Description:
<jats:title>Abstract</jats:title><jats:p>With the aid of isotope labeling and by substituent shifts, the relatively strong <jats:italic>m/z</jats:italic> 91 ion in the mass spectra of 7‐chloro‐2‐methoxy‐5‐phenyl‐3<jats:italic>H</jats:italic>‐1,4‐benzodiazepine and related compounds was shown to contain the 5‐phenyl ring and the 3‐CH<jats:sub>2</jats:sub> group. Mechanisms involving the opening of the 7‐membered ring and the migration of the phenyl ring from C‐5 to C‐3 are postulated for the formation of this ion. This rearrangement ion was also observed in the mass spectra of some 1‐alkyl‐7‐chloro‐1,3‐dhydro‐5‐phenyl‐2<jats:italic>H</jats:italic>‐1,4‐benzodiazepin‐2‐ones.</jats:p>