Monoterpene cyclization mechanisms and the use of natural abundance deuterium NMR—short cut or primrose path?

Abstract

Separate incubations of [2- 2H, 2- 13C]geranyl diphosphate (GPP) with (+)- and (−)-pinene cyclases isolated from common sage ( Salvia officinalis) gave a mixture of acyclic and cyclic monoterpene olefins. Mass spectrometric analysis of the derived labelled monoterpenes established that cyclization had taken place with complete retention of the deuterium label. In a complementary set of experiments, GC-mass spectrometric analysis of the individual olefins arising from incubations of a 1:1 mixture of [2- 2H, 2- 13C]GPP and unlabelled GPP confirmed the complete retention of deuterium and ruled out any intermolecular transfer of the C-2 deuterium of GPP from one substrate to another. These results, which are fully consistent with the currently accepted mechanism of monoterpene cyclization, are used to critically evaluate literature reports based on natural abundance deuterium NMR analysis of cyclic monoterpenes which had indicated substantial depletion of deuterium at the carbon atom corresponding to C-2 of GPP.