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[6.2.1.1^3,6.0^2,7]‐dodec‐2(7)‐ene‐9,10‐exo‐dicarboxylic anhydride

Abstract

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.

Face Selectivity of the Diels‐Alder Additions of Exocyclic Dienes Grafted onto 7‐Oxabicyclo[2.2.1]heptanes

“…-The stereoselectivity of the additions of TCE to the dienes 9, 10, 13 and tetraene 18 and of N-phenyltriazolinedione (NPTAD) to 13 are reported in Table 2. In all five reactions the exoface was preferred in contrast with the endo-face selectivity reported for the DielsAlder additions of cyclopentadiene [4] [5] and furan [2] [7] annulated to bicyclo-[2.2. llheptane systems.…”

“…The syn-1 1 -oxasesquinorbornenes 5 and 7 appeared to the more stable than their anti-isomer 6 and 8 [2] [714). This was attributed [7] to a 'synergic' effect of the polarization of the double bond n-electron density in the exo-face of the 2-bicyclo [2.2. llheptene and 7-oxabicyclo [2.2.1]hept-2-ene subsystems joined together by the same C (2), C (7) double bond [6] [8]. Thus, the kinetic endo-face Diels-Alder selectivity of 4 was parallel to the thermodynamic stereoselectivity, in agreement with the Bell-Evans-Polanyi principle [9].…”

“…This might also be the case with at least some of the additions of 1. To our knowledge, there has been until now no case of diene grafted onto a bicyclic skeleton [5][6][7][ 101 where the kinetic face selectivity of its cycloadditions had been proven not to be governed by the stability of the adducts.…”

Benzocantharidin

“…However, careful examination reveals a slight (ca 2.5 °) bend in the carbocyclic n system if C(1)C(4)C(5)C(6) and C(5)C(6)C(7)C(8) are analyzed separately. Such distortion of the planarity of the n-bond of the bicyclo[2.2.1] system away from the one-atom bridge would be in agreement with somewhat larger distortions which have been observed (Watson, Galloy, Bartlett & Roof, 1981;Hagenbuch, Vogel, Pinkerton & Schwarzenbach, 1981) and theoretically calculated (Wipff & Morokuma, 1980) in norbornene systems, leading to an increase in n-electron density on the exo side. Since this uneven n-electron distribution may play a role in the preferential reactivity of the norbornene double bond at the exo face, the possibility of n-bond non-planarity and the resulting effects on the reactivity of the carbocyclic n-system of benzocantharidin should be considered when examining its chemistry.…”

Face Selectivity of the Diels‐Alder Reaction of 5,6‐Bis((D)methylidene)‐2‐bicyclo[2.2.2]octene

“…Bartlett et al reported, however, that the feeble face selectivity for the cycloadditions of maleic anhydride (MA) to 1 can be reversed by changing the solvent [28]. When applied to the Diels-Alder additions of the furan derivative 2 (endo-face preferred under conditions of kinetic and thermodynamic control) [29] and of the exocyclic dienes 3 (X = CI ((E) and (2) isomer), OCH3 ( ( E ) isomer) and 4 (exo-face preferred) [30], the PMO approach proposed by Paguette & Gleiter gave predictions at variance with the face selectivities observed [3 11. Furthermore, the shape of the numerous subHOMO's that should be considered in this type of analysis were, in our hands [3 11, dependent upon the calculation technique (ab initio STO3G, MNDO, MIND0/3, EHT) and upon small variations in the geometry of the dienes.…”