Benzothiazinones by addition of o-Mercaptobenzamides to acetylene esters

The condensation of o-mercaptobenzamides with methyl acetylenedicarboxylate or methyl propiolate occurs with trans addition of the SH to the alkyne linkage. The resulting vinyl sulfide adducts can be ring closed to 1,3-benzothiazin-4-ones in excellent yield.