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.


■ INTRODUCTION
Recently, we have been interested in the conformational analysis of benzo-fused seven-membered-ring nitrogen heterocycles, which are found as the scaffolds of many drugs. 1 Our continuing interest in the relationship between axial chirality and biological activity 2,3 prompted us to examine the N-acyl/ N-sulfonyl 5H-dibenzo [b,d]azepin-7(6H)-ones (I, II) ( Figure  1), which were reported to have immunosuppressive effects by inhibiting the potassium channel (Kv1.3, IK-1) of T cells. 4 The Ca 2+ -dependent potassium channel IK-1 and the voltage-gated potassium channel Kv1.3 in human T cells play a pivotal role during cell proliferation. Thus, inhibitors of these channels could be expected to be new drug candidates for treating autoimmune diseases such as rheumatoid arthritis and multiple sclerosis. 5 The 5H-dibenzo [b,d]azepin-7(6H)-one moiety has dynamic axial chirality based on the sp 2 −sp 2 axis arising from the biphenyl (axis 1). In addition, N-acylated derivatives (I) have another axial chirality around the Ar−NC(O) (sp 2 −sp 2 ) axis (axis 2) and E/Z-amide rotamers based on the N−C(O) axis (axis 3). Thus, N-acylated derivatives (I) should exist in (aS)/(aR) axial isomers 6 derived from axes 1 and 2, and E/Zamide rotamers derived from axis 3. Similarly, the congener Nsulfonyl derivatives (II) were considered to have atropisomeric properties caused by the biphenyl (axis 1) and Ar−N(SO 2 ) (axis 2). 7 Their complex stereochemical structures are considered to constitute a key core structure of the immunosuppressive activity. Although the conformational change, i.e., ring flip, in molecules without a methyl substituent at the ortho position of the benzene ring (R 1 = H) was anticipated to be too rapid for isolation of the stereoisomers at room temperature, molecules with 4-methyl (R 1 = Me) were expected to freeze the conformations so that relatively stable stereoisomers could be separated. Such investigations should reveal the active structure (eutomer) exerting the inhibitory activity on the potassium channel (Kv1.3, IK-1) of T cell activity. Herein we describe a study of the conformational properties of the N-acyl/N-sulfonyl 5H-dibenzo [b,d]azepin-7(6H)-ones nucleus (I, II), and preliminary results of the blockade of the potassium channel. Through the synthesis, the intramolecular Friedel−Crafts acylation as a crucial step to provide the 5H-dibenzo [b,d]azepin-7(6H)-one nucleus was revisited. It was shown that the electron-withdrawing effect of the N-substituent of the amino acids affects the yield of cyclized compounds.

■ RESULTS AND DISCUSSION
Preparation of 5H-Dibenzo [b,d]azepin-7(6H)-ones. For the preparation of 5H-dibenzo [b,d]azepin-7(6H)-one, we intended to utilize the intramolecular Friedel−Crafts acylation of N-(1,1′)-biphenyl-2-yl-glycine derivatives (1). The cyclization of the aryl amino acids appeared to be an obvious route. According to the procedure reported in a previous paper, 4 the corresponding acid chlorides, prepared from N-(1,1′)biphenyl-2-yl-glycine using thionyl chloride, were treated with anhydrous aluminum chloride. However, the reaction of N-(1,1′)-biphenyl-2-yl-glycine derivatives with an N-acetyl (1Aa), N-p-toluoyl (1Ab) provided complex mixtures (Table  1, entries 1,2). Since further examination of the various reaction conditions was not rewarding, the pioneering work on Friedel−Crafts cyclization of aryl amino acids 8 was reviewed. It was reported that Friedel−Crafts intramolecular acylation of aryl amino acids has little hope of succeeding because it gave a mixture of isoquinoline derivatives, oxazolonium halides, and phenanthridine derivatives as major products. Among them, Paterson and Procter reported that the N-p-tosylated aryl amino acid reacted to give the desired cyclic compound, although other N-acylated ones did not cyclize. 9 In light of this, we focused on the amino-protective groups of the electronwithdrawing property and revisited the intramolecular Friedel−Crafts acylation of aryl amino acids.
