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

Dinesh Kumar, Ch. ; Naresh Chary, V. ; Dinesh, A. ; Reddy, P. S. ; Srinivas, K. ; Gayatri, G. ; Sastry, G. Narahari ; Prabhakar, S. (2011) Differentiation of isomeric 2-aryldimethyltetrahydro-5-quinolinones by electron ionization and electrospray ionization mass spectrometry Rapid Communications in Mass Spectrometry, 25 (19). pp. 2815-2827. ISSN 0951-4198

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Official URL: http://onlinelibrary.wiley.com/doi/10.1002/rcm.517...

Related URL: http://dx.doi.org/10.1002/rcm.5175

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.

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Deposited On:28 Jul 2017 05:48
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