Concomitant C-Ring Expansion and D-Ring Formation in Lanosterol Biosynthesis from Squalene without Violation of Markovnikov's Rule

A mechanism is proposed for the formation of the C and D rings of lanosterol from squalene on the basis of density functional calculations of a model cyclopentylcarbinyl carbocation. The mechanism involves the ring expansion of the cyclopentylcarbinyl carbocation to the six-membered ring C in concert with the formation of the D ring. The mechanism does not involve a violation of Markovnikov's rule as the earlier proposed mechanism does for the formation of rings C and D.