Enzymatic cyclization of 22,23-dihydro-2,3-oxidosqualene into euph-7-en-3β-ol and bacchar-12-en-3β-ol by recombinant β-amyrin synthase

Recombinant β-amyrin synthase from Pisum sativum converted 22,23-dihydro-2,3-oxidosqualene, a substrate analogue lacking the terminal double bond of 2,3-oxidosqualene, into a 4:1 mixture of euph-7-en-3β-ol and bacchar-12-en-3β-ol. This is the first demonstration of the enzymatic formation of the baccharene skeleton with a six-membered D-ring. In the absence of the terminal double bond, the proton-initiated cyclization first generated the tetracyclic dammarenyl cation, followed by a backbone rearrangement with loss of H-7α leading to the formation of euph-7-en-3β-ol, while D-ring expansion to the baccharenyl cation and subsequent 1,2-hydride shifts with H-12α elimination yielded bacchar-12-en-3β-ol. It is remarkable that the formation of the anti-Markovnikov six-membered D-ring did not depend on the participation of the terminal π-electrons. Copyright © 2004 American Chemical Society.