Methyl and ethyl endo- and exo-bicyclo [2.2.2] oct-5-ene-2-carboxylates exhibit different mass spectral behaviour under i-Bu-chemical ionization (CI) conditions and upon collision-induced dissociation (CID) of their MH+ IONS. The endo-esters exhibit m/z 135 [MH MeOH]+ and [MH EtOH]+ ions, which are absent in the i-Bu-CI mass spectra of the exo-isomers. This stereospecific behavior is explained in terms of the proton bridging requirement in the alcohol elimination from MH+ ions of esters. Such proton bridging between the alkoxycarbonyl group and the double bond is possible in the endo-esters, but not in the exo-isomers. Hydrogen-deuterium exchange processes, observed in the CID spectra of the MD+ ion of the endo-esters, support the proposed mechanistic pathway. Hydrogen bridging with the double bond also plays a role in the alcohol elimination processes of stereoisomeric bicyclo [2.2.2] oct-5-ene-2,3-dicarboxylates.
甲基和乙基内型和外型
双环[2.2.2]辛-5-烯-2-羧酸酯在 i-Bu
化学电离 (CI) 条件下及其碰撞诱导解离 (CID) 下表现出不同的质谱行为
MH+ 离子。内酯显示 m/z 135 [
MH → MeOH]+ 和 [
MH → EtOH]+ 离子,这些离子在外型异构体的 i-Bu-Cl 质谱中不存在。这种立体定向行为可以用从酯的
MH+ 离子消除醇时的质子桥接要求来解释。烷氧基羰基和双键之间的这种质子桥联在内酯中是可能的,但在外异构体中则不然。在内酯 MD+ 离子的 CID 光谱中观察到的氢-
氘交换过程支持了所提出的机制途径。双键的氢桥也在立体异构双环[2.2.2]辛-5-烯-2,3-二
羧酸酯的醇消除过程中发挥作用。