Stereochemistry of sterol biosynthesis: interrelationship of the terminal methyl groups in squalene, the C-1,1? methyls in squalene 2,3-oxide, and the C-30,31 methyls in lanosterol

The trans-terminal methyls of squalene derive from C-2 of mevalonic acid, and the conversion of squalene in turn into squalene 2,3-oxide and lanosterol occurs without interchange of isopropylidene methyls, so that C-2 of mevalonic acid provides the 4α-methyl group of lanosterol.