylthiopropanoic acid for the 3-acetylthio-2-methyl propanoic acid of Examples 7-11, the L-3,4-dehydroproline, D,L-3,4-dehydroproline, L-3-hydroxyproline, L-4-hydroxyproline and L-thiazolidine-4-carboxyclic acid derivatives are obtained, following substantially the described procedures. 
     Example 13 
     Synthesis of the N-hydroxysuccinimide ester of cyclopentane carboxylic acid. 
     A cool solution of 11.4 gm of dicyclohexylcarbodiimide in dimethylformamide (DMF) was added drop-wise to a mixture of 5.71 gm of cyclopentanecarboxylic acid and 5.76 gm of N-hydroxysuccinimide in DMF at 0° C. The reaction mixture was stirred at 0° C. for 30 minutes and then at 4° C. overnight. Crystalline dicyclohexylurea was removed by filtration and the precipitate was washed with ethyl acetate. Solvents from the combined filtrate were removed under reduced pressure and the residue was crystallized from benzene and hexane yielding 5.77 gm of white crystals having a melting point of 72.5°-73° C. The infrared absorption spectrum in chloroform gave a typical spectrum of N-hydroxysuccinimide esters. Elemental analysis of the crystals yielded the following results. 
     Calculated: C=56.86; H=6.20; N=6.63; Found: C=56.77: H=6.07; N=6.66. 
     EXAMPLE 14 
     Synthesis of N.sup.α -cyclopentylcarbonyl-N.sup.ε -tert-butyl-oxycarbonyl-L-lysine. 
     A solution of 1.2316 gm of oxycarbonyl-L-lysine and 420 mg of NaHCO 3  in 10 ml of water and 5 ml of THF was cooled in an ice bath with stirring. To this solution was added a cold solution of 1.19 gm of the product from Example 13 in 10 ml of THF. The THF was removed with a rotary evaporator at 35° C. after the reaction mixture had been stirred overnight at room temperature. About 20 ml of water was added to the reaction mixture and the pH was adjusted to 9 with solid NaHCO 3 . The aqueous phase was extracted three times with ethyl acetate and the organic phase was discarded. The aqueous solution was cooled in an ice bath and then acidified to pH 2 with 1N HCl in the presence of ethyl acetate. The organic phase was washed twice with ice water and then twice with a solution of saturated NaCl. The organic solution was dried over anhydrous MgSO 4  and then filtered. The solvent was removed with a rotary evaporator and the residue was crystallized from ether and hexane yielding 1.135 gm of white crystals having a melting point of 104.5°-105.5° C. Elemental analysis of the crystals yielded the following. 
     Calculated: C=59.63; H=8.83; N=8.18; Found: C=59.74; H=8.85; N=8.24. 
     The product was shown to be homogeneous using paper electrophoresis at pH 1.9 and pH 5.0 and using TLC with solvent systems 1, 2 and 3. 
     EXAMPLE 15 
     Synthesis of N.sup.α -cyclopentylcarbonyl-N.sup.ε -tert-butyloxycarbonyl-L-lysine-N-hydroxysuccinimide ester. 
     A solution of 1.027 gm of the product from Example 14 and 346 mg of N-hydroxysuccinimide in 10 ml of CH 2  Cl 2  was cooled to -5° C. To this solution was added with stirring a cold solution of 680 mg of dicyclohexylcarbodiimide in 5 ml of CH 2  Cl 2 . The reaction mixture was stirred at 0° C. for 30 minutes and then at 4° C. overnight. Crystalline dicyclohexylurea was removed by filtration and was washed with ethyl acetate. The combined filtrate was washed twice with 1N NaHCO 3 , twice with water and finally with a solution of saturated NaCl. The organic phase was dried over anhydrous MgSO 4 , filtered, and the solvent was removed with a rotary evaporator. The residue was crystallized from isopropanol yielding 0.844 gm of white crystals having a melting point of 140.5° C. The infrared absorption spectrum in chloroform gave a typical spectrum of N-hydroxysuccinimide esters. Elemental analysis of the crystals yielded the following. 
     Calculated: C=57.39; H=7.57: N=9.56; Found: C=57.10; H=7.57; N=9.61. 
     EXAMPLE 16 
     Synthesis of N.sup.α cyclopentylcarbonyl-N.sup.ε -tert-butyl-oxycarbonyl-L-lysyl-L-phenylalanine. 
     A solution of 220 mg of the product from Example 15 in 2 ml of dioxane was added drop-wise to a mixture of 99.1 mg of L-phenylalanine and 51 mg of NaHCO in a mixture of 2 ml of water and 1 ml of DMF. The reaction mixture was stirred at room temperature overnight and dioxane was removed with a rotary evaporator at 35° C. Ethyl acetate (10 ml) was added to the reaction mixture, cooled, acidified to pH 2 with 0.1N HCl and the aqueous solution was discarded. The organic phase was washed with ice water, a solution of saturated NaCl, and dried over anhydrous MgSO 4  ; and the solvent was removed with a rotary evaporator. The residue was crystallized from ether and petroleum ether (b.p. 30°-60° C.) yielding 95 mg of white solid having a melting point of 90°-92° C. The product was shown to be homogeneous using paper electrophoresis at pH 1.9 and pH 5.0 and using TLC with solvent systems 1, 2, 3 and 4. Elemental analysis, 
     Calculated: C=63.78; H=8.03; N=8.58; Found: C=63.40; H=8.07; N=8.34. 
     EXAMPLE 17 
     Synthesis of N.sup.α -13-(N.sup.ε -cyclopentylcarbonyl-N -tert-butyloxy-carbonyl-L-lysyl-L-phnylalanylthio)-2-D-methylpropanoyl]-L-proline. 
     A solution of 73.5 mg of the product from Example 16 in 0.5 ml redistilled dimethylformamide (DMF) was cooled in an ice-dry ice-acetone bath at -20° C. To this solution was added a cold solution of 26 mg of 1,1&#39;-carbonyldiimidazole in 1.0 ml of DMF. The solution was stirred at -10° C. for two hours and then mixed with a cold solution of 36 mg of 3-mcrcapto-2-D-methylpropanoyl-L-proline in 1 ml of DMF which was neutralized with N-ethyl morpholine. The reaction mixture was stirred at -10° C. for an additional hour and then slowly warmed to room temperature. The solvent was removed under reduced pressure at 40° C. and ethyl acetate was added to the residue. The mixture was cooled in an ice water bath and washed with 1N citric acid and then three times with saturated NaCl solution. The solvent was removed with a rotary evaporator after drying over anhydrous MgSO 4 . The product was purified by LH-20 column chromatography using a 1.2 cm by 95 cm column and eluted with THF:isopropanol, 3:7 (parts by volume). The peak fractions were pooled and the solvent was removed under reduced pressure yielding the named product which may be abbreviated as N.sup.α -[3-(N.sup.α -cpc-N.sup.ε-Boc-L-Lys-L-Phe-thio)- 2-D-methylpropanol]-L-proline. 
     EXAMPLE 18 
     Synthesis of N.sup.α -[3-(N.sup.α -cyclopentylcarbonyl-L-lysyl-L-phenylalanylthio)2-D-methylpropanoyl]-L-proline. 
     The N.sup.ε -Boc group was removed from the lysine by stirring a mixture of 30 mg of the product from Example 17 with 50 μl anisole and 200 μl of anhydrous trifluoroacetic acid (TFA) at room temperature for one hour. Anisole and TFA were removed under reduced pressure at 35° C. and the residue was triturated with anhydrous ether. The residue was purified by liquid chromatography on Sephadex G-10 1  using a 1.2 cm by 95 cm column and eluted with 5% acetic acid. The peak fractions were pooled and freeze-dried yielding the named Product which can be abbreviated as N.sup.α -[3-(N.sup.α -cpc-L-Lys-L-Phe-thio)2-D-methylpropanoyl-L.proline. 
    
