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ENDO isomers of N-phenyl-5-norbornene-2,3-dicarboximide was converted to exo-isomers below retro-Diels-Alder temperature under various reaction conditions. The exo-endo yield ratio was studied by NMR and the 1H and 13C NMR chemical shifts of the isomers are reported. While no intermediate could be isolated using several different radical trapping agents, the results show that 7,5 proton chemical shift is a possible isomerization mechanism.
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Wong et al. (1980) studied this question.
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