Abstract
Cyclase I from Salvia officinalis leaf catalyzes the conversion of geranyl pyrophosphate to the stereo-chemically related bicyclic monoterpenes (+)-alpha-pinene and (+)-camphene and to lesser quantities of monocyclic and acyclic olefins, whereas cyclase II from this plant tissue converts the same acyclic precursor to (-)-alpha-pinene, (-)-beta-pinene and (-)-camphene as well as to lesser amounts of monocyclics and acyclics. These antipodal cyclizations are considered to proceed by the initial isomerization of the substrate to the respective bound tertiary allylic intermediates (-)-(3R)- and (+)-(3S)-linalyl pyrophosphate. [(3R)-8,9-14C,(3RS)-1E-3H]Linalyl pyrophosphate (3H:14C = 5.14) was tested as a substrate with both cyclases to determine the configuration of the cyclizing intermediate. This substrate with cyclase I yielded alpha-pinene and camphene with 3H:14C ratios of 3.1 and 4.2, respectively, indicating preferential, but not exclusive, utilization of the (3R)-enantiomer. With cyclase II, the doubly labeled substrate gave bicyclic olefins with 3H:14C ratios of from 13 to 20, indicating preferential, but not exclusive, utilization of the (3S)-enantiomer in this case. (3R)- and (3S)-[1Z-3H]linalyl pyrophosphate were separately compared to the achiral precursors [1-3H]geranyl pyrophosphate and [1-3H]neryl pyrophosphate (cis-isomer) as substrates for the cyclizations. With cyclase I, geranyl, neryl, and (3R)-linalyl pyrophosphate gave rise exclusively to (+)-alpha-pinene and (+)-camphene, whereas (3S)-linayl pyrophosphate produced, at relatively low rates, the (-)-isomers. With cyclase II, geranyl, neryl, and (3S)-linalyl pyrophosphate yielded exclusively the (-)-isomer series, whereas (3R)-linalyl pyrophosphate afforded the (+)-isomers at low rates. These results are entirely consistent with the predicted stereochemistries and additionally revealed the unusual ability of these enzymes to catalyze antipodal cyclizations when presented with the unnatural linalyl enantiomer.
Highlights
From the Slnstitute of siologica~C! hemistry, Washington State University, Pullman, Washington99164-6340 and the VDepartment of Chemistry, Brown University,Providence, Rhode Island 02912
Pyrophosphate to Cyclic Olefins-As in earlier studies to determine enantiomeric selectivity in the conversion of linalyl pyrophosphate to bornane monoterpenes, initial experiments were directed toward examining the conversion of [(3R)-8,9
Nonenzymatic background), which were analyzed by radioGLC2 (3H-detection) following the addition of appropriate racemic carrier standards.The corrected product distributions generated from linalyl pyrophosphate were somewhat different than those produced with geranyl pyrophosphate, the natural substrate for these enzymes
Summary
ENANTIOSELECTIVITY IN THEENZYMATIC CYCLIZATION OF (+)- AND (-)-LINALYL PYROPHOSPHATE TO (+)- AND (-)-PINENE AND (+)- AND (-)-CAMPHENE*. With cyclase 11, geranyl, neryl, and (3s)-linalyl pyrophosphate yielded exclusively the (-)-isomer series, bound linalyl pyrophosphate intermediate (3).Rotation about the newly generated C2-C3single bond to thecisoidconformer overcomes the original stereochemical impediment tothe whereas (3R)-linalyl pyrophosphate afforded the (+)- cyclization of geranyl pyrophosphate, while subsequent ioniisomers at low rates. These results are entirely con- zation of this tertiaryallylic isomer allows Cl-C6 cyclization sistent with thepredicted stereochemistries and addi- of the anti-endo form, or, alternatively, deprotonationof the tionally revealed the unusual ability of these enzymes resulting ion pair (4) to generate the acyclic olefin myrcene to catalyze antipodal cyclizations when presented w(i5th) (which could be generated from 2).
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