206436-05-7Relevant articles and documents
Stereospecific retro-diels-alder fragmentation of stereoisomeric 3- methoxy- and 3,6-dialkoxytricyclo[6.2.2.02,7]dodeca-9-enes upon electron ionization
Morlender-Vais,Mandelbaum
, p. 229 - 241 (2007/10/03)
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 different behavior under electron ionization (EI): the m/z 80 cyclohexa-1,3-diene 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 O-methylcyclohex-2-en-1-one structure (ion a), which may be obtained by an RDA fragmentation accompanied by a hydrogen migration (RDA - H), with the charge retained in the dienophile moiety. A similar effect 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, afford the most abundant m/z 155 ions via the RDA - 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 stereospecific RDA - 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 stereospecific behavior shows that the thermodynamically favored RDA - 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 - H process is lower than that of RDA. The latter effect is ascribed to anchimeric assistance of the alkoxyl in the initial C - C bond cleavage in the stepwise RAD - H process, which is possible only when at least one alkoxyl has the exo configuration.