     EXAMPLES 19-26 
     By substituting glycine, L-alanine, L-tryptophan, L-tyrosine, L-isoleucine, L-leucine, -L-histidine, or L-valine for the L-phenylalanine in Example 16 and substantially following the procedures of Examples 17 and 18, the following compounds are obtained. 
     
         ______________________________________Ex-ample Compound______________________________________19     N.sup.α --[3-(N.sup.α --cyclopentylcarbonyl-L-lysyl-gly  cyl-  thio)-2-D-methylpropanoyl]-L-proline20     N.sup.α --[3-(N.sup.α --cyclopentylcarbonyl-L-lysyl-L-t  ryptophyl-  thio)-2-D-methylpropanoyl]-L-proline21     N.sup.α --[3-(N.sup.α --cyclopentylcarbonyl-L-lysyl-L-t  yrosyl-  thio)-2-D-methylpropanoyl]-L-proline22     N.sup.α --[3-(N.sup.α --cyclopentylcarbonyl-L-lysyl-L-i  soleucyl-  thio)-2-D-methylpropanoyl]-L-proline23     N.sup.α --[3-(N.sup.α --cyclopentylcarbonyl-L-lysyl-L-l  eucyl-  thio)-2-D-methylpropanoyl]-L-proline24     N.sup.α --[3-(N.sup.α --cyclopentylcarbonyl-L-lysyl-L-h  istidyl-  thio)-2-D-methylpropanoyl]-L-proline25     N.sup.α --[3-(N.sup.α --cyclopentylcarbonyl-L-lysyl-L-v  alyl-  thio)-2-D-methylpropanoyl]-L-proline26     N.sup.α --[3-(N.sup.α --cyclopentylcarbonyl-L-lysyl-L-a  lanylthio)-  2-D-methyl-propanoyl]-L-proline______________________________________ 
    
