Heterocycles by cycloaddition. Part II. Cycloaddition–extrusion reactions of five-membered mesoionic compounds with benzyne: prepartion of benz[c]azole and benzo[c]thiophen derivatives

Abstract

Several five-membered mesoionic compounds react with benzyne to give benzo-fused five-membered heterocycles [(2) and (5)] with a formal quinonoid structure. A bridged intermediate adduct (4c) was isolated from the reaction of benzyne with a mesoionic thiazol-4-one (3c). Pyrolysis of the adduct gave an isobenzothiophen (5a), whereas its photolysis led to an isoquinolone (7). The corresponding adduct S-oxide (4e) gave the isoquinolone (7) by both pyrolysis and photolysis.