Face Selectivity in the Diels‐Alder Additions and Chelotropic Addition of Sulfur Dioxide to 2‐(D)Methylidene‐3‐methylidenebicyclo[2.2.1]heptane

The stereoselectivity of the cycloadditions of 2‐(D)methylidene‐3‐methylidenebicyclo[2.2.1]heptane (4) to various dienophiles has been determined. The exo‐ vs. endo‐face selectivity depends on the type of dienophiles, and for olefinic ones, on the mode of attack (Alder‐ vs. anti‐Alder endo rule). It is > 9:1 with N‐phenyltriazolinedione (NPTAD) and ethylenetetracarbonitrile (TCNE), < 1:9 with dimethyl acetylenedicarboxylate (DMAD), 30 ± 5:70 ± 5 with DMAD in the presence of AlCl3, 15 ± 5:85 ± 5 with dehydrobenzene and 40 ± 5:60 ± 5 with 1O2 generated photochemically (Table 1). With para‐benzoquinone and maleic anhydride, the exo‐ vs. endo‐ face selectivity is < 1:9 and 20 ± 5:80 ± 5, respectively, for their anti‐Alder mode of attack; it is 50 ± 5:50 ± 5 and 55 ± 5:45 ± 5, respectively, for their Alder mode of reaction. Under conditions of kinetic control, the chelotropic addition of SO2 to 4 is endo‐face selective.