Erroneous epimerization at C-22 in sapogenins

• Sapogenin isomerizations catalyzed by BF3·OEt2 do not occur at C-22 but at C-25.
• Sarsasapogenin acetate isomerization with BF3·OEt2 yields smilagenin acetate (not 22-epi-sarsasapogenin acetate).
• Diosgenin acetate isomerization with BF3·OEt2 affords yamogenin acetate in a tiny yield.
• Yamogenin treated with BF3·OEt2 yields diosgenin (not 22-epi-yamogenin).

A recent report [1] about the epimerization of steroidal sapogenins at C-22 by treatment with BF3·OEt2 is incorrect. We proved that the epimerization of sapogenins with BF3·OEt2 occurs at C-25 as in the case of other acid-catalyzed reactions previously studied.