Stereospecific multi-step alcohol elimination involving 1,4-methoxyl migration from the MH+ ions of stereoisomeric 3,6-dialkoxytricyclo[6.2.2.02,7]dodeca-9-enes

Abstract

It has recently been shown that in the 3-methoxytricyclo[6.2.2.02,7]dodeca-9-ene system the 2,3-cis-isomer (endo-1) undergoes a unique multi-step methanol elimination under chemical ionization (CI) and collision-induced dissociation (CID) conditions, involving a 1,4-migration of a methoxy group from position 3 to 10, which is possible only in that particular stereoisomer. The epimeric 2,3-trans-stereoisomer (exo-1) and saturated analogues undergo elimination of MeOH by different pathways. In the present work the mechanistic pathways of alcohol elimination were examined in analogous stereoisomeric protonated mixed ethyl–methyl 3,6-diethers (2) by an extensive deuterium labeling and CID study. The cis-endo-isomer endo-2 with both endo-alkoxy groups undergoes methanol and ethanol elimination partly by the multi-step pathway, involving the 1,4-migration of an alkoxy group from position 3 or 6 to 10 or 9, respectively, under both CI and CID conditions. In the two trans-diethers, the elimination of alcohol involving the endo-alkoxyl occurs mainly via the multi-step pathway involving the 1,4-migration, whereas that involving the exo-alkoxy group takes place by different routes, which have been also investigated. The results of this work show the diversity and often complexity of mechanistic pathways of alcohol elimination from protonated cyclic secondary ethers, where the simple C—O bond cleavage leading to a secondary cation is a relatively high-energy process. Copyright © 1999 John Wiley & Sons, Ltd.