Studies in Mass Spectrometry. XIII.¹ Stereospecific Electron Impact Induced Fragmentation Processes in Some Tricyclic Diesters

endo-Dimethyl esters la, 2a, 3a, and 4a undergo elimination of methanol under electron impact through a seven-centered transition state. Stereoisomeric exo diesters lb, 2b, 3b, and 4b do not appreciably eliminate methanol. Loss of methoxyl radical from the molecular ion has been observed instead. trans-Diesters lc, 2c, 3c, and 4c eliminate methanol under electron impact through a five-centered transition state. An additional stereospecific hydrogen migration accompanying retro-Diels-Alder fragmentation has been observed in the endo isomers.