Syntheses and Diels-Alder cycloaddition reactions of 4H-furo[3,4-b]indoles. A regiospecific Diels-Alder synthesis of ellipticine

Abstract

Seven examples of the novel 4H-furo[3,4-b]indole ring system (3-9)-a stable, synthetic analogue of indole-2,3-quinodimethane have been synthesized in 6-8 steps from simple indoles in overall yiels of 21-28%. These 4H-furo[3,4-b]indoles undergo Diels-Alder reactions with several dienophiles (dimethyl acetylenedicarboxylate N-phenylmaleimide, benzyne), inclunding ethyl acrylate, witch reacts regiospecifically with furoindole 4 to afford a single carbazole ester (59). This result, predicted by molecular orbital calculations, was used to design and execute a regiospecific Diels-Alder synthesis of the antitumor alkaloid ellipticine (63)