Ketalization of gaseous acylium ions.

A novel reaction of gaseous acylium ions: ketalization with diols and analogs, has been systematically studied via pentaquadrupole MS2 and MS3 experiments and ab initio calculations. A variety of alpha,beta-diols and their amino, thiol, ether, and thioether analogs have been tested for reactivity, mechanism evaluation, site selectivity, and for the effects of alpha- and beta-interfunctional separation. As for condensed-phase ketalization of neutral carbonyl compounds followed by hydrolysis, gaseous acylium ions are chemically deactivated in the form of cyclic ionic ketals by ketalization, and are efficiently released via on-line collision-induced dissociation. Ketalization of acylium ions is shown to identify and structurally characterize alpha,beta-diols and their analogs, and to distinguish regioisomers. Diastereomers can also be distinguished, as illustrated for cis and trans 1,2-diaminocyclohexane. The MS2 and MS3 data together with 18O-labeling and ab initio calculations establish for acylium ion ketalization a mechanism of anchimeric assistance with participation of the neighboring acyl group.