SummaryThe face selectivity (endo-face vs. exo-face attack onto the exocyclic s-cis- It is ca. 95/5, 75/5, 70130, 60/40 and 50/50 for N-phenyltriazolinedione (NPTAD), tetracyanoethylene (TCE), dimethyl acetylenedicarboxylate (DMAD), maleic anhydride (MA) and singlet oxygen ('02), respectively. The endo-face preference is probably due to a participation of the homoconjugated double bond at C (2), C (3) which makes the etheno bridge more polarizable than the ethano bridge in 11. The absence of face selectivity with '02 is consistent with an entropy-controlled mechanism involving the intermediacy of an exciplex.
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