Concerted Nature of AB Ring Formation in the Enzymatic Cyclization of Squalene to Hopenes

The cyclization of the A-B rings of squalene to hopene is studied computationally (DFT). A transition structure is found for a concerted, asynchronous pathway for the formation of chair-chair decalin carbocation. The computationally derived conformer leading to this asynchronous transition structure is remarkably similar to the analogous region of 2-azasqualene encapsulated by squalene-hopene cyclase recently reported by Schulz. A concerted A-B ring closure is likely to occur in the cyclization of squalene to hopene. [structure--see text]