Bromoquinone–enaminone annulations: syntheses of murrayaquinone-A and (±)-bismurrayaquinone-A

Abstract

A total synthesis of the carbazolequinone alkaloid, murrayaquinone-A was achieved by an initial annulation of the N-benzyl enamine 8 with 2-methyl-5-bromobenzoquinone 6. Shapiro deoxygenation–olefination followed by heating with DDQ resulted in the exclusive formation of N-benzylmurrayaquinone-A 16. Debenzylation proved very difficult but was finally achieved by heating briefly in trifluoroacetic acid with a catalytic quantity of trifluoromethanesulfonic acid. A single dimeric annulation side product was also formed in the annulation reaction. By using the same synthetic sequence as was employed in the synthesis of 1, the N,N-bis-p-methoxybenzyl dimer 13 was successfully converted to (±)-bismurrayaquinone-A 5.