Isomerization of E-Cinnamamides into Z-Cinnamamides Using a Recycling Photoreactor.

The photocatalytic synthesis of thermodynamically less-stable Z-alkenes has received considerable research attention in recent years. In this study, a recycling photoreactor was applied to the photoisomerization of E-alkenes (cinnamamide and Weinreb amide derivatives) to produce Z-alkenes. The closed-loop recycling system comprises an immobilized photosensitizer to achieve rapid photoisomerization and a high-performance liquid chromatography instrument for separation of the Z/E diastereomers. After 4-10 cycles, the desired pure Z-alkenes were obtained efficiently. In the photoreactor system, a photosensitizer (thioxanthone) was covalently immobilized on silica gel via amide bonding, which led to an enhanced photocatalytic activity compared to the parent thioxanthone. This recycling photoreactor shows promise as an alternative system for the production of Z-alkenes.

Stereochemical properties of quazepam and its affinity for the GABAA receptor.

C9-methylated quazepam 1 was prepared, and its physicochemical properties were investigated. The atropisomers of 1 were isolated as (a1R, a2S) and (a1S, a2R) isomers. Their absolute configurations were determined based on ECD spectra in comparison with those calculated using the time-dependent density functional theory. Preliminary examination of affinity for the GABAA receptor revealed that the (a1R, a2S) isomer of 1 possessed higher activity than its antipode (a1S, a2R) isomer. The active configuration of C9-methylated quazepam 1 is the same as that of 1,4-benzodiazepin-2-ones.

Stereochemical Analysis of Trifluoroacetamide Derivatives Based on Through-Space 1H-19F Spin-Spin Couplings.

In this study, the conformational properties of tertiary trifluoroacetamides in dibenzoazepine (1a and 1b) and benzodiazepine (2a and 2b) derivatives, which exist as equilibrated E- and Z-amide conformers in solution, were investigated by 1H and 19F NMR spectroscopy. In all cases, one of the methylene protons neighboring the nitrogen atom of the minor conformer showed a finely split pattern due to coupling with the trifluoromethyl fluorine atoms, as confirmed by 19F-decoupling experiments. One-dimensional (1D) and two-dimensional (2D) 1H-19F heteronuclear Overhauser spectroscopy (HOESY) experiments were performed to confirm whether these couplings are attributable to through-bond spin-spin couplings (TBCs) or through-space spin-spin couplings (TSCs). HOESY cross-peaks between CF3 (19F) and one of the CH2-N protons of the minor conformers indicate that the two nuclei are spatially close to each other, thus establishing the stereochemistry of the major (E-) and minor (Z-) conformers. The E-amide preferences of the trifluoroacetamides are consistent with the results of density functional theory calculations and X-ray crystallographic analyses. Furthermore, the otherwise incomprehensible 1H NMR spectra were accurately assigned using the HOESY-determined TSCs. The 1H NMR assignments of the E- and Z-methyl signals of N,N-dimethyl trifluoroacetamide, the simplest tertiary trifluoroacetamide, were revised for the first time in half a century.

Conversion of Racemic Alkyl Aryl Sulfoxides into Pure Enantiomers Using a Recycle Photoreactor: Tandem Use of Chromatography on Chiral Support and Photoracemization on Solid Support.

Chiral sulfoxides are valuable in the fields of medicinal chemistry and organic synthesis. A recycle photoreactor utilizing the concept of deracemization, where a racemate is converted into a pure enantiomer, is developed and successfully applied in the syntheses of chiral alkyl aryl sulfoxides. The recycling system consists of rapid photoracemization using an immobilized photosensitizer and separation of the enantiomers via chiral high-performance liquid chromatography, and the desired pure chiral sulfoxides are obtained after 4-6 cycles. The key to the success of the system is the photoreactor site, wherein the photosensitizer 2,4,6-triphenylpyrylium is immobilized on the resin and irradiated (405 nm) to enable the rapid photoracemizations of the sulfoxides. As the green recycle photoreactor requires no chiral components, it should be a useful alternative system for application in producing chiral compounds.

