Structural confirmation of position isomers 2-(2-methylaminoprolyl)benzofuran and 5-(2-methylaminopropyl)benzofuran: a combined mass spectrometric and computational study.

Rationale: Phenylethylamines are one of the most common types of new psychoactive substances, following synthetic cannabinoids and synthetic cathinones. They are regulated in many countries because of their strong hallucinogenic effects, which can cause serious nerve damage. There is a wide variety of phenylethylamines, exhibiting rapid renewal and extremely similar structures, therefore accurate qualitative analysis of isomers is a difficult problem in current drug analysis.
Methods: The dissociation pathways of the position isomers 2-(2-methylaminoprolyl)benzofuran (2-MAPB) and 5-(2-methylaminopropyl)benzofuran (5-MAPB) were investigated by gas chromatography-mass spectrometry and liquid chromatography coupled to high-resolution quadrupole Orbitrap MS. The dissociation patterns of the phenethylamine-based designer drugs 2-MAPB and 5-MAPB were explored and extended in this work based on MS combined with density functional theory studies.
Results: For electron ionization mass spectrometry (EI-MS) analysis, the dissociation patterns of 2-MAPB were similar to those of 5-MAPB. For electrospray ionization mass spectrometry (ESI-MS ⁿ ) analysis, the hydrogen atom on amino group was facile to form a intramolecular hydrogen bond with the oxygen atom on the parent nucleus of benzofuran in the structure of 2-MAPB, leading to higher abundance of the product ion at m/z 58. However, there was a conjugated system between the positive charge formed by the cleavage of the 5-MAPB side chain and the benzofuran ring, enabling the 5-MAPB to generate a product ion at m/z 131. Computational study showed that energy barrier and spin density difference distribution jointly control the selective dissociation in EI-MS, while different types of orbital interaction induced by intramolecular hydrogen bond led to different dissociation results in ESI-MS ⁿ .
Conclusions: These different dissociation patterns could be used to distinguish 2-MAPB from 5-MAPB. This could assist forensic laboratories in the differentiation and characterization of potential isomers in these kinds of compounds, especially in mixtures.
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