Isolation and Characterization of Atropisomers of Seven-Membered-Ring Benzolactams

Bioactive atropisomers: Unraveling design strategies and synthetic routes for drug discovery

Atropisomerism, an expression of axial chirality caused by limited bond rotation, is a prominent aspect within the field of medicinal chemistry. It has been shown that atropisomers of a wide range of compounds, including established FDA-approved drugs and experimental molecules, display markedly different biological activities. The time-dependent reversal of chirality in atropisomers poses complexity and obstacles in the process of drug discovery and development. Nonetheless, recent progress in understanding atropisomerism and enhanced characterization methods have greatly assisted medicinal chemists in the effective development of atropisomeric drug molecules. This article provides a comprehensive review of their special design thoughts, synthetic routes, and biological activities, serving as a reference for the synthesis and biological evaluation of bioactive atropisomers in the future.

Atropodiastereoselective 5N-Acylation of 1,5-Benzodiazepin-2-ones with (S)-2-Phenylpropanoyl and (S)-2-Phenylbutanoyl Chlorides

5N-Acylation of 1N-methyl-1,5-benzodiazepin-2-ones with (S)-2-phenylpropanoyl and (S)-2-phenylbutanoyl chlorides afforded the (a¹S,a²S,S)-atropisomer (I) diastereoselectively over the (a¹R,a²R,S)-isomer (II) in the ratio of 1:0.06-0.15. The preferential formation of I may be explained by the thermodynamically preferable π-π stacking interaction between two benzene rings in the benzodiazepine ring and the acyl chloride during the reaction. Analysis using ab initio calculations (RI-MP2/6-31+G(d) level of theory) for the acylation reaction indicated the π-π stacking interaction in the transition state. Furthermore, isomer I was shown to be thermodynamically more stable than II. The higher stability of I may be caused by the folded form of the two benzene rings, which was revealed by NMR, X-ray, and computational analyses.

Stereogenic and conformational properties of medium-ring benzo-fused N-heterocycle atropisomers

Medium-ring (7-9-membered) benzo-fused N-heterocycles - a core structure in several important pharmaceuticals - have a diverse range of interesting conformational and stereochemical properties which arise from restricted bond rotation in the non-aromatic ring. The atropisomers of these pharmaceutically relevant N-heterocycles typically exhibit different biological activities, warranting the need to deeply understand the factors controlling the conformation and stereochemistry of the systems. Beginning with a brief introduction to atropisomer classification, this review will detail a number of medium-ring benzo-fused N-heterocycle systems from the recent literature to provide an overview of structural factors which can affect the atropisomeric nature of the systems by altering the overall conformation and rate of stereo-inversion. As well as general factors such as ring-size and sterics, the impact of additional stereocentres in these systems will be addressed. This includes the differences between sulfur, nitrogen and carbon stereocentres, and the consequences of stereocentre placement around the N-heterocycle ring. Further, conformational stabilisation via non-covalent intramolecular bonds will be explored. As such, this review represents a significant resource for aiding in the design, synthesis and study of new and potentially bioactive medium-ring benzo-fused N-heterocycles.

Low Temperature Dynamic Chromatography for the Separation of the Interconverting Conformational Enantiomers of the Benzodiazepines Clonazolam, Flubromazolam, Diclazepam and Flurazepam

Benzodiazepines (BZDs) are an important class of psychoactive drugs with hypnotic-sedative, myorelaxant, anxiolytic and anticonvulsant properties due to interaction with the GABAa receptor in the central nervous system of mammals. BZDs are interesting both in clinical and forensic toxicology for their pharmacological characteristics and potential of abuse. The presence of a non-planar diazepine ring generates chiral conformational stereoisomers, even in the absence of stereogenic centers. A conformational enrichment of BZD at the binding sites has been reported in the literature, thus making interesting a stereodynamic screening of a wide range of BZDs. Herein, we report the investigation of three stereolabile 1,4-benzodiazepine included in the class of “designer benzodiazepines” (e.g., diclazepam, a chloro-derivative of diazepam, and two triazolo-benzodiazepines, flubromazolam and clonazolam) and a commercially available BZD known as flurazepam, in order to study the kinetic of the “ring-flip” process that allows two conformational enantiomers to interconvert at high rate at room temperature. A combination of low temperature enantioselective dynamic chromatography on chiral stationary phase and computer simulations of the experimental chromatograms allowed us to measure activation energies of enantiomerization (ΔG‡) lower than 18.5 kcal/mol. The differences between compounds have been correlated to the pattern of substitutions on the 1,4-benzodiazepinic core.

