Abstract
The eilmination of methanol from the MD + ion of 2,3-cis-3-methoxytricyclo[6.2.2.0 2,7 ]dodeca-9-ene, endo-2, upon chemical ionization (CI) gives rise to both [MD - MeOD] + and [MD - MeOH] + ions. Only the [MD - MeOH] + ion is formed under collision-induced dissociation (CID) conditions. This is in contrast with the behavior of the 2,3-trans-stereoisomer exo-2 and with saturated analogs which undergo exclusive elimination of MeOD. The unusual elimination of MeOH from endo-2 indicates transfer of the external deuteron in the MD + ion from the oxygen atom to the interior of the organic moiety and a back transfer of a hydrogen from the organic moiety to the oxygen atom prior to the C-O bond dissociation step. A deuterium labeling study showed that the hydrogen atom involved in this elimination process originates at position 3 (formal 1,1-elimination). These results suggest a multi-step mechanism for this unique stereospecific methanol elimination, initiated by a proton transfer from the methoxy group to the double bond followed by a 1,4-methoxyl migration from C-3 to C-10. The proposed mechanism finds support in the Cl and CID study of deuterium-labeled analogs which have a methoxy group at position 10.
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