Stereochemistry of N-Acyl-5H-dibenzo[b,d]azepin-7(6H)-ones.

The stereochemical properties of N-acyl-5H-dibenzo[b,d]azepin-7(6H)-ones (2a-c), which inhibit potassium channels in T cells, were examined by freezing their conformational change due to 4-methyl substitution. N-Acyl-5H-dibenzo[b,d]azepin-7(6H)-ones exist as pairs of enantiomers [(a1R, a2R), (a1S, a2S)], and each atropisomer is separable at room temperature. An alternate procedure for preparing 5H-dibenzo[b,d]azepin-7(6H)-ones involves the intramolecular Friedel-Crafts cyclization of N-benzyloxycarbonylated biaryl amino acids. Consequently, the N-benzyloxy group was removed during the cyclization reaction to produce 5H-dibenzo[b,d]azepin-7(6H)-ones suitable for the subsequent N-acylation reaction.

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.

Photoisomerization of Sulindac and Ozagrel Hydrochloride by Vitamin B2 Catalyst Under Visible Light Irradiation.

PURPOSE: Photoisomerization of the E/Z-alkene structures of drugs is a matter of concern as it could result in potency loss and adverse side effects. This study focused on light-induced isomerization of sulindac and ozagrel hydrochloride catalyzed by concomitant vitamin B2 under light-emitting diode (LED) or fluorescent light. METHODS: In the presence of 0.05/0.03 equivalents of vitamin B2/flavin adenine dinucleotide (FAD), sulindac or ozagrel hydrochloride was irradiated with LED light (405 nm) or fluorescent light. The photoisomerization in CD3OD and D2O was monitored by 1H NMR spectroscopy. RESULTS: Sulindac and ozagrel hydrochloride isomerized in the presence of a catalytic amount of vitamin B2 or FAD under irradiation of 405 nm LED light and fluorescent light. Irradiation with LED light was found to be more effective than fluorescent light irradiation. The rate of photoisomerization was affected by the solvent, and the reaction in CD3OD proceeded faster than in D2O. Furthermore, ozagrel hydrochloride was more prone to isomerization than sulindac. CONCLUSION: The catalytic activity of vitamin B2 or FAD was demonstrated in the photoisomerization reaction of sulindac and ozagrel hydrochloride. Considering that the rate of photoisomerization in D2O is very slow, the possibility of the occurrence of photoisomerization during clinical use is low. However, this study suggests that the interfusion of vitamin B2 or FAD under excessive light exposure should be avoided as a caution during intravenous administration of sulindac or ozagrel hydrochloride.

Axial chirality and affinity at the GABAA receptor of triazolobenzodiazepines.

Triazolobenzodiazepines substituted with a methyl group at the C1- and C10-positions and chloro group at C2' of pendant-phenyl were prepared and their physicochemical properties were investigated. The atropisomers of 1,10-disubstituted triazolobenzodiazepines, 1d and 1f, were isolated as (a1R, a2S) and (a1S, a2R) isomers. Their absolute configurations were determined on the basis of CD spectra in comparison with those of stereochemically defined 9-methyl-1,4-benzodiazepin-2-ones. Examination of the affinity at the human GABAA receptors revealed that each (a1R, a2S) isomer of 1d and 1f possessed higher activity than its antipode (a1S, a2R) isomer. It was also found that 1a, which behaves achirally due to the rapid conformational change, had the highest GABAA affinity, equal to that of triazolam. Considering that each eutomer of 1d and 1f is (a1R, a2S), the conformation of 1a at the binding site of the GABAA receptor is expected to be (a1R, a2S).

Nobiletin and 3'-Demethyl Nobiletin Activate Brown Adipocytes upon ß-Adrenergic Stimulation.

