Formation of the C Ring in the Lanosterol Biosynthesis from Squalene

[reaction: see text] Ab initio calculations were performed on a cyclohexane derivative to elucidate the mechanism of the formation of the five-membered C ring in the biosynthesis of lanosterol from squalene. A conformational analysis of the side chain containing the double bond indicated that the conformer that should give rise to the cyclized C ring is not a minimum on the potential surface. Consequently, it is suggested that it is very likely that C-ring formation occurs in concert with formation of the A and B rings.