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The reactions of LR (Lawesson’s Reagent) with 4-oxocarboxylic acid derivatives I, N-acyl aminoacid derivatives III, and N-acyl-N′-ethoxycarbonylhydrazines V, smoothly produce substituted thiophenes II, thiazoles IV, and 1,3,4-thiadiazole-2(3H)-thiones VI.

We herein report an improved synthetic pathway to 5‐thioxo‐L‐proline (2) (using R,S nomenclature, the L form here possesses the S configuration). Compound 2 is synthesized from naturally occurring 5‐oxo‐L‐proline (1) at room temperature with 2,4‐bis(4‐methoxyphenyl)‐1,3,2,4‐dithiadiphosphetane 2,4‐disulfide (3) (Lawesson′s reagent, LR). The synthesis is an improved version of our previously published method. The optical purity of 2 has been proven by thin‐layer chromatography based on chiral ligand exchange. The synthesis of N‐acyl and S‐alkyl derivatives of 5‐thioxo‐L‐proline (Top‐OH), 5‐thioxo‐L‐proline methyl ester (Top‐OCH3), and the pseudodipeptide Boc‐L‐Phe‐Top‐OCH3 is also described.