GC-MS analysis of the regioisomeric methoxy- and methyl-benzoyl-1-pentylindoles: Isomeric synthetic cannabinoids

GC/MSn analysis of the crude reaction mixtures from Friedel-Crafts acylation: Unambiguous identification and differentiation of 3-aroylbenzofurans from their 4- and 6-regioisomers

RATIONALE

3-Aroylbenzofurans and their 2-nitrophenyl derivatives constitute fundamental intermediates for the synthesis of target compounds with pharmaceutical properties. However, their preparation via the Friedel-Crafts acylation of 2-phenylbenzofurans, using Lewis acid as catalyst, often leads to mixtures of regioisomeric aroylbenzofurans that can be challenging to distinguish, thus preventing the reaction characterization.

METHOD

We report a method for the unambiguous identification and differentiation of the desired 3-benzoyl isomers from their 4- and 6-regioisomers in a crude reaction mixture using gas chromatography coupled to multiple-stage mass spectrometric (GC/MSⁿ ) analysis performed in collision-induced dissociation (CID) mode.

RESULTS

Upon electron ionization, each set of isomers displayed nearly identical mass spectra. MSⁿ revealed fragmentation patterns that varied in the location of the benzoyl group on the benzofuran scaffold: CID experiments performed on the molecular ion allowed the distinction of the 3-acyl isomers from the 4- and 6-regioisomers; CID experiments on the [M - Ar]⁺ ion allowed the distinction of the 4-benzoyl from the 6-benzoyl regioisomer, when the nitro group is located on the 2-phenyl ring. Moreover, the unusual loss of OH^• radical allowed ascertaining the position of the nitro group in 3-acyl regioisomers bearing the NO₂ group. The origin of the diagnostic OH^• loss was investigated through MSⁿ experiments using ¹⁸ O-labelled 3-benzoyl derivatives.

CONCLUSIONS

The method allows the rapid characterization of crude reaction mixtures of benzoylbenzofurans using solely GC/MSⁿ analysis, simplifying the workflow of extensive isolation and purification for structure elucidation.

GC-MS and GC-IR Analyses of the Methoxy-1-n-pentyl-3-(1-naphthoyl)-indoles: Regioisomeric Designer Cannabinoids

The indole ring regioisomeric methoxy-1-n-pentyl-3-(1-naphthoyl)-indoles represent indole ring-substituted analogs of the synthetic cannabinoid JWH-018. The electron ionization mass spectra show equivalent regioisomeric major fragments resulting from cleavage of the groups attached to the central indole nucleus. The characteristic (M-17)+ fragment ion at m/z 354 resulting from the loss of OH group is significant in the mass spectra of all four compounds. Fragmentation of the naphthoyl and/or pentyl groups yields the cations at m/z 314, 300, 244 and 216. The vapor-phase infrared spectra provide a number of characteristic absorption bands to identify the individual isomers. Gas chromatographic separations on a capillary column containing a film of trifluoropropylmethyl polysiloxane (Rtx-200) provided excellent resolution of these compounds, their precursor indoles and intermediate pentylindoles. The elution order appears related to the degree of crowding of indole ring substituents.

Analytical studies on the 2-naphthoyl substituted-1-n-pentylindoles: Regioisomeric synthetic cannabinoids

The six 1-n-pentyl-2-, 3-, 4-, 5-, 6- and 7-(2-naphthoyl)-indoles each have the same substituents attached to the indole ring, identical elemental composition (C₂₄H₂₃NO) yielding identical nominal and accurate masses. The electron ionization mass spectra of the 2-naphthoyl substituted isomers share equivalent major fragment ions resulting from cleavage of the groups attached to the central indole nucleus with some differences in relative abundances. These six regioisomers were successfully resolved on an Rtx-5 and Rxi-17Sil MS stationary phases and the molecules having both substituent groups on the same side of the indole ring (1,2- and 1,7-substituents) show the least retention. The more linear molecules have higher relative retention properties. A comparison of the GC properties of the 1-naphthoyl- and 2-naphthoyl groups attached at identical positions of the indole ring showed higher GC retention for the 2-naphthoyl substituted isomer in all cases evaluated. The amide inverse isomers (1-naphthoyl-3-n-pentylindoles) were separated from the 1-n-pentyl-3-naphthoyl-indoles on an Rtx-200 stationary phase. The two inverse amide isomers having the 1- and 2-naphthoyl groups substituted at the 1-position of the indole ring elute before either of the N-alkyl-indole isomers having the 1- and 2-naphthoyl groups substituted at the 3-position of the indole ring. The amide inverse isomers yield EI mass spectra easily distinguishing these amides from the ketone isomers having the naphthoyl groups at the indole 3-position.

