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 Suárez cleavage of the A-ring with a subsequent SmI(2)-mediated pinacol-type coupling to reclose the A-ring following removal of the C-24 carbon by oxidative cleavage.
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Deng et al. (2002) studied this question.
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