One-Pot Synthesis of 5‑Hydroxy‑4H‑1,3-thiazin-4-ones: Structure Revision, Synthesis, and NMR Shift Dependence of Thiasporine A

An annulation of arylthioamides with 3-bromopyruvic acid chloride to 5-hydroxy-4H-1,3-thiazin-4-ones has been developed. The initial condensation affords two regioisomeric thiazolinone intermediates in a temperature-dependent manner. The synthesis of the 2-amino­phenyl­thiazinone derivative led to the revision of the previously proposed structure of thiasporine A. Synthesis of the revised structure and NMR analysis revealed that thiasporine A had been isolated as a carboxylate.