Bisabolyl-Derived Sesquiterpenes from Tobacco 5-Epi-aristolochene Synthase-Catalyzed Cyclization of (2Z,6E)-Farnesyl Diphosphate

摘要

We report the structures and stereochemistry of seven bisabolyl-derived sesquiterpenes arising from an unprecedented 1,6-cyclization (cisoid pathway) efficiently catalyzed by tobacco 5-epi-aristolochene synthase (TEAS). The use of (2Z,6E)-farnesyl diphosphate as an alternate substrate for recombinant TEAS resulted in a robust enzymatic cyclization to an array of products derived exclusively (≥ 99.5%) from the cisoid pathway, whereas these same products account for ca. 2.5% of the total hydrocarbons obtained using (2E,6E)-farnesyl diphosphate. Chromatographic fractionations of extracts from preparative incubations with the 2Z,6E substrate afforded, in addition to the acyclic allylic alcohols (2Z,6E)-farnesol (6.7%) and nerolidol (3.6%), five cyclic sesquiterpene hydrocarbons and two cyclic sesquiterpene alcohols: (+)-2-epi-prezizaene (44%), (-)-α-cedrene (21.5%), (R)-(-)-β-curcumene (15.5%), a-acoradiene (3.9%), 4-epi-a-acoradiene (1.3%), and equal amounts of a-bisabolol (1.8%) and epi-α-bisalolol (1.8%). The structures, stereochemistry, and enantiopurities were established by comprehensive spectroscopic analyses, optical rotations, chemical correlations with known sesquiterpenes, comparisons with literature data, and GC analyses. The major product, (+)-2-epi-prezizaene, is structurally related to the naturally occurring tricyclic alcohol, jinkohol (2-epi-prezizaan-7β-ol). Cisoid cyclization pathways are proposed by which all five sesquiterpene hydrocarbons are derived from a common (7R)-β-bisabolyl~+/pyrophosphate~- ion pair intermediate. The implications of the "cisoid" catalytic activity of TEAS are discussed.