GC-MS differentiation of the six regioisomeric dimethoxybenzoyl-1-pentylindoles: Isomeric cannabinoid substances

Abstract

The six regioisomeric 1-pentyl-3-dimethoxybenzoylindoles can be differentiated by a combination of EI-MS and FT-IR spectra. The six regioisomeric 1-n-pentyl-3-(dimethoxybenzoyl)-indoles represent potential designer modifications in the synthetic cannabinoid drug category. The analytical properties and methods of regioisomeric differentiation were developed in this study. The base peaks in these six spectra allow these compounds to be subdivided into three groups of two compounds each, the m/z 334 ion is the base peak for the 2,4- and 2,6-dimethoxybenzoyl isomers (compounds 2 and 4), the 2,3- and 2,5-dimethoxybenzoylindole isomers (compounds 1 and 3) show the m/z 200 ion of base peak intensity and the 3,4- and 3,5-isomers (compounds 5 and 6) show the molecular ion as the base peak, m/z 351. The four isomers having a methoxy group substituted at the ortho position show a unique fragment ion occurring at [M-17](+). An interesting fragment ion at m/z 200 is significant in the 2,3 and 2,5 isomers and completely absent in the 3,4 and 3,5 isomers. Minor peaks for m/z 200 appear in the mass spectra of the 2,4 and 2,6-isomers. This set of regioisomeric compounds was well resolved by capillary gas chromatography on a dimethylpolysiloxane stationary phase. The elution order appears related to the degree of substituent crowding in the dimethoxybenzoyl group. FTIR spectra provide useful data for differentiation among these regioisomeric compounds. Infrared absorption spectral data provide distinguishing and characteristic information to individualize the regioisomers in this set of compounds.

Keywords: FTIR; Forensic chemistry; GC–MS; Indole derivatives; Regioisomers; Synthetic cannabinoids.