The chemistry and pharmacology of synthetic cannabinoid SDB-006 and its regioisomeric fluorinated and methoxylated analogs

Synthetic cannabinoids are the largest and most structurally diverse class of new psychoactive substances, with manufacturers often using isomerism to evade detection and circumvent legal restriction. The regioisomeric methoxy- and fluorine-substituted analogs of SDB-006 (N-benzyl-1-pentyl-1H-indole-3-carboxamide) were synthesized and could not be differentiated by gas chromatography-mass spectrometry (GC-MS), but were distinguishable by liquid chromatography-quadrupole time-of-flight-MS (LC-QTOF-MS). In a fluorescence-based plate reader membrane potential assay, SDB-006 acted as a potent agonist at human cannabinoid receptors (CB1 EC50 = 19 nM). All methoxy- and fluorine-substituted analogs showed reduced potency compared to SDB-006, although the 2-fluorinated analog (EC50 = 166 nM) was comparable to known synthetic cannabinoid RCS-4 (EC50 = 146 nM). Using biotelemetry in rats, SDB-006 and RCS-4 evoked comparable reduction in body temperature (~0.7°C at a dose of 10 mg/kg), suggesting lower potency than the recent synthetic cannabinoid AB-CHMINACA (>2°C, 3 mg/kg).

Molecular recognition of pseudodistamine isomeric precursors trans-3(4)-aminopiperidin-4(3)-ols by EI mass spectrometry

Synthesis of drugs, biologically active compounds or their derivatives always requires precise and reliable method of their identification, including differentiation of the possible isomers. Pseudodistamines and their precursors became a matter of elevated attention due to their different enzymatic inhibition. This paper deals with one of the groups of the pseudodistamine precursors - trans-3(4)-aminopiperidin-4(3)-ols. Their synthesis brings to a mixture of 2 regioisomers, resulting in the necessity of their reliable recognition. NMR spectroscopy commonly used by organic chemists requires advance knowledge and experience to analyse the spectra of these regioisomers. Therefore, we herein proposed a simpler way to recognize trans-3(4)-aminopiperidin-4(3)-ols using mass spectrometry with electron ionization. Fragmentation of 4 pairs of aminopiperidinol regioisomers with variation of amine moiety was studied. The obtained results allowed defining a group of 3 ions ([M-18]^+., [M-19]⁺, [M-43]⁺) related only to the structure of trans-4-aminopiperidin-3-ols and 1 ion (m/z 100) related to the structure of trans-3-aminopiperidin-4-ols. Besides, interrogation of intensity of ions common for spectra of both regioisomers allows making differentiation as well.

A Novel Tetrahydrocannabinol Electrochemical Nano Immunosensor Based on Horseradish Peroxidase and Double-Layer Gold Nanoparticles

In the current study, a novel double-layer gold nanoparticles-electrochemical immunosensor electrode immobilized with tetrahydrocannabinol (THC) antibody derived from Balb/c mice was developed. To increase the fixed quantity of antibodies and electrochemical signals, an electrochemical biosensing signal amplification system was utilized with gold nanoparticles-thionine-chitosan absorbing horseradish peroxidase (HRP). In addition, a transmission electron microscope (TEM) was used to characterize the nanogold solution. To evaluate the quality of the immunosensor, the amperometric I-t curve method was applied to determine the THC in PBS. The results showed that the response current had a good linear correlation with the THC concentration range from 0.01~10³ ng/mL with a correlation coefficient of 0.9986. The lowest detection limit for THC was 3.3 pg/mL (S/N = 3). Moreover, it was validated with high sensitivity and reproducibility. Apparently, the immunosensor may be a very useful tool for monitoring the THC.

Analytical differentiation of quinolinyl- and isoquinolinyl-substituted 1-(5-fluoropentyl)-1H-indole-3-carboxylates: 5F-PB-22 and its ten isomers

Differentiation among regioisomers of synthetic cannabinoids in forensic drug analysis is a crucial issue, since all isomers are not regulated by law. New equivalent analogs obtained via minor modification of their preexisting molecules keep on emerging. Isomers formed via substitutional exchange are also a cause for concern. This study is focused on the isomeric molecules that stem from minor modifications of 5F-PB-22. The analytical properties of these molecules and methods of differentiation are reported. Scan mode analysis using gas chromatography-electron ionization-mass spectrometry (GC-EI-MS) was performed using the authentic 5F-PB-22 standard, five regioisomeric quinolinyl ester indoles, and five regioisomeric isoquinolinyl ester indoles. Because it was not possible to separate 5F-PB-22 from the 5-hydroxyquinoline isomer using GC and all analytes showed similar EI mass spectra, liquid chromatography (LC)-tandem mass spectrometry analysis was performed. Using LC, a successful separation of 5F-PB-22 from all isomers could be achieved. Based on the electrospray ionization-mass spectra, the protonated molecular ion at m/z 377.2 was selected as the precursor ion for the regioisomeric and structural isomeric differentiation. Collision-induced dissociation provides relative intensity differences in the product ions among the isomers, enabling mass spectrometric differentiation of the isomers. To our knowledge, this is the first report on mass spectrometric differentiation of 5F-PB-22 and its ten isomers.