     Example 27 
     Similarly, the L-3,4-dehydroproline, D,L-3,4-dehydroproline, L-3-hydroxyproline, L-4-hydroxyproline and L-thiazolidine-4-carboxylic acid derivatives are obtained by substituting the products of Examples 3-12 for the 3-mercapto-2-D-methylpropanoyl-L-proline in Examples 17 and 19-26 and substantially following the procedures of Examples 17 and 18. 
     EXAMPLE 28 
     By substituting pyro-glutamic acid for the cyclopentanecarboxylic acid in Example 13 and substantially following the procedures of Examples 13-15, N.sup.α -pyro-glutamyl-N.sup.ε -tertbutyloxycarbonyl-L-lysine-N-hydroxysuccinimide ester is obtained. By substituting this product for the N.sup.α -cpc-N.sup.ε -Boc-L-Lys-N-hydroxysuccinimide ester in Example 16 and substantially following the procedures of Examples 16-27, the pyro-glutamayl derivatives are obtained. 
     Example 29 
     The inhibitory potency of N.sup.α -[3-(N.sup.ε -cpc-N.sup.ε -Boc-L-Lys-L-Phe-thio)-2-D-methylpropanoyl]-L-proline (A) and N.sup.α -13-(N.sup.ε cpc-L-Lys-L-Phe-thio)-2-D-methylpropanoyl]-L-proline (B) in vitro were measured in the assay described in Example 1. The enzyme preparation was ACE purified from human urine as described by Ryan, J. W., et al., Tissue and Cell 10, 555 (1978). The I 50  for (A) was found to be 2.5×10 -8  M and for (B), 1.5×10 -8  M. 
     EXAMPLE 30 
     Oral effectiveness of N.sup.α -[3-(N.sup.α -cyclopentylcarbonyl-L-lysyl-L-phenylalanylthio)-2-D-methylpropanoyl]-L-proline. 
     Rats (210-290 g body weight) were fasted overnight and then anesthetized with intraperitoneal pentobarbital, 50-60 mg/kg. Tracheostomy was performed and the animals were ventilated mechanically. A cannula was inserted into a femoral vein for injection of angiotensin I, and a second cannula was inserted into a common carotid artery for direct measurement of arterial blood pressure. Heparin, 1,000 units, was injected via the femoral vein to prevent coagulation. Blood pressure was measured with a pressure transducer connected to a polygraph. The rats were injected with 400 ng/kg of angiotensin I in 20 μl of 0.9 g % NaCl; an amount of angiotensin I sufficient to raise mean arterial blood pressure by 39 mm Hg. After the responsiveness of a given rat to angiotensin I was established, the named compound at 17 μmol/kg (drug dissolved in 0.15 ml of H 2  O plus 10 μl of 1N NaHCO 3 ), was given via a stomach tube. At timed intervals, the effects of 400 ng/kg of angiotensin I on mean arterial blood pressure were tested. Results are shown below: 
     
         ______________________________________Time After Oral         Blood Pressure Response toAdministration         400 ng/kg of Angiotensin I(minutes)     (% of Control)______________________________________-5            100         (39 mm Hg)+5            5910            4415            4431            4437            3845            4455            4481            3693            36104           38______________________________________ 
    
     EXAMPLE 31 
     Intravenous Effectiveness of N.sup.α -[3-(N.sup.α -cyclopentylcarbonyl-L-lysyl-L-phenylalanylthio)-2-D-methylpropanoyl]-L-proline. 
     Anesthetized rats were prepared as described in Example 30. After the responsiveness of a given rat to angiotensin I was established, the named compound at 2 μmol/kg, in a volume of 15 μl of 0.01N sodium bicarbonate, was injected via a femoral vein. At time intervals, the effects of angiotensin I, 400 ng/kg, on mean arterial blood pressure were tested. Results are shown below: 
     
         ______________________________________Time After Oral         Blood Pressure Response toAdministration         400 ng/kg of Angiotensin I(minutes)     (% of Control)______________________________________-5            100         (39 mm Hg)+3            8 9            1514            2319            2324            3337            4145            4654            4677            4687            51______________________________________ 
    
     While the invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modifications and this application is intended to cover any variations, uses or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth, and as follows in the sccpe of the appended claims.