DIELS-ALDER REACTIONS OF 1,5-DIHYDROPYRANO[3,4-B]PYRROL-5(1H)-ONES, PYRROLE-2,3-QUINODIMETHANE ANALOGS - A NEW SYNTHESIS OF INDOLES

The pyrano[3,4-b]pyrrol-5(1H)-ones 7 are stable cyclic analogues of pyrrole-2,3-quinodimethane, and undergo Diels-Alder reaction with a range of acetylenes (dimethyl acetylenedicarboxylate, ethyl propiolate, ethyl trimethylsilylpropynoate, benzyne and the acetylene equivalent, phenyl vinyl sulfoxide), to give, after loss of carbon dioxide, indoles. The Diels-Alder reaction can be extended to the intramolecular variant to give cycloalka-[e]- and [g]-indoles.