Synthetic pathway to 22,23-dioxocholestanic chain derivatives and their usefulness for obtaining brassinosteroid analogues.

Recognizing the functionality of the pentacyclic steroidal derivative 7a as important synthon to obtain new brassinosteroid analogs, we have accomplished the derivatization of hecogenin, a sapogenin from the 25R serie containing a carbonyl group at C-12, to a 22,23-dioxocholestanic chain derivative. Starting from hecogenin acetate (5a) or hecogenin tosylate (5b), we obtained two pentacyclic derivatives (7a and 7b) which were subjected to an oxidation reaction on the double bond at C-12(23) to obtain a 22,23-dioxocholestanic chain, with the regeneration of the carbonyl group at C-12. Reduction of the carbonyl groups lead to the 20-epi-12,23-dihydroxy-22-oxo system 11a-b. The absolute configuration of compound 11a was established by X-ray diffraction analysis.