Stereospecific synthesis and rearrangement of 22S-23-acetylsapogenins

The BF₃·Et₂O-catalysed acetolysis of steroid sapogenins diosgenin, sarsasapogenin and tigogenin in dichloromethane as the solvent instead of acetic anhydride afforded (20S)- and (20R)-22,26-epoxycholestanes (compounds 1 and 2). 22S-23-Acetylsapogenins (compounds 4) were synthesized stereospecifically from 20R-22,26-epoxycholestanes (compounds 2) in good yield. The rearrangement of 22S-23-acetylsapogenins (compounds 4) afforded novel disubstituted dihydropyran furostanol frameworks. Exhaustive NMR characterization of the obtained compounds is provided. Additionally, the structures of the critical compounds (6a and 7a) were unequivocallyconfirmed by single crystal X-ray diffraction studies.

Keywords: 22-epi-steroid sapogenins; BF(3)·Et(2)O-catalysed rearrangement; Dihydropyran furostanol frameworks; X-ray diffraction.