Epoxy-20-oxosteroids

The application of Westphalen and backbone rearrangement reaction conditions (MeN02, Bi(0Tf)3xH20 catalyst) [118] to 16a,17a-epoxy-20-oxosteroids has also been studied [124]. At room temperature and 0.05 eq. of catalyst, low conversion of 16a,17a-epoxy-20-oxopregn-5-en-3p-yl acetate was observed. However, on increasing the temperature to 50 °C, 16a- and 16p-hydroxy-17p-methyl-A13-18-nor pregnane derivatives were formed in near similar amounts from 16a,17a-epoxy-20-oxosteroids (Scheme 33). 

Scheme 33 Bi(OTf)3-xH20-catalyzed rearrangement of 16a,17a-epoxy-20-oxosteroids...

Henbest [ 4] has discussed a variety of reactions where the stereochemistry of the product is apparently controlled or influenced by the electrostatic effects of remote substituents. As an example, the isomer ratio of the steroid alcohols produced by reduction of a 12-keto function is sensitive to both the nature and configuration of a substituent at C(s) (p. 141). We may also cite the conversion of A -3-oxosteroids into their 4,5-epoxy-derivatives by alkaline hydrogen peroxide, where the proportion of /3-epoxy derivatives varies, according to substitution at C(iv>, from ca. 100% in the unsubstituted androst"4-en-3-one to 70% (- - 30% of a-epoxide) in the 17/ -hydroxy derivative. 

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