A biomimetic approach to the synthesis of an antiviral marine steroidal orthoester.

Orthoesterol B, a marine natural product exhibiting antiviral activities, contains a [3.2.1]-bicyclic orthobutyrate bridging the steroid side chain and ring D. A biosynthetic reaction was proposed by which rearrangement of an epoxy ester results in the formation of the orthoester moiety. Steroidal model compounds incorporating 16-butyroxy and 20,22-epoxy groups were synthesized from tigogenin and were shown to rearrange to orthoesters under mild acidic catalysis.