Electron ionization-induced fragmentation of bridgehead-substituted norbornan-2-ones derived from fenchone

Abstract

Camphor- and fenchone-based bridgehead-substituted norbornan-2-ones constitute an interesting class of chiral synthons with applications in stereoselective synthesis, due to the wide variety of stereocontrolled transformations which they can undergo. Despite their synthetic value, few mass spectral studies have been realized for these compounds due to the lack of general procedures for their preparation. In this paper we have carried out an electron ionization (EI) mass spectral study of a series of heteroatomically bridgehead-substituted 7,7-dimethylnorbornan-2-ones. Their cleavage mechanisms have been rationalized on the basis of the substituent shifts, as well as on the identification of relevant peaks through accurate mass measurements and collision-induced dissociation (CID) tandem mass-spectrometric experiments. The driving force for the main fragmentation pathways of C(1)-O- and C(1)-N-substituted 7,7-dimethylnorbornan-2-ones is the α-cleavage of the C(1)C(2) bond directed by the bridgehead substituent. Further rearrangements and fragmentations of the distonic ions initially formed lead to the major ions in the spectra of all the compounds. A comparison with the known behaviour of their isomeric 3,3-dimethylnorbornan-2-ones reveals that the key bridgehead heteroatom effect controlling the fragmentation pathways can be modulated by the position of the gem-dimethyl group. As a consequence, the corresponding fragmentation patterns of both isomeric series are very different. In contrast, the fragmentation patterns of 7,7-dimethyl-2-oxo-1-norbornanesulphonamides, whose main fragmentation pathway involves homolytic SN bond cleavage, are almost identical to those displayed by their 3,3-dimethylated isomers.