Stereospecific elimination of dihydropyran from protonated tetrahydropyranyl (THP) difunctional derivatives upon chemical ionization and collision-induced dissociation: intramolecular interactions in polyfunctional ions

Abstract

Tetrahydropyranyl (THP) ethers of cis-1-methoxymethyl-, 1-benzyloxymethyl-, and 1-hydroxymethyl-4-cyclohexane methanols and of cis-1,4-dihydroxycyclohexane, and THP ester of cis-1,3-cyclohexane dicarboxylic acid (cis-3–cis-7, respectively) exhibit low abundance MH+ ions and undergo efficient elimination of dihydropyran (DHP) upon isobutane chemical ionization (CI). In contrast to this behavior, the trans isomers give rise to highly abundant MH+ ions, whereas the elimination of dihydropyran affords low abundance [MH−DHP]+ ions. A similar stereospecific behavior has been also observed under collision-induced dissociation (CID) conditions. Tetrahydropyranylium ion (m/z 85), obtained by a simple C-O bond dissociation, is abundant in the CI and CID mass spectra of both stereoisomers in all the examined systems. The high stereoselectivity suggests intermediacy of internal proton bridging of the two adjacent basic sites in the formation of the [MH−DHP]+ ions from the MH+ ions of the cis isomers. Thermochemical analysis indicates stabilized proton bridged structures for the [MH−DHP]+ ions. A mechanistic pathway has been proposed for the DHP elimination, based on the stereospecificity of this process and on its thermochemistry.