Mass spectrometric studies on 4‐aryl‐1‐cyclopropyl‐1,2‐dihydropyridinyl derivatives: an examination of a novel fragmentation pathway

Examination of the electrospray ionization product ion spectra of 1,2‐dihydropyridinyl and 4‐aryl‐1,2‐dihydropyridinyl derivatives bearing a 1‐cyclopropyl or 1‐ trans ‐2‐phenylcyclopropyl group has led to the characterization of unexpected fragment ions. For example, the base peak at m / z 156 present in the product ion spectrum of trans ‐1‐(2‐phenylcyclopropyl)‐4‐phenyl‐1,2‐dihydropyridine proved not to be the expected 4‐phenylpyridinium species but rather the isomeric 3‐phenyl‐5‐azoniafulvenyl species. The results of studies with a series of structural and isotopically labeled analogs require a novel fragmentation pathway to account for the formation of this and related fragment ions. One possible pathway is based on an initial 1,5‐sigmatropic shift of a cyclopropylmethylene hydrogen atom that is accompanied by opening of the cyclopropyl ring. The resulting eniminium intermediates then fragment to yield the 5‐azoniafulvenyl species. Copyright © 2006 John Wiley & Sons, Ltd.