Stereospecific Ion-Molecule Reactions of Nucleophilic Gas-phase Reagents with Protonated Bifunctional Tetracyclic Terpene Epimers in the Triple Quadrupole Collision Cell

Abstract

Various nucleophilic reagents (methanol, acetone, ammonia and trimethylamine), characterized by different proton affinities, were introduced into the collision cell of the triple quadrupole as reactive gases for collisionally activated reaction (CAR) studies on stereoisomeric tetracyclic terpenes containing acetal and cyclobutanol functions. Under low-energy (Elab∽3 eV) and multiple collision conditions, proton transfer from protonated terpenes (MH+, selected by the first quadrupole) to the reagent gas was observed in each case, at varying efficiencies. At the same time, collision-induced decompositions (CIDs) were observed as competing processes in each tandem mass spectrum. For methanol, the reaction gas having the lowest proton affinity, CID processes were heavily favored, whereas CAR processes dominated only for trimethylamine (highest proton affinity) reacting with the exo terpene epimer. The latter reagent gas receives the proton in an exothermic transfer which is strikingly stereospecific. The underlying stereochemical effect is attributed not only to proton affinity differences which favor transfer from the exo epimer, but also to steric and kinetic factors which are evidently highly unfavorable for the endo configuration. © 1997 by John Wiley & Sons, Ltd.