β-Amyrin biosynthesis: the critical role of steric volume at C-19 of 2,3-oxidosqualene for its correct folding to generate the pentacyclic scaffold.

The effect of the steric volume at C-19 of (3S)-2,3-oxidosqualene 1 on the polycyclization reaction by beta-amyrin synthase was examined. The substrate analogs, in which the methyl group at C-19 of 1 was substituted by an ethyl group and hydrogen atom, were converted into the following three new compounds: (17 beta-H, 20S)-20-ethyl-dammara-12,24-diene 9, beta-amyrin homologue 10, and the 6,6,6,6-fused tetracycle 11. The folding conformations leading to these products are discussed.