The reaction of 1-cyclopentenylpyrrolidino (I) with methyl 2-acetylamino-3-chloropropionate (II) in DMF gives methyl 2-acetylamino-3-(2-oxocyclopentyl)propionate (III), which is cyclized in refluxing concentrated HCl and reduced with H2 over Pd/C in acetic acid yielding cis,endo-2-azabicyclo[3.3.0]octan-3-carboxylic acid (IV). Esterification of (IV) with SOCl2 and benzyl alcohol affords the corresponding benzyl ester (V), which is condensed with N-(1-ethoxycarbonyl-3-phenylpropyl)alanine (VI) by means of dicyclohexylcarbodiirnide (DCC) in DMF affording benzyl N-(1-ethoxycarbonyl-3-phenylpropyl)alanyl-cis,endo-2-azabicyclo[3.3.0]octan-3-carboxylate (VII). Finally, this compound is debenzylated by hydrogenation with H2 over Pd/C in ethanol.

The reaction of 1-cyclopentenylpyrrolidino (I) with methyl 2-acetylamino-3-chloropropionate (II) in DMF gives methyl 2-acetylamino-3-(2-oxocyclopentyl)propionate (III), which is cyclized in refluxing concentrated HCl and reduced with H2 over Pd/C in acetic acid yielding cis,endo-2-azabicyclo[3.3.0]octan-3-carboxylic acid (IV). Esterification of (IV) with SOCl2 and benzyl alcohol affords the corresponding benzyl ester (V), which is condensed with N-(1-ethoxycarbonyl-3-phenylpropyl)alanine (VI) by means of dicyclohexylcarbodiirnide (DCC) in DMF affording benzyl N-(1-ethoxycarbonyl-3-phenylpropyl)alanyl-cis,endo-2-azabicyclo[3.3.0]octan-3-carboxylate (VII). Finally, this compound is debenzylated by hydrogenation with H2 over Pd/C in ethanol.