The cyclization of 1-benzyloxycarbonyl-4-oxoproline methyl ester (I) with ethanedithiol (II) by means of p-toluenesulfonic acid in refluxing acetic acid gives methyl 7-benzyloxycarbonyl-1,4-dithia-7-azaspiro[4.4]nonane-8-carboxylate (III), which is deprotected with 20% HBr to give methyl 1,4-dithia-7-azaspiro[4.4]nonane-8-carboxylate (IV). The condensation of (IV) with N-benzyloxycarbonyl alanine N-hydroxysuccinimide ester (V) affords the corresponding dipeptide methyl ester (VI), which is hydrolyzed with NaOH to the free acid (VII), and deprotected with 20% HBr as before to 7-alanyl-1,4-dithia-7-azaspiro[4.4]nonane-8-carboxylic acid (VIII). Finally, this compound is condensed with ethyl 2-oxo-4-phenylbutyrate (IX) by means of sodium cyanoborohydride in methanol.

The condensation of (IX) with alanine benzyl ester (X) with sodium cyanoboohydride as before gives N-[1-(ethoxycarbonyl)-3-phenylpropyl]alanine benzyl ester (XI), which is submitted to fractional crystallization with maleic acid yielding N-[1(S)-(ethoxycarbonyl)-3-phenylpropyl]-(S)-alanine benzyl ester (XII), which is debenzylated by hydrogenation with H2 over Pd/C in ethanol yielding the free acid (XIII). Finally, this compound is condensed with spirane (IV) through the N-hydroxysuccinimide ester, in the usual way.

The cyclization of 1-benzyloxycarbonyl-4-oxoproline methyl ester (I) with ethanedithiol (II) by means of p-toluenesulfonic acid in refluxing acetic acid gives methyl 7-benzyloxycarbonyl-1,4-dithia-7-azaspiro[4.4]nonane-8-carboxylate (III), which is deprotected with 20% HBr to give methyl 1,4-dithia-7-azaspiro[4.4]nonane-8-carboxylate (IV). The condensation of (IV) with N-benzyloxycarbonyl alanine N-hydroxysuccinimide ester (V) affords the corresponding dipeptide methyl ester (VI), which is hydrolyzed with NaOH to the free acid (VII), and deprotected with 20% HBr as before to 7-alanyl-1,4-dithia-7-azaspiro[4.4]nonane-8-carboxylic acid (VIII). Finally, this compound is condensed with ethyl 2-oxo-4-phenylbutyrate (IX) by means of sodium cyanoborohydride in methanol.

The condensation of (IX) with alanine benzyl ester (X) with sodium cyanoboohydride as before gives N-[1-(ethoxycarbonyl)-3-phenylpropyl]alanine benzyl ester (XI), which is submitted to fractional crystallization with maleic acid yielding N-[1(S)-(ethoxycarbonyl)-3-phenylpropyl]-(S)-alanine benzyl ester (XII), which is debenzylated by hydrogenation with H2 over Pd/C in ethanol yielding the free acid (XIII). Finally, this compound is condensed with spirane (IV) through the N-hydroxysuccinimide ester, in the usual way.

A new synthesis of spirapril has been described: The cyclization of N-benzyloxycarbonyl-4-oxo-(S)-proline (I) with ethanedithiol by means of boron trifluoride ethearate in glacial acetic acid gives 7-(benzyloxycarbonyl)-1,4-dithia-7-azaspiro[4.4]nonane-8(S)-carboxylic acid (II), which is deprotected with HBr in acetic acid to yield 1,4-dithia-7-azaspiro[4.4]nonane-8(S)-carboxylic acid (III). This compound is then condensed with N-[1(S)-(ethoxycarbonyl)-3-phenylpropyl]-(S)-alanine (IV) by means of N-hydroxysuccinimide (NOS) and 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide (DMCC) in DMF, or by means of N,N-disuccinimidyl carbonate and pyridine.

The protected hydroxyproline tosylate (I) was condensed with the sodium derivative of thiophenol (II) to produce thioether (III). After saponification of the methyl ester function of (III), the N-benzyloxycarbonyl group of (IV) was removed by means of HBr in refluxing HOAc, yielding cis-4-(phenylthio)-L-proline (V). Acylation of (V) with (S)-3-(benzoylthio)-2-methylpropionyl chloride (VI) afforded zofenopril (VII), which was finally converted to the corresponding calcium salt by treatment with calcium oxide in EtOH/H2O. The intermediate (S)-3-(benzoylthio)-2-methylpropionyl chloride (VI) was obtained by enzymatic resolution of racemic 3-(benzoylthio)-2-methylpropionic acid (VIII) with Pseudomonas fluorescens (Amano lipase P-30) to yield (S)-3-(benzoylthio)-2-methylpropionic acid (IX), which was activated with oxalyl chloride to afford the chiral acyl chloride (VI).

The Grignard condensation of N-(benzyloxycarbonyl)-4-oxo-L-proline (XXIX) with phenylmagnesium bromide gives the 4-hydroxy-4-phenylproline derivative (XXX), which is dehydrated with TFA yielding the dehydroproline (XXXI). The hydrogenation of (XXXI) with Li in liquid ammonia affords the previously reported trans-4-phenyl-L-proline (XIX), which is hydrogenated to the corresponding cyclohexyl derivative (XI) with H2 over PtO2.

A new synthesis of spirapril has been reported: The cyclization of N-benzyloxycarbonyl-4-oxo-(S)-proline (I) with ethandithiol by means of boron trifluoride ethearate in dichloromethane gives 7-(benzyloxycarbonyl)-1,4-dithia-7-azaspiro[4.4]nonane-8(S)-carboxylic acid (II), which is treated with concentrated HCl to yield 1,4-dithia-7-azaspiro[4.4]nonane-8(S)-carboxylic acid (III). The protection of (III) with tert-butoxycarbonyl chloride and with trimethylsilylethanol in the usual manner affords N-(tert-butoxycarbonyl)-1,4-dithia-7-azaspiro[4.4]nonane-8(S)-carboxyli c acid 2-(trimethylsilyl)ethyl ester (IV), which is condensed with N-[1(S)-(ethoxycarbonyl)-3-phenylpropyl]-(S)-alanine (V) by a first treatment with p-toluenesulfonic acid and a condensation step with dicyclohexylcarbodiimide (DCC) and hydroxybenzotriazole (HBT), giving spirapril 2-(trimethylsilyl)ethyl ester (VI). Finally, this ester is hydrolyzed with tetrabutylammonium fluoride trihydrate in DMF.