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 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 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 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.