Mechanism of clerosterol biosynthesis in Ajuga hairy roots: stereochemistry of C-28 methylation of 24-methylene sterol

Biosynthesis of clerosterol, (24S)-ethylcholesta-5,25-dien-3β-ol (1), involves transfer of the methyl group from S-adenosylmethionine to the C-28 position of a 24-methylene-sterol precursor. The resulting C-24 cationic species undergoes migration of hydrogen from C-25 to C-24, followed by deprotonation from C-26 to form 1. We have now investigated the steric course of the methylation in hairy roots of Ajuga reptans var. atropurpurea. Feeding of [28E-2H]- and [28Z-2H]-24-methylenecholesterols and 2H NMR analysis of clionasterol obtained by partial hydrogenation of the biosynthesized clerosterol have revealed that the methylation takes place from the 28-si face.