Atropisomerism in 3-arylthiazolidine-2-thiones. A combined dynamic NMR and dynamic HPLC study

Diaryl-Pyrano-Chromenes Atropisomers: Stereodynamics and Conformational Studies

The dynamic scenario of di-aryls-pyrano-chromenes was investigated using DFT calculations. The symmetry of the chromene scaffold and the presence of two ortho-substituted aryls substituents can generate two syn/anti diastereoisomers and conformational enantiomers with different rotational barriers. The relative conformations and configurations were derived using NOESY-1D experiments. Depending on the energies related to the conformational exchange, the experimental energy barriers were determined through Dynamic NMR, Dynamic HPLC or kinetic studies. The atropisomeric pairs were resolved in the latter scenario, and their absolute configuration was assigned using the ECD/TD-DFT method.

Atropisomerism in Drug Discovery: A Medicinal Chemistry Perspective Inspired by Atropisomeric Class I PI3K Inhibitors

Atropisomerism is a type of axial chirality resulting from hindered rotation about a σ bond that gives rise to nonsuperimposable stereoisomers (termed "atropisomers"). The inversion of chirality of an atropisomeric axis is a time- and temperature-dependent dynamic process occurring by simple bond rotation. For this reason, the rotational energy barrier (ΔE_rot) and the interconversion rate between an atropisomeric pair of biologically active molecules are important parameters to consider in drug discovery.Many compounds with atropisomeric axes advance into development every year. The vast majority of them have low rotational energy barriers (ΔE_rot lower than 20 kcal/mol), meaning they are rapidly equilibrating conformers and considered achiral (class 1 atropisomers). Compounds in class 2 (ΔE_rot = 20 to 30 kcal/mol) can be challenging to develop given that the stereochemical integrity of the atropisomeric axes can be compromised over time. It has been recommended that small molecule drug candidates containing one or more atropisomeric axes with rotational energy barriers greater than 30 kcal/mol (class 3 atropisomers) should be developed as single atropisomers.In medicinal chemistry, a σ bond with restricted rotation is engineered into a bioactive molecule primarily to limit its number of accessible conformations, thereby minimizing entropic and/or enthalpic energy penalties associated with biological target binding. In addition to enhanced pharmacology, potential positive outcomes of introducing atropisomerism include improved physicochemical properties and superior pharmacokinetics/ADME profiles. The application of atropisomerism in medicinal chemistry has become increasingly enabled due to recent advances in synthesis, purification, and analysis, as described in this special issue and recent review articles.Herein, we discuss two case studies from our own work in which restricting rotation about axes of atropisomerism led to significant improvements in pharmacological, physicochemical, and ADME properties for different series of PI3K inhibitors. In the first instance, a restricted axis of rotation was designed to mitigate an acid-mediated hydrolytic degradation pathway observed in a series of PI3Kδ inhibitors. The conformational constraint disrupts conjugation between a quinazolinone and a pyridine, leading to improved chemical stability under acidic conditions. In the second case study, introduction of a restricted axis of rotation between two heteroaromatic systems in a series of PI3Kβ inhibitors generated pairs of atropisomeric compounds with significantly different biological activities. Advanced profiling also demonstrated clear substrate stereospecificity in regard to metabolism by aldehyde oxidase. Gratifyingly, the eutomer (more active atropisomer) shows significantly less susceptibility for oxidative metabolism relative to the distomer (less active atropisomer). The improvements in potency, selectivity, chemical stability, and metabolic stability discussed in this manuscript are all directly related to the concept of atropisomerism.

Solvent dependent hindered rotation versus epimerization in axially chiral thiohydantoin derivatives: an experimental and a computational study

5-Benzyl-3-(o-aryl)-2-thiohydantoin and 5-isobutyl-3-(o-aryl)-2-thiohydantoin derivatives (o-aryl = o-tolyl and o-bromophenyl) have been synthesized by reacting o-aryl isothiocyanates with S-phenylalanine methyl ester hydrochloride or with S-leucine methyl ester hydrochloride in the presence of triethylamine (TEA). The synthesized compounds have a chirality center at C5 of the heterocyclic ring and a chirality axis, the N3-C_(aryl) bond. The axially chiral compounds were shown to exist in unequal amounts of SM, SP, RM and RP stereoisomeric forms with a high prevalence of the P isomers over the M isomers. The isomeric assignments were done by comparing the ¹H NMR spectra with the HPLC chromatograms. The stereoisomers were resolved micropreparatively by HPLC on chiral stationary phases and the interconversion of the single isomers has been investigated. The conversion type has been determined as epimerization or rotation by the HPLC analyses. It has been found that although the stereoisomers converted to each other only by rotation in toluene, in ethanol epimerization (racemization at C5 of the heteroring) was accompanied with rotation depending on the duration, temperature of the thermal interconversion experiment and the nature of the ortho substituent. The occurrence of epimerization was also proved through H/D exchange reactions via¹H NMR experiments done in CD₃OD. The rotation and epimerization mechanisms of synthesized compounds were further elucidated by Density Functional Theory (DFT) calculations at M062X/6-311 + G** level of theory and the results were shown to be in harmony with experimental findings.

A nonneglectable stereochemical factor in drug development: Atropisomerism

Chirality is one of the key factors affecting the medicinal efficacy of compounds. In addition to central chirality, sterically hindered chiral axes commonly appear in drugs and the resulting chirality is known as atropisomerism. With developments in medicinal chemistry, atropisomerism has attracted increasing attention. This review discusses the classification, biological activity, pharmacokinetics, toxicity and side effects of atropisomers, and can serve as a reference in the research and development of potential chiral drugs.

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.

Conformational and Stereodynamic Behavior of Five- to Seven-Membered 1-Aryl-2-iminoazacycloalkanes

The stereodynamic behavior of 1-arylpyrrolidin-2-imines, having a C_aryl-N stereogenic axis, has been studied by means of dynamic nuclear magnetic resonance and density functional theory calculations, evaluating the steric effect of ortho-aryl substituents. The rotational barrier due to E/Z isomerism about the -C=N-H bond was also determined. The dynamic stereochemistry of homologous six- and seven-membered iminoazacycloalkane rings and their oxo-analogues was also comparatively investigated, evidencing a ring size effect. It was found that the seven-membered heterocycle shows additional dynamic features because of ring inversion.