Synthesis and absolute configuration of cyclic synthetic cathinones derived from α-tetralone
Chirality, Volume 36, Issue 2: Synthesis and absolute configuration of cyclic synthetic cathinones derived from α-tetralone
The goal of this study is to explore and characterize new cyclic α-tetralone-based derivatives of synthetic cathinones, which have both pharmaceutical potential and implications for drug regulation. Given the continuous emergence of new synthetic cathinones as legal alternatives to controlled substances, this research aims to synthesize novel derivatives and verify their chemical structures.
The study employs high-performance liquid chromatography (HPLC) with a circular dichroism (CD) detector to achieve chiral separation and directly obtain CD spectra of the enantiomers. Additionally, density functional theory (DFT) calculations are used to determine stable conformers in solution, enabling accurate simulation of their spectra. By combining experimental and computational approaches, the study successfully assigns the absolute configuration of six previously uncharacterized synthetic cathinones, contributing valuable knowledge to both pharmaceutical research and forensic science.
The original article
Synthesis and absolute configuration of cyclic synthetic cathinones derived from α-tetralone
Martin Paškan, Kristýna Dobšíková, Martin Kuchař, Vladimír Setnička, Michal Kohout
Chirality, Volume 36, Issue 2, February 2024
https://doi.org/10.1002/chir.23646
licensed under CC-BY 4.0
In recent years, new psychoactive substances (NPS) are of growing concern worldwide. Over 880 new substances have been reported by European Monitoring Centre for Drugs and Drug Addiction (EMCDDA).1 Increasing diversity on the drug market is also signaled by the availability and use of the non-controlled synthetic cathinones. At the end of 2021, EMCDDA monitored a total number of 162 cathinones, making them the second largest category of NPS after synthetic cannabinoids.1 Compared to 2019, the number of production sites for synthetic cathinones in Europe increased threefold in the year 2020, mainly found in Poland and the Netherlands.2
Synthetic cathinone are derivatives of (S)-cathinone, an alkaloid naturally occurring in Catha edulis (Khat) leaves.3, 4 While the abuse of Khat leaves is mainly limited to certain ethnic groups at the Horn of Africa and the Arabian Peninsula, the synthetic cathinone derivatives are spread worldwide. Firstly, derivatives containing N-alkyl, alkyl, or halogen substituent at any position of an aromatic ring emerged.5 Nowadays, one-third of all seized synthetic cathinones is attributable to N-ethylhexedrone and one-quarter to 3-methylmethcathinone (3-MMC) and 3-chloromethcathinone (3-CMC).1 The synthetic cathinones are associated with serious health problems to their users. Severe intoxications and deaths related to their use have been reported.6-8 During intoxication, a wide range of adverse side effects has been observed, ranging from cardiovascular and neurological to psychiatric effects, such as arterial hypertension, tachycardia, hallucinations, agitation, hyperthermia, and changes in locomotor behavior.9-12 Due to the chiral center in their structure, cathinones can occur in two enantiomers, with one enantiomer typically exhibiting a greater psychological effect on the human biological system than the other.13, 14 For instance, studies demonstrated that (S)-methcathinone possesses a stronger stimulation effect than (R)-methcathinone.15 This fact could be important for the development of new pharmaceuticals based on the cathinone structure, as some synthetic modification can be sought to suppress psychoactivity and enhance pharmaceutical activity. Nevertheless, the focus is primarily put on the full characterization of NPS in order to speed up their regulation.
Despite the varied legal actions already in place by many governments to regulate NPS including the synthetic cathinones, the number of these substances continues to grow and diversify worldwide, in attempts to circumvent existing legislations. Based on current trends, the aromatic ring is one of the most popular sites in the cathinone scaffold being explored by designer-type modifications. Determining the absolute configuration is crucial as it provides essential information about the three-dimensional (3D) arrangement of atoms in a molecule. The absolute configuration specifies the spatial orientation of substituents at chiral centers, distinguishing between enantiomers and aiding in the comprehensive understanding of a molecule's chemical and biological properties. Various analytical methods, including X-ray crystallography,16, 17 circular dichroism (CD) spectroscopy,18-20 and nuclear magnetic resonance21, 22 can be employed to establish absolute configuration.
The most common methods presently available for the determination of synthetic cathinones and related drugs include mainly high-performance liquid chromatography (HPLC),23 gas chromatography (GC),24 and supercritical fluid chromatography (SFC)25 with mass detection (MS). Although these methods offer insight into the molecular structure, they are incapable of discerning between the enantiomers of the substance. The coupling of HPLC to a CD detector is one of the most powerful hyphenated techniques for stereochemical investigations. HPLC-CD can distinguish between enantiomers for a large variety of chemical compounds26-30 and provides a highly selective detection of compounds possessing optically active absorption bands in the 220–420 nm wavelength range.31, 32 This method was proven in recent years by Rebizi et al.,33 demonstrating its efficacy in characterization and enantioresolution of racemic mixtures containing four iminoflavan derivatives. The absolute configuration assignment was accomplished through on-line HPLC-CD spectra measurement, followed by comparison with CD curves obtained via quantum chemical simulation using TD-DFT.
In this study, we designed and synthesized novel synthetic cathinones based on α-tetralone substituted with halogen, alkyl, and methoxy groups, and resolved the corresponding enantiomers by HPLC-CD. This method was supported by density functional theory (DFT) calculations at the TD-B3LYP/6-311++G(d,p) level of theory, and a high similarity index between experimental and simulated spectra allowed us to determine not only their absolute configuration but also to describe their 3D structure including the stable conformers in solution. This complex study provides important insight into the 3D structure characterization of the substances in solution.
