7 alpha-Alkylation, 7,7-Bisalkylation, and Reduction of the 20-Oxo Group of Poststerone in Reactions with Alkyl Halides in Lithium-Ammonia Solution

摘要

Reactions of poststerone with methyl iodide, allyl bromide, and propargyl bromide in lithium-ammonia solution resulted in its completely stereoselective 7 alpha-alkylation accompanied by reduction of the 20-oxo group with formation of equimolar amounts of 20R- and 20S-hydroxy derivatives. The reaction of poststerone with excess allyl bromide afforded 7,7-bis-allyl 20R- and 20S-alcohols at a ratio of 3 : 1. The reduction of the 20-oxo group in the alkylation of poststerone with excess propargyl bromide led to the formation of an equimolar mixture of Delta(8(14))-and Delta(8(9))-isomers with exclusive S configuration of C-20.