Regio- and stereoselective cleavage of steroidal 22-oxo-23-spiroketals catalyzed by BF3·Et2O.

The regioselective opening of the F ring of 22-oxo-23-spiroketals using BF3·OEt2 in acetic anhydride yielded novel cholestanic frameworks with pyranone E ring 20-23. The structures of the new derivatives of botogenin, diosgenin, hecogenin and tigogenin thus obtained were established using one and two dimensional (1)H, (13)C experiments (DEPT, COSY, HETCOR, HMBC). The X-ray diffraction analysis unequivocally confirmed the R configuration at C-23 in the starting 22-oxo-23-spiroketal 18 and the Z configuration of the C23-C24 double bond in the reaction product 20.