Abstract
The reduction of Δ 4-androsten-3, 17 dione 1 and of progesterone 2 by nBu 4NBH 4 is highly chemioselective: in THF only the a-enone moiety is reduced, the saturated C 17 or C 20 keto group being kept unchanged. When TMEDA is added, saturated alcohols are obtained, without any allylic alcohol when the reaction goes to completion. However this reduction is poorly stereoselective as 70:30 mixtures of A/B cis and trans ring junction compounds are obtained. In MeOH, the saturated keto group is more than 90% selectively reduced. However, the reduction of 1 and 2 by LiBH 4 and Zn(BH 4) 2 is poorly chemioselective. These results are interpreted in terms of competition between electrophilic assistance and steric effects.
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