Claisen rearrangement in phenyl allenyl ethers on electron impact

Phenyl allenyl ethers undergo Claisen rearrangement under electron impact conditions producing intense [M  CO] ions. Another competing fragmentation pathway in these compounds is an intramolecular aromatic substitution followed by the loss of H˙ or the substituent to give rise to fragments corresponding to benzopyrilium cations. Electron‐withdrawing substituents on the phenyl ring favour the cyclization reaction. An interesting ortho interaction of the nitro group with the allenic moiety leads to the expulsion of CO2 from the molecular ion. The proposed fragmentation pathways and the ion structures are established with the help of linked‐scan spectra (B/E, B2/E), collision activation decomposition spectra and high‐resolution data. Copyright © 1992 John Wiley & Sons, Ltd.