TiCl4 catalyzed cleavage of (25R)-22-oxo-23-spiroketals. Synthesis of sapogenins with furostanol and pyranone E rings on the side chain

[Display omitted] •Diastereoselective opening of 22-oxo-23-spiroketals provided the novel furostanols 11a–11d.•Regioselective cleavage of the F ring of 22-oxo-23-spiroketals with TiCl4/Ac2O was attained.•The structure of compounds 11a–d was established using one and bidimensional NMR techniques.•The X-ray analysis of compound 11a confirmed the 22α-hydroxyl orientation at C-23. The regioselective opening of the F ring of 22-oxo-23-spiroketals 7a-d using TiCl4 in acetic anhydride yielded the novel furostanols 11a-d along with cholestanic derivatives 8a-d with pyranone E ring. The structures of the new derivatives thus obtained were established using one- (DEPT) and two-dimensional 1H, 13C NMR experiments (COSY, HSQC, HMBC, NOESY). The 22α-hydroxyl orientation in compounds 11a-d was proposed by comparison of the 13C chemical shifts with those of other aglycone members of this family, and confirmed by combined NOESY and X-ray diffraction analysis of compound 11a.