The π‐Anisotropy of the Double Bond of a syn‐11‐Oxasesquinorbornene Derivative.. Stereoselectivity of the Diels–Alder additions of (2‐norborneno)[c]furan. Crystal structure of 11‐oxa‐endo‐tetracyclo[6.2.1.13,6.02,7]‐dodec‐2(7)‐ene‐9,10‐exo‐dicarboxylic anhydride

The reaction of (2‐norborneno)[c]furan (4) with maleic anhydride gave 11‐oxa‐endo‐tetracyclo[6.2.1.13,6.02,7]dodec‐2(7)‐ene‐9,10‐exo‐dicarboxylic anhydride (5) and, with methyl acetylenedicarboxylate, methyl 11‐oxa‐endo‐tetracyclo [6.2.1.13,6.02,7]dodeca‐2(7),9‐diene‐9,10‐dicarboxylate (7). The syn‐11‐oxa‐sesquinorbornenes 5 and 7 could be equilibrated with their cycloaddents. They are at least 2 kcal/mol more stable than the corresponding anti‐sesquinorbornenes 6 and 8. The structure of 7 was deduced from its spectral data, by epoxidation with air or a peracid to give the exo‐epoxide 13 and by catalytic hydrogenation to give 14. The structure of 5 was established by single‐crystal X‐ray diffraction. A dihedral angle of 163° was measured between the C(1,2,7,8) and C(2,3,6,7) planes in 5. This important deviation from planarity for the C(2,7) double bond is attributed to (π, ω)‐repulsive interactions that make the π‐electron density of 2‐norbornene and 7‐oxa‐2‐norbornene derivatives preferentially polarized toward the exo‐face. This finding is discussed in relation with the relative stability of the syn‐ and anti‐ 11‐oxasesquinorbornenes and with the endo‐stereoselectivity of the cycloadditions of the norbornenofuran 4.