Abstract
In order to elucidate the mechanism of D-homoannulation of pregnanediol 20-sulfate, solvolysis of [13C-20]-5β-pregnane-3α, 20α-diol disulfate (3) in refluxing 3N hydrochloric acid was investigated. The resulting D-homosteroids, 17α-methyl-D-homo-5β-androstane-3α, 17aβ-diol (8) and 17α-methyl-17aβ-chloro-D-homo-5β-androstan-3α-ol (10), contained a quantitative amount of 13C only at C-17, indicating that the ring-enlargement reaction of the 20α-ol sulfate proceeded with stereospecific migration of the C16-C17 bond. The same result was obtained from isomeric [13C-20]-5β-pregnane-3α, 20β-diol disulfate (6). Based on these results, the D-homoannulation of pregnanediol 20-sulfate was concluded to proceed by a stepwise mechanism through the C-20 carbocation. The stereochemistry of this Wagner-Meerwein type rearrangement reaction is also discussed.
