A two-step synthesis of solasodine pivalate from diosgenin pivalate is described. The key transformation involves the reaction of diosgenin pivalate with benzyl carbamate (CbzNH₂) promoted by TMSOTf. During the reaction the F-ring of the spiroketal moiety opens up with a simultaneous introduction of a Cbz-protected amino group in position 26. A one-pot deprotection of 26-amine with AcBr/BuOH followed by the N-cyclization affords solasodine pivalate in 45% overall yield.
Keywords: diosgenin; ketal cleavage; solasodine; spirostanes; steroids; tomatidinol.