Bismuth(III) triflate-catalyzed rearrangement of 16α,17α-epoxy-20-oxosteroids. Synthesis and structural elucidation of new 16α-substituted 17α-alkyl-17β-methyl-Δ13-18-norsteroids

Bismuth(III) Reagents in Steroid and Terpene Chemistry

“…Thus the 5β,6β-stereochemistry is essential for the development of 1,3- syn -diaxial interactions with the angular 10β-methyl group, that are relieved during the rearrangement. In another related study, it has been shown that the “backbone” rearrangement of the 5β,6β-epoxide group occurs preferencially to the Wagner-Meerwein rearrangement of the 16α,17α-epoxy-20-oxo moiety ( Scheme 26 ) [ 86 ].…”

“…The application of Westphalen and “backbone” rearrangement reaction conditions [ 83 ] to 16α,17α-epoxy-20-oxosteroids has also been studied [ 86 ]. At room temperature, with 0.05 eq.…”

“…of catalyst, low conversion of 16α,17α-epoxy-20-oxopregn-5-en-3β-yl acetate was observed. However, on increasing the temperature to 50 ºC, 16α- and 16β-hydroxy-17β-methyl-Δ 13 -18-nor pregnane derivatives were formed in near similar amounts from 16α,17α-epoxy-20-oxosteroids ( Scheme 27 ) [ 86 ].…”

“…16β-Epimers can be formed due to an acid-catalyzed reverse-aldol equilibrium involving the 16-hydroxy-20-keto function of the rearranged steroid, under the reaction conditions employed, which is responsible for the epimerization at C16. ( Scheme 28 ) [ 86 ].…”

3.16 The Semipinacol Rearrangements

“…It should be noted that the migrating group may include those which are not directly connected to the epoxy carbon, which is actually a typical Wagner-Meerwein rearrangement process. [166][167][168] These above-mentioned features explain not only most of the rearrangement procedures, but also the design of related substrates, which usually have different numbers or types of substituents at each side of the epoxy moiety. Therefore, these semipinacol rearrangements, in principle, go through H migration or alkyl (acyl) migration, leading to the corresponding ketones or aldehydes (Scheme 125).…”