2 MATERIALS AND METHODS
2.2 NMR and MS analyses
1H, 13C, 19F NMR spectra, and all 2D experiments were carried out using Varian Gemini 400 Hz DDR2 and JEOL-ECZL400G with deuterated solvents (chloroform and methanol). Every sample was measured at 25°C. Deuterated solvents were purchased from Chemstar (Plzeň, Czech Republic). Mass spectrometry spectra were measured on a Thermo Scientific LTQ Orbitrap Velos, Hybrid Mass Spectrometer Ultimate Confidence (Bremen, Germany) using electrospray ionization in the positive mode.
2.3 Chiral separation
The method for chiral resolution of the synthesized compounds was developed on an Agilent 1100 Series instrument (Agilent Technologies, Waldbronn, Germany), composed of a solvent tray, a degasser, a binary pump, an autosampler, and a UV–VIS detector (Figure S7, Table S1). HPLC-CD experiments were performed using the following system: LCP 5020 HPLC Pump (ECOM, Prague, Czech Republic) and Jasco CD-4095 detector (CD and UV signal). The system was controlled by Clarity software (DataApex, Prague, Czech Republic). For chiral separation, Chiral ART Amylose-C column (250 × 4.6 mm, 5 μm; YMC Europe GmbH, Dinslaken, Germany) based on amylose tris(3,5-dimethylphenylcarbamate; Figure 1) was used. As a mobile phase, heptan and propan-2-ol (iPrOH), (90/10, v/v) + 0.1% HCOOH + 0.1% isopropylamine (IPA) was used. The flow rate of the mobile phase was 1 mL min−1 and the injection volume was 5 μl. All samples were diluted in the mobile phase in the concentration of 10 g l−1.
Chirality, Volume 36, Issue 2: FIGURE 1 - Chemical structure of the polysaccharide-based selector present in the chiral stationary phase used for chiral resolution of the target compounds.
2.4 Simulated ECD and UV spectra
The conformational study of (S)-aminoketones hydrochlorides (Figure 2) was performed by the Conformer-Rotamer Ensemble Sampling Tool (CREST), a semi-empirical tight-binding (TB) quantum chemistry code, designed for fast conformational search. All the DFT calculations were performed using the Gaussian 16 program package.34 The dispersion corrected functional theory TD-B3LYP-D3/6-311++G(d,p) with Grimmes's dispersion was used for all geometry optimizations, and the frequencies were calculated. This combination was proven to be powerful by a number of recent rotational spectroscopic studies of flexible organic molecules.35-37 The CREST conformer candidates (relative energy < 3 kJ mol−1) were reoptimized at the B3LYP/6-311++G(d,p) level (Figure 3) and their relative abundances calculated using the Boltzmann distribution based on the Gibbs free energies (T = 273 K). The solvent effect of heptane (ε = 1.9113) was modeled using a conductor-like polarizable continuum model. For the ECD and UV absorption spectra, the 30 lowest singlet states of each conformer were calculated, and the Gaussian band profiles with a 12 nm bandwidth were assumed. Subsequently, the simulated ECD and UV spectra were compared with the experimental spectra using the CDSpecTech program package38, 39 to gain the scaling factor for the simulated spectra and the similarity index (SimEA, SimECD). The experimental and simulated electronic spectra were compared using the similarity factor defined by Bultinck et al.; they proposed similarity factors to address the comparison of IR and VCD spectra effectively.40, 41 These factors, grounded in the cosine similarity equation, can be readily modified for the comparison of UV and ECD spectra.42
\(\mathrm{S}=\frac{\int \mathrm{f}\left(\mathrm{x}\right)\mathrm{g}\left(\mathrm{x}\right)\mathrm{dx}}{\sqrt{\int {\mathrm{f}}^2\left(\mathrm{x}\right)\mathrm{dx}\int {\mathrm{g}}^2\left(\mathrm{x}\right)\mathrm{dx}}},\)
where f(x) represents a normalized predicted spectrum and g(x) is the normalized experimental spectrum, both showing similarity in overlap integral. For a perfect agreement of the experimental and calculated spectra, the similarity index would be 1.0.
Chirality, Volume 36, Issue 2: FIGURE 2 - The structure of the hydrochloride 9f with the labeled dihedral angle α. The characteristic torsion angle is defined as α ≤ (1, 2, 3, 4).
Chirality, Volume 36, Issue 3: FIGURE 3 - The visualization of the most stable conformers of compounds 9a–f at the B3LYP/6-311++G(d,p) level in heptane including the stabilizing hydrogen bond. Color code (on-line) nitrogen is represented in blue, oxygen in red, chlorine in green, carbon in gray, and hydrogen in white.
4 CONCLUSIONS
Our study has been focused on the synthesis and chiral separation of novel cyclic synthetic cathinones structurally related to α-tetralone using chiral HPLC with CD detection. The enantioseparated fraction of each substance were characterized in the combination with DFT calculations. As a result, one stable conformer was identified for compounds 9a–e and two for 9f. Their Boltzmann averaged spectra were simulated, and very good agreement with the experiment allowed us not only to determine the absolute configuration of eluting enantiomers but also to describe their 3D structure in detail revealing a stabilizing hydrogen bond. The correctness of results obtained from HPLC-CD was proven by measuring and comparing ECD and UV spectra for substance 9a, which was chirally resolved on the multi-milligram scale. Using the preparative chiral resolution of enantiomers, we could also observe a reversal of elution order in comparison to acyclic synthetic cathinones. These results may be useful for the future biochemical studies in which we will focus not only on the toxicity and receptor binding studies of racemates but also of individual enantiomers.
