The extent of electron-impact-induced ring contraction in the formation of [M − CH3] fragments from N-acetylpiperidine

In N-acetylpiperidine, α-carbon atoms (C-2 or C-6) of the ring have been recently identified as a source of loss of CH₃ radicals from the molecular ion in addition to β-carbon atoms (C-3 or C-5) and the acyl substituent (C-8). Skeletal rearrangement by ring contraction to a five-membered cyclic intermediate was invoked to be responsible for the expulsion of C-2 or C-6. Deuterium labelling suggested major and approximately equal contributions from the two losses of ring atoms as compared to an only minor contribution from loss of C-8. ¹³C-labelling of the latter now establishes the correctness of this inference, demonstrating that only 17% of the total loss of methyl arise from this position. Together with results of previous deuterium labelling this figure indicates that ring contraction contributes to the formation of M ? CH₃] fragments to an extent of approximately 40%.