BF3·Et2O-induced stereoselective aldol reaction with benzaldehyde, and steroid sapogenins and its application to a convenient synthesis of dinorcholanic lactones

Treatment of steroid sapogenins with benzaldehyde and BF3·Et2O cleanly produces E-23(23′)-benzylidenspirostanes in good yields in a reaction pathway which consists on an aldol reaction followed by a dehydration step. The obtained E-23(23′)-benzylidenspirostanes can be easily converted to dinorcholanic lactones by treatment with CrO3 in acetic acid. The synthetic sequence to dinorcholanic lactones is compatible with the presence of double bonds and carbonyl groups in the steroid framework.

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► Reaction of spirostanes with benzaldehyde produces E-23(23′)-benzylidenspirostanes.
► The reaction pathway consists of an aldol reaction followed by a dehydration step.
► Benzylidenspirostanes are oxidized to dinorcholanic lactones with CrO3 in acetic.
► X-ray studies of the obtained compounds confirmed the proposed structures.