Synthesis and Structure of Cyclopropano-Annelated Homosesquinorbornene Derivatives Containing Pyramidalized Double Bonds: Evidence for the Sterical Effect of a Cyclopropyl Group on the Degree of CC Double-Bond Pyramidalization

endo- and exo-2,3,4,7-tetrahydro-1H-1,4-methanobenzocycloheptene-7-carboxylic acid ethyl esters
have been synthesized, and their Diels−Alder cycloaddition reactions with maleic anhydride,
dimethyl acetylenedicarboxylate and singlet oxygen have been investigated. The X-ray analysis of
four adducts indicated the pyramidalization of the central double bond. Density functional theory
calculations on the isolated products and model compounds showed excellent agreement between
the experimental and theoretical determined butterfly angles. Furthermore, it has been shown
that a cyclopropyl group fused to [2.2.2] system decreases significantly the degree of the
pyramidalization which is attributed to the steric interactions between the cyclopropyl group and
ethano bridge of the norbornene systems. Due to the instability of the bicyclic endoperoxides, their
X-ray analysis could not be carried out. DFT calculations on model compounds showed increased
bending in the case of the product obtained by the addition of singlet oxygen to endo-2,3,4,7-tetrahydro-1H-1,4-methanobenzocycloheptene-7-carboxylic acid ethyl ester.

内型（endo）和外型（exo）-2,3,4,7-四氢-1H-1,4-亚甲基苯并环庚烯-7-羧酸乙酯已被合成，并研究了它们与马来酸酐（maleic anhydride）、乙炔二羧酸二甲酯（dimethyl acetylenedicarboxylate）以及单线态氧（singlet oxygen）之间的狄尔斯-阿尔德（Diels−Alder）环加成反应。四种加合物的X射线衍射分析（X-ray analysis）表明，其中心双键存在金字塔化畸变（pyramidalization）。对分离得到的产物及模型化合物进行的密度泛函理论（DFT）计算显示，实验测定的蝶形角（butterfly angle）与理论计算值之间具有极佳的一致性。此外，研究发现稠合至[2.2.2]环系的环丙基（cyclopropyl group）可显著降低金字塔化程度，该现象归因于环丙基与降冰片烯系统（norbornene systems）的亚乙基桥之间的空间位阻相互作用（steric interactions）。由于双环内过氧化物（bicyclic endoperoxides）不稳定，未能对其开展X射线衍射分析。对模型化合物的DFT计算显示，当单线态氧与内型（endo）-2,3,4,7-四氢-1H-1,4-亚甲基苯并环庚烯-7-羧酸乙酯发生加成反应所得产物的弯曲程度有所增加。