Equilibration of 24,25-dibromides of lanosterol and derivatives.

Bromination of lanosteryl acetate under conditions of kinetic control gives two 24,25-dibromides. A and B, in equal amounts. Thermodynamically controlled equilibration of these dibromides results in a 5:1 mixture of the two. The driving force is considered to be interaction between the 20-methyl group and the 24-bromine atom in the 24R-configuration. Configurations have been assigned to the dibromides from the results of an X-ray crystal structure determination on dibromide A.