Preparation and Diels–Alder reactivity of thieno[2,3-c]-and thieno[3,2-c]-pyran-3-ones. Stable 2,3-dimethylenethiophene derivatives; synthesis of benzothiophenes

The thieno[2,3-c]pyran-3-ones (7) and the isomeric [3,2-c]pyranones, e.g.(26) are stable derivatives of 2,3-dimethylenethiophene (3). When heated with alkynes they undergo Diels–Alder reaction to give, after loss of carbon dioxide, benzothiophenes. With unsymmetrical alkynes, the Diels–Alder reactions exhibit varying degrees of regioselectivity. Intramolecular Diels–Alder reactions of the thieno[2,3-c]pyran-3-ones (17) and (21) give cycloalka[g]- and cycloalka[e]-benzothiophenes respectively.