Yield of benzothiazine derivatives and catalysis by heteropolyacids

An efficient heteropolyacid-catalyzed reaction for the synthesis of benzothiazine derivatives is reported. In this transformation, H₃PW₁₂O₄₀ was employed as a catalyst in a reaction involving 2-aminobenzenethiols, acetylenic esters, and malonate esters. Optimum conditions are developed in i-PrOH at 50˚C for 7 h.