Selectivite de la reduction de l'androsten-4 dione-3,17 et de la progesterone par divers borohydrures: role de l'assistance electrophile par le cation ou un solvant hydroxyle

The reduction of Δ⁴-androsten-3, 17 dione 1 and of progesterone 2 by nBu₄NBH₄ is highly chemioselective: in THF only the a-enone moiety is reduced, the saturated C₁₇ or C₂₀ 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 LiBH₄ and Zn(BH₄)₂ is poorly chemioselective. These results are interpreted in terms of competition between electrophilic assistance and steric effects.