The stereoisomeric 2,3-cis-and 2,3-trans-3-methoxytricyclo [ 6.2.2.02,7 ] dodeca-9-enes endo-1 and exo-1 (endo and exo refer to the methoxy group) exhibit di †erent behavior under electron ionization (EI) : the m/z 80 cyclohexa-1,3diene radical cation formed by retro-Diels-Alder (RDA) fragmentation is the most abundant ion in the 70 eV mass spectrum of endo-1, whereas exo-1 exhibits preferential formation of an m/z 111 ion corresponding to the Omethylcyclohex-2-en-1-one structure (ion a), which may be obtained by an RDA fragmentation accompanied by a hydrogen migration (RDA Ô H), with the charge retained in the dienophile moiety. A similar e †ect has been observed in the EI mass spectra of the four stereoisomeric 3-ethoxy-6-methoxytricyclo[ 6.2.2.02,7 ] dodeca-9-enes 2 ; endo-2, with both endo-alkoxy groups, gives rise to the most abundant m/z 80 ion via the regular RDA process, whereas the other three stereoisomers, with at least one exo-alkoxy group, a †ord the most abundant m/z 155 ions via the RDA Ô H process, which correspond to the 4-alkoxy-substituted analogues of the m/z 111 ion a obtained from exo-1. Collision-induced dissociation measurements and a deuterium labeling study showed that the m/z 155 ions obtained from the two trans-diethers (trans-2a and trans-2b) have isomeric structures b and c (a mixture of b and c is formed in the case of exo-2), and that the highly stereospeciÐc RDA Ô H process involves a double hydrogen transfer, one from position 4 to the diene moiety and the other from position 3 to 4. The above stereospe-ciÐc behavior shows that the thermodynamically favored RDA Ô H process has a higher activation energy than the regular RDA fragmentation in the case of endo-1 and endo-2. In all other isomers, which have at least one exo-alkoxyl, the activation energy of the RDA Ô H process is lower than that of RDA. The latter e †ect is ascribed to anchimeric assistance of the alkoxyl in the initial CÈC bond cleavage in the stepwise RDA Ô H process, which is possible only when at least one alkoxyl has the exo conÐguration.