A number of sulfoxides derived from 3-benzylthiopropionic acid, S-benzylcysteine, and S- phenylcystelne have been synthesized and exposed to typical Pummerer reaction conditions. Cyclization of the S-benzyl sulfoxides to six-membered or seven-membered heterocyclic rings (1, 3-thiazin-4-ones and 1, 3, 6-oxathiazepines) is observed only in acetic anhydride solvent and only after conversion of the carboxyl group to an amide or peptide. ...