STEROIDS AND RELATED NATURAL PRODUCTS: XXX. SELECTIVE REDUCTION OF ESTERS. PART A. BENZOATES

Previously, reagents such as sodium borohydride – boron trifluoride were shown to be capable of reducing typical aliphatic esters and lactones to the corresponding ether derivatives. Under the same reaction conditions, a variety of benzoate esters (Ib, IIb, III, and V) were essentially unaffected. Resistance to attack was illustrated by conversion, with sodium borohydride – boron trifluoride, of 3-oxo-17β-benzoyloxy-5α-androstane (III) into 3β-hydroxy-17β-benzoyloxy-5α-androstane (IV) and of reserpine (V) into alcohol VI. With aryl–carbon–oxygen bonds of the ketone VII or alcohol IXa type, reaction with sodium borohydride– boron trifluoride was shown, in general, to cause reductive fission of the benzyl–oxygen linkage (e.g. X → XI). An exception was noted in the case of tertiary benzyl alcohols containing an available β-hydrogen (e.g. IXc). Here, dehydration to yield, for example, olefin XIIb appears to be the more favorable reaction course. The marked difference in the reactivity of sodium borohydride – boron trifluoride toward benzoyl–oxygen vs. benzyl–oxygen bonds has been demonstrated.