Studies on quinones. Part 21. Regioselective synthesis of tetracyclic quinones related to rabelomycin

Abstract

The Diels–Alder reaction of the hydroxyquinones 5,11 and 16 with (E)-1 -trimethylsiloxybuta-1,3-diene 3 afforded the corresponding mixture of the regioisomers 6a,b, 12a,b and 17a, b in the ratios 9:1, 8:1 and 11:1, respectively. From these mixtures, the quinones 8, 14 and 19 were obtained by hydrolysis and subsequent oxidation.

The preparation of the diene 22 by chlorotrimethylsilylation of the anion of the ester 21 is described. Diels–Alder reaction of diene 22 with the quinones 23 and 24 yielded the corresponding 6-ethoxybenz[a]anthracenequinones 25 and 26, together with the 6-hydroxybenz[a]anthracenequinones 19 and 20. The quinones 19 and 20, which were isolated in mixtures with the ester 21, undergo selective aerial oxidation under basic conditions to give the corresponding benz[a]anthracene-1,7,12-triones 27 and 28.