Efficient Synthesis of the Tetracyclic Aminoquinone Moiety of Marmycin A

An efficient four-step route to the tetracyclic aminoquinone moiety of marmycin A that proceeds in 41% overall yield from 5-nitronaphthoquinone and 5-methyl-1-vinylcyclohexene will facilitate preparation of marmycin A analogues for biological evaluation. The Diels−Alder reaction gave exclusively the desired adduct that is favored by steric considerations rather than the regioisomeric adduct that is favored by electronic considerations.