N-Benzoyl- and N-Sulfonyl-1,5-benzodiazepines: Comparison of Their Atropisomeric and Conformational Properties

Chirality Transfer Intramolecular Pauson-Khand Reaction with N-C Axially Chiral Sulfonamides Bearing an Ene-Yne Structure

An intramolecular Pauson-Khand reaction with enantioenriched N-C axially chiral N-allyl-N-(2-alkynylphenyl)sulfonamide derivatives proceeded with complete chirality transfer from axial chirality (P configuration) to central chirality (R configuration), affording chiral nitrogen-containing tricyclic compounds (tetrahydrocyclopentaquinolin-2-one derivatives).

Synthesis and Atropisomeric Properties of Benzoazepine-Fused Isoindoles

Atropisomeric, bench-stable benzoazepine-fused isoindoles were synthesized via oxidation from isoindoline precursors. Using the isoindoles 5d-f as models, the stereochemistry and conformational folding of the systems were examined. Chiral UHPLC was used to analyze the rate of racemization and calculate the Gibbs free energy of enantiomerization (ΔG^‡_Enant). X-ray crystallography, ¹H NMR spectroscopy, and DFT calculations were used to elucidate the three axes of chirality and clarify the structural factors contributing to ΔG^‡_Enant. Tandem rotation around the axes of chirality precludes the formation of diastereomers, with rotational restriction of the C_aryl-N_sulfonamide bond determined as the moderator of atropisomeric stability in the system, affected primarily by steric hindrance as well as by π-stacking interactions facilitated by the folded conformation of the sulfonamide over the isoindole moiety.

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.

Catalytic Enantioselective Synthesis of N-C Axially Chiral N-(2,6-Disubstituted-phenyl)sulfonamides through Chiral Pd-Catalyzed N-Allylation

Recently, catalytic enantioselective syntheses of N-C axially chiral compounds have been reported by many groups. Most N-C axially chiral compounds prepared through a catalytic asymmetric reaction possess carboxamide or nitrogen-containing aromatic heterocycle skeletons. On the other hand, although N-C axially chiral sulfonamide derivatives are known, their catalytic enantioselective synthesis is relatively underexplored. We found that the reaction (Tsuji-Trost allylation) of allyl acetate with secondary sulfonamides bearing a 2-arylethynyl-6-methylphenyl group on the nitrogen atom proceeds with good enantioselectivity (up to 92% ee) in the presence of (S,S)-Trost ligand-(allyl-PdCl)₂ catalyst, affording rotationally stable N-C axially chiral N-allylated sulfonamides. Furthermore, the absolute stereochemistry of the major enantiomer was determined by X-ray single crystal structural analysis and the origin of the enantioselectivity was considered.

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.

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.

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.

Deformative transition of the Menschutkin reaction and helical atropisomers in a congested polyheterocyclic system

A 4,7-phenanthroline polycyclic 1A designed for probing the limits of the Menschutkin reaction was synthesized in a six-step sequence. The rotational barrier of the phenyl ring nearby the N-methyl group in rac-2A was estimated to be ≫ 18.1 kcal/mol from VT-NMR experiments, making them a new type of helical atropisomer. The methylation rate constants of 9 and 1A with MeI was found to be 2.22 × 10(-4) and 9.62 × 10(-6) s(-1) mol(-1) L, respectively; thus, the formation rate of (P/M)-2A is one of the slowest rates ever reported for a Menschutkin reaction. The N-methyl protons in (P/M)-2A exhibit a significant upfield shift (Δδ 1.0 ppm) in its (1)H NMR, compared to those without a nearby phenyl, indicating a strong CH-π interaction is involved. Conformational flexibility in dipyridylethene 9 is clearly shown by its complexation with BH3 to form helical atropisomers (P,P/M,M)-10. The pKa values of the conjugate acids of 1A and 9 in acetonitrile were determined to be 4.65 and 5.07, respectively, which are much smaller compared to that of pyridine 14a (pKa = 12.33), implying that the basicity, nucleophilicity, and amine alkylation rates of 1A and 9 are markedly decreased by the severe steric hindrance of the flanking phenyl rings in the polyheterocycles.