Figure 82.

Class II cyclization reaction of squalene (2) is initiated by general acid D376 (AH) and proceeds through a cascade of carbon–carbon bond-forming reactions to yield the 6–6–6–6–5 hopenyl cation (15), which undergoes proton elimination with the assistance of a water molecule general base (B:) to yield hopene (16). Alternatively, the hopenyl cation can be quenched by addition of a water molecule to yield minor side product diplopterol (17). Reprinted with permission from ref (455). Copyright 2005 Wiley-VCH.