1) The reaction of tert-butoxycarbonyl-L-prolinal (I) with benzyl propionate (II) by means of lithium diisopropylamide (LDA) in THF gives a mixture of isomers that is separated by flash chromatography, yielding the (2R,3R)-isomer (III). The methylation of (III) with diazomethane and boron trifluoride ethearate or NaH and methyl iodide affords the methoxy derivative (IV), which is deprotected with HCl in dioxane, giving (V). The condensation of (V) with tripeptide (VI) by means of diethyl phosphorocyanidate (DEPC) in DMF yields the tetrapeptide benzyl ester (VII), which is finally debenzylated by hydrogenolysis over Pd/C in tert-butanol and amidated with 2-phenylethylamine (VIII) and DEPC and triethylamine in DMF.

Intermediate (VI) has been obtained as follows: The condensation of benzyloxycarbonyl-L-isoleucine (IX) with malonic acid monomethyl ester potassium salt (X) by means of carbonyldiimidazole (CDI) and MgCl2 in THF gives the ketoester (XI), which is reduced to the hydroxyester (XII) with NaBH4 in methanol. The methylation of (XII) with methyl iodide and silver oxide in DMF affords the N-methyl methoxyester (XIII). The hydrolysis of (XIII) with NaOH in dioxane/water followed by reesterification with isobutylene gives the tert-butyl ester (XIV), which is deprotected by hydrogenation over Pd/C, yielding the amino acid (XV). The condensation of (XV) with benzyloxycarbonyl-L-valine (XVI) by means of DCC in dichloromethane gives the protected dipeptide (XVII), which is debenzylated as usual, yielding (XVIII). The condensation of (XVIII) with N,N-dimethyl-L-valine (XIX) affords the tripeptide ter-butyl ester (XX). Finally, (XX) is hydrolyzed by treatment with trifluoroacetic acid to the free acid intermediate (VI).

2) The reaction of 3(R)-[1-(tert-butoxycarbonyl)-2(S)-pyrrolidinyl]-3-methoxy-2(R)-methylpropionic acid (XXI) with 2-phenylethylamine (VIII) by means of DEPC in dichloromethane gives the amide (XXII), which is deprotected with trifluoroacetic acid to yield the amide (XXIII). Finally, this compound is condensed with the previously described intermediate (VI) by means of DEPC in dichloromethane.

1) The reaction of tert-butoxycarbonyl-L-prolinal (I) with benzyl propionate (II) by means of lithium diisopropylamide (LDA) in THF gives a mixture of isomers that is separated by flash chromatography, yielding the (2R,3R)-isomer (III). The methylation of (III) with diazomethane and boron trifluoride ethearate or NaH and methyl iodide affords the methoxy derivative (IV), which is deprotected with HCl in dioxane, giving (V). The condensation of (V) with tripeptide (VI) by means of diethyl phosphorocyanidate (DEPC) in DMF yields the tetrapeptide benzyl ester (VII), which is finally debenzylated by hydrogenolysis over Pd/C in tert-butanol and amidated with 2-phenylethylamine (VIII) and DEPC and triethylamine in DMF.

Intermediate (VI) has been obtained as follows: The condensation of benzyloxycarbonyl-L-isoleucine (IX) with malonic acid monomethyl ester potassium salt (X) by means of carbonyldiimidazole (CDI) and MgCl2 in THF gives the ketoester (XI), which is reduced to the hydroxyester (XII) with NaBH4 in methanol. The methylation of (XII) with methyl iodide and silver oxide in DMF affords the N-methyl methoxyester (XIII). The hydrolysis of (XIII) with NaOH in dioxane/water followed by reesterification with isobutylene gives the tert-butyl ester (XIV), which is deprotected by hydrogenation over Pd/C, yielding the amino acid (XV). The condensation of (XV) with benzyloxycarbonyl-L-valine (XVI) by means of DCC in dichloromethane gives the protected dipeptide (XVII), which is debenzylated as usual, yielding (XVIII). The condensation of (XVIII) with N,N-dimethyl-L-valine (XIX) affords the tripeptide ter-butyl ester (XX). Finally, (XX) is hydrolyzed by treatment with trifluoroacetic acid to the free acid intermediate (VI).

2) The reaction of 3(R)-[1-(tert-butoxycarbonyl)-2(S)-pyrrolidinyl]-3-methoxy-2(R)-methylpropionic acid (XXI) with 2-phenylethylamine (VIII) by means of DEPC in dichloromethane gives the amide (XXII), which is deprotected with trifluoroacetic acid to yield the amide (XXIII). Finally, this compound is condensed with the previously described intermediate (VI) by means of DEPC in dichloromethane.

1) The reaction of tert-butoxycarbonyl-L-prolinal (I) with benzyl propionate (II) by means of lithium diisopropylamide (LDA) in THF gives a mixture of isomers that is separated by flash chromatography, yielding the (2R,3R)-isomer (III). The methylation of (III) with diazomethane and boron trifluoride ethearate or NaH and methyl iodide affords the methoxy derivative (IV), which is deprotected with HCl in dioxane, giving (V). The condensation of (V) with tripeptide (VI) by means of diethyl phosphorocyanidate (DEPC) in DMF yields the tetrapeptide benzyl ester (VII), which is finally debenzylated by hydrogenolysis over Pd/C in tert-butanol and amidated with 2-phenylethylamine (VIII) and DEPC and triethylamine in DMF.

Intermediate (VI) has been obtained as follows: The condensation of benzyloxycarbonyl-L-isoleucine (IX) with malonic acid monomethyl ester potassium salt (X) by means of carbonyldiimidazole (CDI) and MgCl2 in THF gives the ketoester (XI), which is reduced to the hydroxyester (XII) with NaBH4 in methanol. The methylation of (XII) with methyl iodide and silver oxide in DMF affords the N-methyl methoxyester (XIII). The hydrolysis of (XIII) with NaOH in dioxane/water followed by reesterification with isobutylene gives the tert-butyl ester (XIV), which is deprotected by hydrogenation over Pd/C, yielding the amino acid (XV). The condensation of (XV) with benzyloxycarbonyl-L-valine (XVI) by means of DCC in dichloromethane gives the protected dipeptide (XVII), which is debenzylated as usual, yielding (XVIII). The condensation of (XVIII) with N,N-dimethyl-L-valine (XIX) affords the tripeptide ter-butyl ester (XX). Finally, (XX) is hydrolyzed by treatment with trifluoroacetic acid to the free acid intermediate (VI).

2) The reaction of 3(R)-[1-(tert-butoxycarbonyl)-2(S)-pyrrolidinyl]-3-methoxy-2(R)-methylpropionic acid (XXI) with 2-phenylethylamine (VIII) by means of DEPC in dichloromethane gives the amide (XXII), which is deprotected with trifluoroacetic acid to yield the amide (XXIII). Finally, this compound is condensed with the previously described intermediate (VI) by means of DEPC in dichloromethane.