Brown adipose tissue (BAT) specifically regulates energy expenditure via heat production. Nobiletin (NOB), a natural polymethoxylated flavone present in citrus fruits, can activate thermogenesis in the BAT of high-fat diet-induced obese mice. The activity of BAT is directly regulated by ß-adrenergic stimulation. In this study, we report the effects of NOB on BAT activation using ß-adrenergic agonists. We observed that when HB2 brown adipocyte cell lines are stimulated with ß-adrenergic agonists, NOB enhances the expression of uncoupling protein 1 (UCP1), which is associated with the mitochondrial energy metabolism in these cells. Moreover, NOB increases the mRNA expression of the brown adipokines neuregulin-4 (Nrg4) and fibroblast growth factor-21 (FGF-21) and the secretion of FGF-21. These results suggest that NOB can enhance the thermogenic functions of brown adipocytes and promote brown adipokine secretion due to enhanced ß-adrenergic stimulation. In addition, 3'-demethyl nobiletin (3'-DMN), an NOB CYP-enzyme metabolite, can increase UCP1 mRNA expression. Both NOB and 3'-DMN significantly promoted mitochondrial membrane potential in HB2 adipocytes following ß-adrenergic stimulation. Therefore, we believe that NOB could be a promising candidate for activating BAT under ß-adrenergic stimulation and preventing the onset of obesity.

Atropisomeric Properties of N-Alkyl/Aryl 5H-Dibenz[b,f]azepines.

The atropisomeric properties of N-alkyl and N-aryl 4-substituted 5H-dibenz[b,f]azepines were investigated. The N-alkylation and N-arylation of 4-Cl or 4-Me substituted compounds was performed; however, none of the atropisomers produced were separated by chiral HPLC. Notably, we observed that the rotation of the four axes (ax. 1-4) in the 4-substituted 5H-dibenz[b,f]azepine structure is so rapid that N-alkylation or N-arylation is not sufficient to freeze it at room temperature. Additionally, the X-ray crystal structures of N-aryl compounds 13b and 14a indicated that the N atom in the triphenyl amine moiety in their structures shows sp2-like property.

Atropisomeric Properties of N-Acyl/N-Sulfonyl 5H-Dibenzo[b,d]azepin-7(6H)-ones.

The stereochemistry of N-acyl/N-sulfonyl 5H-dibenzo[b,d]azepin-7(6H)-ones (I, II) was examined in detail by freezing the conformation with a methyl group at the C-4 of dibenzoazepine. Because the two axes (axis 1, axis 2) move together concertedly, I and II exist only as a pair of enantiomers [(a1R, a2R) and (a1S, a2S)], which was confirmed by X-ray analysis of IIBc. It was elucidated that the amide derivatives I exist in equilibrium with the E/Z-amide (100:2-100:34), which means that the exocyclic bond (axis 3) is not in concert with the endocyclic axes (axis 1, axis 2). For the preparation of 5H-dibenzo[b,d]azepin-7(6H)-one, the intramolecular Friedel-Crafts acylation of N-(1,1')-biphenyl-2-yl-glycine derivatives was revisited. It was revealed that the electron-withdrawing property of the amino-protective group was a key to the success of seven-membered cyclization.

Rapid Photoracemization of Chiral Alkyl Aryl Sulfoxides.

The photoracemization of chiral alkyl aryl sulfoxides with a photosensitizer has not been sufficiently investigated thus far. Therefore, in this study, a rapid photoracemization reaction of enantiopure alkyl aryl sulfoxides using 1 mol % 2,4,6-triphenylpyrylium tetrafluoroborate (TPT+) was developed. Various substitution patterns were tolerated and every racemization reaction proceeded extremely fast (k2 = 1.77 × 104-6.08 × 101 M-1 s-1, t1/2 = 0.4-114 s). Some chiral sulfoxides with easily oxidizable functional groups are not appropriate for this photoisomerization. The electrochemical potentials of the functional groups, determined via cyclic voltammetry, are useful for predicting the reactive or nonreactive groups in this photoracemization reaction. A theoretical study was conducted to clarify the sp2-like nature of S of the sulfoxide cation radical, which makes photoracemization easier.

Conformational Preference of 2'-Fluoro-Substituted Acetophenone Derivatives Revealed by Through-Space 1H-19F and 13C-19F Spin-Spin Couplings.

