Preparation of a 24-nor-1,4-dien-3-one triterpene derivative from betulin: A new route to 24-nortriterpene analogues

A new route to 24-nortriterpene derivatives with 2-hydroxy-Delta(1,4)-cyclohexadien-3-one A-rings from triterpene precursors has been demonstrated beginning with betulin to prepare derivatives of betulinic acid. The key steps in the transformation are a Suarez cleavage of the A-ring with a subsequent SmI2-mediated pinacol-type coupling to reclose the A-ring following removal of the C-24 carbon by oxidative cleavage.