Acylation of L-proline methyl ester (I) with methoxalyl chloride (II) afforded oxamate (III), which was condensed with 1,1-dimethylpropyl magnesium chloride to produce ketoamide (IV). Hydrolysis of the methyl ester of (IV) with LiOH gave carboxylic acid (V). This was finally coupled with 3,3-diphenylpropyl mercaptan (VI) using DCC and dimethylaminopyridine to give the title thioether.