Squalene-Hopene Cyclase: Mechanistic Insights into the Polycyclization Cascades of Squalene Analogs Bearing Ethyl and Hydroxymethyl Groups at the C-2 and C-23 Positions.

Squalene-hopene cyclase (SHC) catalyzes the conversion of squalene 1 into 6,6,6,6,5-fused pentacyclic hopene 2 and hopanol 3. To elucidate the binding sites for the terminal positions of 1, four analogs, having the larger ethyl (Et) and the hydrophilic CH2 OH groups at the 23E or 23Z positions of 1, were incubated with SHC. The analog with the Et group at the 23E position (23E-Et-1) yielded two tetra- and three pentacyclic products; however, the analog possessing the Et group at the 23Z position (23Z-Et-1) gave two hopene homologs and the neohopane skeleton, but no hopanol homologs. Hopene homolog (C31 ) was generated from 23E-Et-1 by deprotonation from 23Z-Me (normal cyclization cascade). Intriguingly, the same homolog was also generated from the geometrical isomer 23Z-Et-1, indicating C-C bond rotation about the C-21-C-22 axis of the hopanyl cation and the more compact nature of the binding domain at 23Z compared with 23E. On the other hand, analogs with the CH2 OH group gave novel hopane skeletons having 1-formylethyl and 1-hydroxyprop-2-en-2-yl residues at C-21. Products bearing an aldehyde group were generated in higher yield from 23Z-CH2 OH-1 (89 %), than from 23E-CH2 OH-1 (26 %). The significant yield (26 %) of the aldehyde products from 23E-CH2 OH-1 indicated that C-C bond rotation had occurred owing to the absence of hydrophobic interactions between the hydrophilic 23E-CH2 OH and its binding site. The polycyclization mechanisms of the four different analogs are discussed.