The effect of the steric volume at C-19 of (3S)-2,3-oxidosqualene 1 on the polycyclization reaction by β-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: (17β-H, 20S)-20-ethyl-dammara-12,24-diene 9, β-amyrin homologue 10, and the 6,6,6,6-fused tetracycle 11. The folding conformations leading to these products are discussed.
