Recombinant
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-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-3
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-ol and bacchar-12-en-3
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-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-7
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leading to the formation of euph-7-en-3
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-ol, while D-ring expansion to the baccharenyl cation and subsequent 1,2-hydride shifts with H-12
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elimination yielded bacchar-12-en-3
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-ol. It is remarkable that the formation of the anti-Markovnikov six-membered D-ring did not depend on the participation of the terminal
![](/images/gifchars/pi.gif)
-electrons.