The conformational properties of 2'-fluoro-substituted acetophenone derivatives were elucidated based on Hα-F and Cα-F through-space spin-spin couplings (TS-couplings), which occur between two atoms constrained at a distance smaller than the sum of their van der Waals radii. This study revealed that 2'-fluoro-substituted acetophenone derivatives in solutions form exclusively s-trans conformers by analyzing their NMR spectra focused on the TS-couplings. The magnitudes of the coupling constants 5J (Hα, F) and 4J (Cα, F) correlate linearly with the value of the dielectric constant of the solvents. Furthermore, s-trans conformations of the two derivatives were confirmed by X-ray crystallographic analysis. These conformational preferences were consistent with the DFT calculations. The s-cis conformer, in which fluorine and oxygen atoms lie in a syn-periplanar mode, may be subject to strong repulsion between the two polar atoms and become unstable. The s-trans preference of the 2'-fluoro-substituted acetophenone derivatives may be utilized in drug design.

Elucidation of the Active Conformation of Antiproliferative Sulfonamides, 5N-Arylsulfonyl-1,5-benzodiazepin-2-ones.

The 5N-arylsulfonyl-1,5-benzodiazepin-2-ones with antiproliferative activity were prepared and successfully separated into the (a1R,a2R)- and (a1S,a2S)-atropisomers with extraordinary stability (ΔG⧧ = ∼130 kJ/mol) by freezing the conformation around the sp2-sp2 axis in an Ar-N(SO2) moiety with a C6-methyl group. Also, by introducing a C3-methyl group (central chirality) into the 1,5-benzodiazepine nucleus, the stereochemistry at the axis was biased to take solely one diastereomer with a relative stereochemistry of (a1R*,a2R*,3R*). The (a1S) stereochemistry was crucial for exerting the antiproliferative activity.

4-Substituted carbamazepine derivatives: Conformational analysis and sodium channel-blocking properties.

The physicochemical properties of 4-substituted carbamazepine derivatives were investigated. It was elucidated that the 4-substitution is not effective in reducing the rotations (E/Z) about the N-C1' axes around the outer carbamoyl moiety. However, the atropisomers were isolated with high stereochemical stability, meaning that the 4-substitution reduced the butterfly motion of the tricyclic ring system efficiently. The Cl/CH3-substituted carbamazepine derivatives showed greater inhibitory effects on hNav1.2 channel currents compared with carbamazepine, although no difference in the activity between enantiomers was observed.

Elucidation of the E-Amide Preference of N-Acyl Azoles.

The conformational properties of N-acyl azoles (imidazole, pyrazole, and triazole) were examined. The N-2',4',6'-trichlorobenzoyl azoles were stable in methanol at room temperature, and no hydrolyzed products were observed over 7 days in the presence of 5% trifluoroacetic acid or 5% triethylamine in CDCl3. The high stability may be explained by the double-bond amide character caused by the steric hindrance due to the ortho-substituents in the benzoyl group. While specific E-amide preferences were observed in N-acyl pyrazoles/triazoles, the amides of the imidazoles gave a mixture of E and Z. One of the conceivable ideas to rationalize this conformational preference may be repulsive interaction between two sets of lone-pair electrons on the pyrazole 2-nitrogen (nN) and the carbonyl oxygen atoms (nO) in the Z-conformation of N-acyl pyrazoles/triazoles. However, analysis of orbital interactions suggested that in the case of the E-conformation of N-acyl pyrazoles, such electron repulsion is small because of distance. The interbond energy calculations suggested that the Z-conformer is involved in strong vicinal σ-σ repulsion along the amide linkage between the σN1N2 and σC1C2 orbitals in the anti-periplanar arrangement and between the σN1C5 and σC1C2 orbitals in the syn-periplanar arrangement, which lead to the overwhelming E-preference in N-acyl pyrazoles/triazoles. In the case of N-acyl imidazoles, similar vicinal σ-σ repulsions were counterbalanced, leading to a weak preference for the E-conformer over the Z-conformer. The chemically stable and E-preferring N-acyl azoles may be utilized as scaffolds in future drug design.

