The role of the CC double bond in alcohol elimination from MH+ ions of unsaturated bicyclic esters upon chemical ionization

✍ Scribed by N. Khaselev; A. Mandelbaum

• Publisher: John Wiley and Sons
• Year: 1995
• Tongue: English
• Weight: 412 KB
• Volume: 30
• Category: Article
• ISSN: 1076-5174
• DOI: 10.1002/jms.1190301103

✦ Synopsis

Abstract

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.