The Preparation of the Spirostanic Analogues of Brassinolide and Castasterone

Methods for the preparation of the spirostanic analogues of brassinosteroids (25R)-5alfa- spirostan-6-one-2alfa,3alfa-diol and (25R)-B-homo-5alfa-spirostan-6-oxo-7-oxalactone- 2alfa,3alfa-diol, starting from diosgenin, were examined. The best preparative route was via diosgenin tosylation, isosteroidal rearrangement with potassium acetate in aqueous acetone, oxidation with Jones reagent, cyclopropyl ring opening with hydrobromic acid, hydrogen bromide elimination with lithium bromide and carbonate, dihydroxylation with osmium tetroxide and N-methylmorpholine N-oxide, producing (25R)-5alfa- spirostan-6-one-2alfa,3alfa-diol in 57.3% overall yield and lactonization with trifluoroperoxyacetic acid producing (25R)-B-homo-5alfa-spirostan-6-oxo-7-oxalactone- 2alfa,3alfa-diol in 24.6% overall yield from diosgenin. The shortest route to (25R)-5alfa-spirostan-6-one-2alfa,3alfa-diol results in only 39.4% overall yield.