Molecular Recognition of the HPLC Whelk-O1 Selector towards the Conformational Enantiomers of Nevirapine and Oxcarbazepine

The presence of stereogenic elements is a common feature in pharmaceutical compounds, and affording optically pure stereoisomers is a frequent issue in drug design. In this context, the study of the chiral molecular recognition mechanism fundamentally supports the understanding and optimization of chromatographic separations with chiral stationary phases. We investigated, with molecular docking, the interactions between the chiral HPLC selector Whelk-O1 and the stereoisomers of two bioactive compounds, the antiviral Nevirapine and the anticonvulsant Oxcarbazepine, both characterized by two stereolabile conformational enantiomers. The presence of fast-exchange enantiomers and the rate of the interconversion process were studied using low temperature enantioselective HPLC and VT-NMR with Whelk-O1 applied as chiral solvating agent. The values of the energetic barriers of interconversion indicate, for the single enantiomers of both compounds, half-lives sufficiently long enough to allow their separation only at critically sub-ambient temperatures. The chiral selector Whelk-O1 performed as a strongly selective discriminating agent both when applied as a chiral stationary phase (CSP) in HPLC and as CSA in NMR spectroscopy.

Photochemical mechanism of 1,5-benzodiazepin-2-one: electronic structure calculations and nonadiabatic surface-hopping dynamics simulations

Due to the significant applications in bioimaging, sensing, optoelectronics etc., photoluminescent materials have attracted more and more attention in recent years. 1,5-Benzodiazepin-2-one and its derivatives have been used as fluorogenic probes for the detection of biothiols. However, their photochemical and photophysical properties have remained ambiguous until now. In this work, we have adopted combined static electronic structure calculations and nonadiabatic surface-hopping dynamics simulations to study the photochemical mechanism of 1,5-benzodiazepin-2-one. Firstly, we optimized minima and conical intersections in S0 and S1 states; then, we proposed three nonadiabatic decay pathways that efficiently populate the ground state from the Franck-Condon region based on computed electronic structure information and dynamics simulations. In the first pathway, upon photoexcitation to the S1 state, the system proceeds with an ultrafast excited-state intramolecular proton transfer (ESIPT) process. Then, the molecule tends to rotate around the C-C bond until it encounters keto conical intersections, from which the system can easily decay to the ground state. The other two pathways involve the enol channels in which the S1 system hops to the ground state via two enol S1/S0 conical intersections, respectively. These three energetically allowed S1 excited-state deactivation pathways are responsible for the decrease of fluorescence quantum yield. The present work will provide detailed mechanistic information of similar systems.

Visible Light-Promoted Beckmann Rearrangements: Separating Sequential Photochemical and Thermal Phenomena in a Continuous Flow Reactor

The Beckmann rearrangement of oximes to amides typically requires strong acids or highly reactive, hazardous electrophiles and/or elevated temperatures to proceed. A very attractive alternative is the in situ generation of Vilsmeier-Haack reagents, by means of photoredox catalysis, as promoters for the thermal Beckmann rearrangement. Investigation of the reaction parameters for this light-induced method using a one-pot strategy has shown that the reaction is limited by the different temperatures required for each of the two sequential steps. Using a continuous flow reactor, the photochemical and thermal processes have been separated by integrating a flow photoreactor unit at low temperature for the electrophile generation with a second reactor unit, at high temperature, where the rearrangement takes place. This strategy has enabled excellent conversions and yields for a diverse set of oximes, minimizing the formation of side products obtained with the original one-pot method.

An efficient synthesis of novel dibenzoxdiazepine-fused heterocycles through a multicomponent reaction

A one-pot synthesis of 6-oxa-2,2a¹,11-triazadibenzo[cd,g]azulenes by a three-component reaction of a 2-aminoheterocycle, aldehydes, and 2-isocyanophenyl acetate is presented.

Is nevirapine atropisomeric? Experimental and computational evidence for rapid conformational inversion

The non-nucleoside reverse transcriptase inhibitor nevirapine displays in its room temperature (1)H-NMR spectrum signals characteristic of a chiral compound. Following suggestions in the recent literature that nevirapine may display atropisomerism-and therefore be a chiral compound, due to slow interconversion between two enantiomeric conformers-we report the results of an NMR and computational study which reveal that while nevirapine does indeed possess two stable enantiomeric conformations, they interconvert with a barrier of about 76 kJ mol(-1) at room temperature. Nevirapine has a half life for enantiomerisation at room temperature of the order of seconds, is not atropisomeric, and cannot exist as separable enantiomers.