Cyclization of amino acid (I) using 3-ethyl-1-(3-dimethylaminopropyl)carbodiimide (EDC) at a pH of 5.6 produced the azaphosphorinane (II) as a mixture of diastereoisomers. Subsequent reaction of (II) with diphenyldiazomethane yielded the bis(benzhydryl) ester (III) as a mixture of isomers, which were separable by column chromatography. Further refluxing of either isomer in EtOH removed selectively the phosphinic ester to give (IV) as an isomeric mixture. Coupling of (IV) with benzhydryl mercaptan by means of diethyl cyanophosphonate produced a mixture of cis (V) and trans (VI) phosphinothioic esters, which were separated by fractional crystallization from EtOAc. Finally, deprotection of both benzhydryl groups from the cis isomer (V) by means of trifluoroacetic acid yielded the title diacid.