Stereochemistry of N-Benzoyl-5-substituted-1-benzazepines Revisited: Synthesis of the Conformationally Biased Derivatives and Revision of the Reported Structure.

The syn (aR*,5R*) and anti (aS*,5R*) diastereomers of N-benzoyl-C5-substituted-1-benzazepines originating in the chiralities at C5 and the Ar-N(C═O) axis were first stereoselectively synthesized by biasing the conformation with a substituent at C6 and C9, respectively. Detailed examination of the stereochemistry (i.e., conformation and configuration) of these N-benzoyl-1-benzazepines by X-ray crystallographic analysis, VT NMR, and DFT calculations revealed new physicochemical aspects of these heterocycles including revision of the stereochemistry previously reported.

N-benzoyl- and N-sulfonyl-1,5-benzodiazepines: comparison of their atropisomeric and conformational properties.

The atropisomeric and conformational properties of 1,5-benzodiazepines with an N-sulfonyl (p-tosyl/mesyl) group (IIa/b) were investigated by comparison with those of the N-benzoyl congeners (I). Similar to I, when the Ar-N(SO2) axis was frozen by a C9-substitution in the molecules, IIa/b were separated into the (aR)- and (aS)-atropisomers. The conformation of IIa/b revealed that the substituent (p-tolyl/methyl group) in the sulfonyl moiety occupies the position over the diazepine ring (folded form) in both the solid and solution states [e.g., (+)-(aR)-N-p-tosyl-1,5-benzodiazepin-2-one (IIa-2)], whereas that of I is anti to the diazepine ring [e.g., (-)-(aR)-N-benzoyl-1,5-benzodiazepin-2-one (I-2)], which was further supported by a computational study. The stereochemical stability also differed between the two congeners (e.g., ΔG(‡): 104 kJ/mol for I-2 and 132 kJ/mol for IIa-2).

Crystallization-induced diastereomeric transformation of N-2'-t-butyl-6'-iodobenzoyl-3-bromocarbazole.

We previously reported the atropisomeric properties of 2'-t-butyl-6'-iodo-substituted N-benzoyl-3-bromocarbazole, i.e., the steric or electronic effects of the substituents restricted the rotation about the N-C7' and C7'-C1' bonds to separate four stereoisomers (cis/trans for the N-C7' axis, aR/aS for the C7'-C1' axis). Furthermore, the 2'-t-butyl-6'-iodo-substituted N-benzoyl 3-bromocarbazole was confirmed to be a gear molecule, in which the rotation about the C7'-C1' bond was in perfect concert with that about the N-C7' bond. Herein, we report a unique crystallization-induced diastereomeric transformation found in this molecule. In the isolation process, where the product is recrystallized from the diastereoisomeric mixture, in situ isomerization and selective crystallization could lead to a diastereomeric transformation, and a mixture of diastereomers (trans/cis=54 : 46) was converted to trans-isomer-enriched crystals (trans/cis>96 : 4) in 95% yield. Conformational analysis clarified the preference for the trans versus cis isomer.

Fentanyl-Type Antagonist of the µ-Opioid Receptor: Important Role of Axial Chirality in the Active Conformation.

In recent years, synthetic opioids have emerged as a predominant cause of drug-overdose-related fatalities, causing the "opioid crisis." To design safer therapeutic agents, we accidentally discovered µ-opioid receptor (MOR) antagonists based on fentanyl with a relatively uncomplicated chemical composition that potentiates structural modifications. Here, we showed the development of novel atropisomeric fentanyl analogues that exhibit more potent antagonistic activity against MOR than naloxone, a morphinan MOR antagonist. Derivatives displaying stable axial chirality were synthesized based on the amide structure of fentanyl. The aS- and aR-enantiomers exerted antagonistic and agonistic effects on the MOR, respectively, and each atropisomer interacted with the MOR by assuming a distinct binding mode through molecular docking. These findings suggest that introducing atropisomerism into fentanyl may serve as a key feature in the molecular design of future MOR antagonists to help mitigate the opioid crisis.