Steroidal A-Ring Functionalization

Introduction of oxygen at C-1 in the A ring of the steroid nucleus usually is accomplished by basic a-epoxidation of the corresponding 3-keto-l-ene derivative. The scheme of Narwid et al. [scheme (14)] illustrates this approach in which the 

6-double bond is protected by conversion to 6-hydroxy or keto functions. Reduction of the epoxide and regeneration of the 5-ene gives the la-hydroxylated product in ca. 18% yield. 

The approach of Hesse and Barton [scheme (15)] to the la-hydroxy-5-cholestene system involves li/NH3/NH4Cl reduction of the lot,2a-epoxy4,6-dien-3- 

A third approach [scheme (16)] involves deconjugation of a 1,4-dien-3-one to the 1,5-diene followed by selective hydroboration of the 1,2-olefin. This scheme gives both the la-hydroxy and 2a-hydroxy cholesterol derivatives. Application of this method to l,4,6-cholestatrien-3-one leads directly to the la-hydroxy provitamin D skeleton  

A fourth procedure [scheme (17)] is quite different from the previous examples. Oxygen is introduced at C-1 by the transannular photochemical 2+2 cycli- 

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