Configurational assignment of fragment ions by CID. Stereochemistry and mechanism of retro‐Diels–Alder fragmentation accompanied by hydrogen transfer in gas‐phase cations

Low energy CID mass spectra of m / z 173, C 8 H 13 O 4 + , obtained from the diethyl ester of cis,syn,cis ‐l,2,3,3a,4,5,5a,6,7,8‐decahydroindacene‐4,5‐dicarboxylic acid and cis,syn,cis ‐l,2,3,4,5,6,7,8,8a,9,10,10a‐dodecahydrophenanthrene‐9,10‐dicarboxylic acid indicate they have the structure of protonated diethyl maleate. This finding together with previous deuterium labelling results suggest that the formation of this ion from both precursors takes place by migration of a hydrogen atom from an allylic δ‐position followed by the concerted cleavage of two allylic CC bonds in analogy to the ground state retro‐Diels–Alder fragmentation.