Abstract: A highly convenient and efficient process for economically producing in a high yield high-quality captopril which is remarkably reduced in the content of impurities and has a high melting point and intermediates for synthesizing the same which contain only a small amount of precursors as impurities and have excellent qualities. The process comprises subjecting an acid halide and an L-proline to the Schotten-Baumann reaction and eliminating the impurities formed as the by-products in the form of the precursors represented by general formula (5) or (6) by treating, during or after the Schotten-Baumann reaction, the aqueous medium solution with active carbon or crystallization followed by deacylation. In the formula, R.sup.1 represents acyl and n represents an integer of from 2 to 4.

Abstract: There is described a process for preparing 3-(2-thienylthio)butyric acid by converting a compound having the formula (III): ##STR1## wherein R is a straight chain or branched C.sub.1 to C.sub.4 alkyl group, whereby the production of 3-(3-thienylthio)butyric acid as a by-product which is the position isomer can be controlled to at most 0.1 mol %. 3-(2-Thienylthio)butyric acid is a useful compound as an intermediate for a medicinal compound.

Abstract: There is described a process for preparing 3-(2-thienylthio)butyric acid by converting a compound having the formula (III): ##STR1## wherein R is a straight chain or branched C.sub.1 to C.sub.4 alkyl group, whereby the production of 3-(3-thienylthio)butyric acid as a by-product which is the position isomer can be controlled to at most 0.1 mol %. 3-(2-Thienylthio)butyric acid is a useful compound as an intermediate for a medicinal compound.