Diels-Alder adducts of medium-ring carbocyclic dienes prepared by rearrangement of catalytically generated cyclic oxonium ylides

The novel bicyclic and tricyclic systems dimethyl (4aS*,6S*)-6-methoxy-7-ox o-4a,5,6,7,8,9-hexahydro-2H-benzocycloheptene-3,4-dicarboxylate, C16H20O6, (I), dimethyl (4aS*,6R*)-6-methoxy-7-oxo-4a,5,6,7,8,9-hexahydro-2H-benzocyc loheptene-3,4-dicarboxylate, C16H20O6, (II), (3aS*,9R*,10aS*,-10bR*)-9-meth oxy-2-oxa-1,3a,4,6,7,8,9,10,10a,10b-decahydro-3H-cyclohepta[e]indene-1,3,8- trione, C14H16O5, (III), and (1S*,2R*,9S*,10aR*)-9-methoxy-8-oxo-1,2,3,5,6, 7,8,9,10,10a-decahydrobenzocyclooctene-1,2-dicarboxylic acid, C15H20O6, (IV ), have been crystallographically characterized, allowing the determination of the relative configuration of the stereogenic centres. The poor quality of the dicarboxylic acid crystals necessitated the use of synchrotron radi ation.