Proposed Mechanism for the Reaction Catalyzed by a Diterpene Cyclase, Aphidicolan-16β-ol Synthase: Experimental Results on Biomimetic Cyclization and Examination of the Cyclization Pathway by ab Initio Calculations
Lewis acid-catalyzed cyclizations and rearrangements. Isolation of these products in an aphidicolin-producing fungus led us to speculate that the mechanism of the ACS-catalyzed cyclizationreaction is the same as that of a nonenzymatic reaction. Ab initio calculations of the acid-catalyzed reaction intermediates and the transition states indicate that the overall reactioncatalyzed by ACS is an exothermic