MONOTERPENE CYCLIZATION MECHANISMS AND THE USE OF NATURAL-ABUNDANCE DEUTERIUM NMR - SHORT-CUT OR PRIMROSE PATH

Separate incubations of [2-H-2, 2-C-13]geranyl diphosphate (GPP) with (+)- and (-)-pinene cyclases isolated from common sage (Salvia officin alis) gave a mixture of acyclic and cyclic monoterpene olefins. Mass s pectrometric analysis of the derived labelled monoterpenes established that cyclization had taken place with complete retention of the deute rium label. In a complementary set of experiments, GC-mass spectrometr ic analysis of the individual olefins arising from incubations of a 1: 1 mixture of [2-H-2, 2-C-13]GPP and unlabelled GPP confirmed the compl ete retention of deuterium and ruled out any intermolecular transfer o f the C-2 deuterium of GPP from one substrate to another. These result s, which are fully consistent with the currently accepted mechanism of monoterpene cyclization, are used to critically evaluate literature r eports based on natural abundance deuterium NMR analysis of cyclic mon oterpenes which had indicated substantial depletion of deuterium at th e carbon atom corresponding to C-2 of GPP.