N1CCC[C@H]1C(O)=O.C(SC[C@@H](C)C(Cl)=O)(=O)C.[OH-].[K+].P([O-])([O-])([O-])=O.[K+].[K+].[K+].C([S:32][CH2:33][C@@H:34]([CH3:45])[C:35]([N:37]1[CH2:44][CH2:43][CH2:42][C@H:38]1[C:39]([OH:41])=[O:40])=[O:36])(=O)C.[OH-].[Na+]>>[CH3:45][C@@H:34]([C:35]([N:37]1[C@H:38]([C:39]([OH:41])=[O:40])[CH2:42][CH2:43][CH2:44]1)=[O:36])[CH2:33][SH:32]It has been found that the compound of the formula I can be prepared from L-proline in two reaction steps with a total yield of 67% through the acylation of L-proline, if L-proline is acylated with (R)-3-acetylthio-2-methylpropanoyl chloride of formula III: ##STR3## in the presence of KOH and potassium phosphate buffer at pH 7.5-8.5, and at a temperature of between about 0°-5° C. The resulting compound is 1-[3-acetylthio-2-(S)-methylpropanoyl]-L-proline, ##STR4## Compound IV, which also can be termed "S-acetyl captopril", can be acidified and then deblocked with sodium hydroxide to give captopril in yield of 67% relative to starting material of formula III, without any unwanted byproducts.
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