Isomeric differentiation of conjugated diene epoxides by polar [4+2(+)] Diels-Alder cycloaddition of acylium ions in an ion trap mass spectrometer

Polar [4 + 2(+)] Diels-Alder cycloaddition is performed using acylium catio ns as reagent ions for the characterization and differentiation of isomeric conjugated diene epoxides, The [M + RCO](+) cycloadduct that is produced i s unstable and dissociates not by retrocycloaddition, but by pathways favor ed by the epoxide ring that lead mainly to substituted pyrilium ions. These product ions display m/z ratios that correlate with either the epoxide (R- 1) or diene (R-2) substituents, thus serving as structurally diagnostic ion s for isomeric differentiation. The multiple tandem mass spectrometric (MSn ) capabilities of the quadrupole ion trap mass spectrometer (QITMS) were pa rticularly useful for designing multistep procedures sometimes required for reagent-ion preparation and isolation. They were also appropriate when an additional collisionally induced dissociation (CID) stage was desired to pr ovide further information on any product ion or for additional selectivity. (Int J Mass Spectrom 190/191 (1999) 253-264) (C) 1999 Elsevier Science B.V .