The formula III starting material may be prepared by alkylating an anion derived from an alkyl or (substituted)aralkylalkoxyphosphinyl acetic acid, alkyl ester, that is ##STR100## by a primary halide derivative of R3 (e.g., R3 --Br) which contains suitable protection for the R3 group, in the presence of a base such as sodium hydride and an inert solvent such as tetrahydrofuran to form ##STR101## S