Procedure for the Synthesis of N- (mercaptoacyl) -amino acids from alpha-substituted acids.

Process for preparing a compound of formula (I): ** ** FORMULA in which: R1 represents: a phenyl group; or a 3, 4-methylenedioxyphenyl; R2 represents a hydrogen atom or an alkyl group of C1 to C6; R3 represents a hydrogen atom, C1-C6 alkyl or a C1-C6 group phenylalkylene group; and R4 represents an acyl radical linear or branched, or an aromatic acyl radical. said method comprising a step (B) consisting of carrying out a Michael addition of a thioacid of formula (IV). R4SH (IV) wherein R4 is as defined in formula (I), a derivative of alpha-substituted acrylic amide of formula (V): ** ** FORMULA in which R1, R2 and R3 are as defined in formula (I).