Syntheses of Angucyclinones Related to Ochromycinone. II. Regio- and Stereo-selective Reduction of a Tetrahydroangucyclinone

Abstract

The reduction of racemic 8-hydroxy-6-methoxy-3,3-dimethyl-3,4,5,6,6a,12a-hexahydrobenz[a]anthracene- 1,7,12(2H)-trione, with either (i) sodium borohydride and cerium(III) chloride in ethanol (the Luche reagent) or (ii) sodium borohydride in ethanol, was shown (by n.m.r. and X-ray crystallographic data) to yield the same racemic product, viz. 7,8-dihydroxy-6-methoxy-3,3-dimethyl-2,3,4,5,6,6a,7,12a-octahydrobenz[a]anthracene-1,12-dione. The 7-OH and 6-OMe groups are cis to each other. The observed regiochemistry can be rationalized in both cases by preferential interaction of respectively (i) the cerium(III) chloride of the Luche reagent, and (ii) sodium borohydride with the 8-OH group of (3) (the 8-OH group is more acidic than the solvent), while the cis stereochemistry is consistent with hydride delivery from the less hindered face in (3).