The remarkable influence of steroid A/B-ring junction on the Wittig olefination reaction of the 11-oxo group: towards the synthesis of 5 alpha- and 5 beta-oriented delta 3-isomers of desogestrel.

ABSTRACT

The 5 alpha- and 5 beta-oriented delta 3-double bond isomers 8, 9 of the widely used progestin desogestrel (7) were synthesized. Wittig olefination reaction of the 5 alpha-intermediate 12 showed a dramatically reduced reaction rate compared with the olefination of the 5 beta-intermediate 13. Computational studies suggest that different energies of the intermediary 1,2-oxaphosphetanes may, at least partially, have been the reason for this phenomenon.