Aristolochene Synthase. Elucidation of the Cryptic Germacrene A Synthase Activity Using the Anomalous Substrate Dihydrofarnesyl Diphosphate

Germacrene A ( 3 ) has been proposed to be an intermediate in the cyclization of farnesyl diphosphate ( 1 ) to the sesquiterpene hydrocarbon (+)-aristolochene ( 2 ). Under normal circumstances, however, neither 3 nor any other intermediate is released from the cyclase active site. When the anomalous substrate (7 R )-6,7-dihydrofarnesyl diphosphate ( 5 ) is incubated with aristolochene synthase from Aspergillus terreus, the resultant dihydrogermacrene ( 6 ) cannot be further cyclized and is released from the active site. The structure of the abortive cyclization product, m / z 206, was confirmed as dihydrogermacrene ( 6 ) by direct GC−MS comparison with an authentic synthetic sample prepared from (−)-(3 S )-β-citronellol.