Total synthesis of a D-ring indole analogue of daunomycin

The strong base-induced cycloaddition of 4-methoxy-5-methylpyrano[4,3-b]indole-1,3(4H,5H)-dione (3a) to 2-chloro-6-oxo-5,6,7,8-tetrahydro-1,4-naphthoquinone ethylene acetal (4) constitutes a regiospecific and convenient route to the D-ring indole analogue (2) of daunomycin.