Birch reduction intramolecular protonation

Stereoselective l.4-reduction oi the 1.3-butadiene system to olefin 57 lakes place tinder the conditions of the Birch reduction. Intramolecular protonation of the intermediate carbanion at the 18-position to give 57 occurs with high selectivity syn to the hydroxymethy-iene group Conversion into phosphoric acid derivative 58 and cleavage of the phosphoric acid amide group under (he conditions of the Bcnkeser reduction provides compound 5921 Fluonde ion causes the release of free p-amyrin (1) in a final step I Li, NH3(iyTllF (1/1.75), -78 C 93%. 

Intramolecular protonation on the more hindered face of a steroid from a neighbouring hydroxyl group best explains a reversal of diastereoselectivity in the Birch reduction of styrene double bonds.266 The kinetics and product distribution of lithium metal reduction of benzaldehyde to benzyl alcohol in THF have been studied electron transfer from Li to PhCHO occurs in a slow step, but absorption of the PhCHO onto the metal surface is also crucial in determining the overall kinetics. The proposed mechanism successfully accounts for the formation of minor products, benzoin and... 

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