Differentiation of isomeric 2-aryldimethyltetrahydro-5-quinolinones by electron ionization and electrospray ionization mass spectrometry

Abstract

A series of isomeric 2-aryl-6,6-dimethyltetrahydro-5-quinolinones (set I) and 2-aryl-7,7-dimethyltetrahydro-5-quinolinones (set II) were studied under positive ion electron ionization (EI) and electrospray ionization (ESI) techniques. Under EI conditions, the molecular ions were found to be less stable in set I isomers, and they resulted in abundant fragment ions, i.e., [M–CH₃]⁺, [M–CO]^+., [M–HCO]⁺, [M–(CH₃,CO)]⁺, and [M–(CH₃,CH₂O)]⁺, when compared with set II isomers. In addition, the set I isomers showed specific fragment ions corresponding to [M–OH]⁺ and [M–OCH₃]^+. The retro-Diels-Alder (RDA) product ion was always higher in set II isomers. The ESI mass spectra produced [M ⁺ H]⁺ ions, and their decomposition showed favorable loss of CH₃ radical, CH₄ and C₂H₆ molecules in set I isomers. The set II isomers, however, showed predominant RDA product ions, and specific loss of H₂O. The selectivity in EI and ESI was attributed to the instability of set I isomers by the presence of a gem-dimethyl group at the α-position, and it was supported by the data from model compounds without a gem-dimethyl group. Density functional theory (DFT) calculations successfully corroborated the fragmentation pathways for diagnostic ions. This study revealed the effect of a gem-dimethyl group located at the α-position to the carbonyl having aromatic/unsaturated carbon on the other side of the carbonyl group.