STEREOSPECIFIC ION-MOLECULE REACTIONS OF NUCLEOPHILIC GAS-PHASE REAGENTS WITH PROTONATED BIFUNCTIONAL TETRACYCLIC TERPENE EPIMERS IN THE TRIPLE QUADRUPOLE COLLISION CELL

Various nucleophilic reagents (methanol, acetone, ammonia and trimethy lamine), characterized by different proton affinities, were introduced into the collision cell of the triple quadrupole as reactive gases fo r collisionally activated reaction (CAR) studies on stereoisomeric tet racyclic terpenes containing acetal and cyclobutanol functions. Under low-energy (E-lab similar to 3 eV) and multiple collision conditions, proton transfer from protonated terpenes (MH+, selected by the first q uadrupole) to the reagent gas was observed in each case, at varying ef ficiencies. 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 pro cesses 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 t ransfer which is strikingly stereospecific. The underlying stereochemi cal effect is attributed not only to proton affinity differences which favor transfer from the exo epimer, but also to steric and kinetic fa ctors which are evidently highly unfavorable for the endo configuratio n. (C) 1997 by John Wiley & Sons, Ltd.