GC‐MS and GC‐IRD studies on the six‐ring regioisomeric dimethoxybenzylpiperazines (DMBPs)

Abstract

Gas chromatography with infrared detection (GC-IRD) provides direct confirmatory data for the differentiation between the six regioisomeric aromatic ring substituted dimethoxybenzylpiperazines (DMBPs). These regioisomeric substances are resolved by GC and the vapour-phase infrared spectra clearly differentiate among the six dimethoxybenzyl substitution patterns. The mass spectra for these regioisomeric substances are almost identical. With only the 2,3-dimethoxy isomer showing one unique major fragment ion at m/z 136. Thus mass spectrometry does not provide for the confirmation of identity of any one of these compounds to the exclusion of the other isomers. Perfluoroacylation of the secondary amine nitrogen for each of the six regioisomers gave mass spectra showing some differences in the relative abundance of fragment ions without the appearance of any unique fragments for specific confirmation of structure. Gas chromatography coupled with time-of-flight mass spectrometric detection (GC-TOF) provided an additional means of confirmation of the elemental composition of the major fragment ions in the mass spectra of these compounds.

Gas chromatography/mass spectrometry analysis of the six‐ring regioisomeric dimethoxybenzyl‐N‐methylpiperazines (DMBMPs)

“…The mass spectrum in Fig. provides support that the fragmentation pathway for these N ‐methylpiperazines in this study is equivalent to the previously reported monosubstituted dimethoxybenzylpiperazines . The mass spectrum in Fig.…”

“…This ion is a characteristic fragment for the piperazine ring and is almost universal in all N ‐1‐monosubstituted piperazines (N‐4 substituent is hydrogen). This m/z 56 ion yielded a mass shift of +6 Da when d 8 ‐piperazine was used to label all the four carbons of the piperazine ring . Thus, the m/z 56 ion was confirmed to contain six hydrogen atoms and three of the carbons from the original piperazine portion of these molecules; this confirmation was based on studies in the dimethoxybenzylpiperazines (N‐4 = H).…”

“…(A) and shows that the m/z 56 species did not undergo a mass shift as a result of the N‐CD 3 group. Thus, the N‐4 methyl group is not a part of the m/z 56 ion and indicates that the methyl group migration is consistent with the N‐H migration in the monosubstituted series . The use of piperazine‐D 8 in the monosubstituted series produced a mass shift of +6 Da for the ion in question yielding the C 3 D 6 N + cation at m/z 62.…”

“…This ion essentially represents the molecular equivalent of a methyl group loss from the m/z 151 cation to form a peak of intensity approximately equal to the base peak for the 2,3‐dimethoxybenzyl regioisomer. Previous carbon‐13‐ and deuterium‐labeling studies in the monosubstituted (N‐4 = H) dimethoxybenzylpiperazines showed that the methyl group lost to form the m/z 136 species is from the methoxy group at the 3‐position of the aromatic ring. The resonance‐stabilizing effects of the 2‐methoxy group favor the loss of the methyl group from the 3‐position.…”

“…The resonance‐stabilizing effects of the 2‐methoxy group favor the loss of the methyl group from the 3‐position. A possible mechanistic pathway for the formation of the m/z 136 ion has been described . The mass spectrum in Fig.…”