Short-step stereoselective synthesis of 2α,3α,22-triacetoxy-23,24-dinor-5α-cholan-6-one: key intermediate for the preparation of 24-norbrassinolide, dolicholide and dolichosterone
Abstract
A stereoselective synthesis of 6-oxo-23,24-dinor-5α-cholan-2α,3α,22-triyl triacetate 9 was achieved in 44% overall yield in nine steps starting from 3β-hydroxyandrost-5-en-17-one 8. The important features of this synthesis are the modified Wittig reaction on the 17-oxo steroid 8 and the stereospecific generation of the chiral centre at C-20 by a resin-catalysed ene reaction on (Z)-3β, p-tolylsulfonyloxypregna-5,17(20)-diene 11.