Stereochemistry of N-Acyl-5H-Dibenzo [b,d]azepin-7(6H)-ones. N-Acyl 5H-dibenzo [b,d]azepin-7(6H)-ones (IA) (R 1 = H) and (IB) (R 1 = Me) should have chirality based on the sp 2 −sp 2 axis arising from the biphenyl (axis 1). In addition, another axial chirality arising from the sp 2 −sp 2 axis of the benzene-amide bond (axis 2) should exist as well as E/Zamide diastereomers around the N−C(O) bond (axis 3). It was therefore anticipated that IA and IB exist as complicated stereoisomers. However, our preceding studies on this dibenzoazepinone nucleus revealed that axes 1 and 2 move concertedly to form the stable relative configuration. 11 Thus, we presumed that the configuration of the enantiomers should be (a 1 R, a 2 R) and (a 1 S, a 2 S), respectively. Additionally, E/Zamide diastereomers around the N−C(O) bond (axis 3) were assumed to exist. The conformational properties of IA and IB are highlighted in Figure 2.
First, the conformational properties of IAg-h (R 1 = H) in the solution state were investigated precisely using 1 H NMR spectroscopy ( Figure 3). Compounds IAg and IAh were shown to exist as an equilibrium mixture of diastereomers in solution (CDCl 3 ) at the ratios 100:7 [ Figure 3b] and 100:30 [ Figure 3c], respectively. In each spectrum, one of the two  diastereotopic H-6 proton resonances in the major amide diastereomer is located at about 5.6 ppm (IAg) and 5.4 ppm (IAh), each 1.5 ppm, 1.3 ppm downfield from its partner, respectively. This downfield shift was also previously observed by Hassner 12a and Qadir et al., 12b who ascribed the phenomenon to coplanarity between the exocyclic amide carbonyl bond and the equatorial proton on the adjacent carbon (C-6). Based on this anisotropic effect of the carbonyl group, we presumed that both IAg and IAh exist in the Eamide in preference to the Z-amide. It is clear that the two endocyclic axes (axes 1 and 2) move together concertedly, although the exocyclic axis (axis 3) does not move in concert with them. Viewed in this light, it was assumed that the sevenmembered ring 5H-dibenzo [b,d] In need of solid evidence for the determination of the E/Zamide stereochemistry, investigation using NOE spectra seemed promising. However, the trifluoroacetyl group in IAg was not observed in 1 H NMR, and the methoxy carbonyl group in IAh was inadequate because of the flexibility of the −O−Me bond. Thus, the N-acetylated compound IAa was prepared for this purpose from IAg through two steps (hydrolysis and acetylation) (Scheme 1).
IAa as well as IAg−h was shown to exist as an equilibrium mixture of E/Z-amide diastereomers in solution (CDCl 3 ) at the ratio 100:2 [ Figure 3a]. Additionally, one of the two diastereotopic H-6 proton resonances in the major amide diastereomer is located at about 5.7 ppm, 1.7 ppm downfield from its partner. Irradiation of the dominant CH 3 resonance of acetyl in the major amide diastereomer led to 2.65% enhancement of the 4-H proton of benzene (Scheme 1). Therefore, the preference of the E-amide in IAa was determined. Based on this, the preference of the E-amide observed in IAg and IAh was confirmed. It was also revealed that IBg-h (R 1 = Me) showing similar spectra (see Supporting Information) preferred the E-amide to the Z-amide in solution (Table 2). Furthermore, the following computational studies were carried out to study the conformational preferences of N-acyl 5H-dibenzo [b,d]azepin-7(6H)-ones of IAa, IAg, IAh, IBg, and IBh. First, the conformational ensembles of IAa, IAg, IAh, IBg, and IBh were generated from 2D chemical structures as the initial structures for the density functional theory (DFT) calculations. These conformations were generated and optimized with the RDKit using the universal force field (UFF) and clustered using a tolerance of 0.2 Å root-meansquare deviation. For each conformer, the Hartree−Fock (HF) calculations were carried out to obtain optimized geometries and energies at the RHF/6-31G(d) levels. For each conformer excluding atropisomers, DFT calculations were carried out to obtain optimized geometries and energies at the RB3LYP/6-31G (d) and the RmPW1PW91/6-311+G(d,p) levels. The relative energy differences of the two conformers were estimated on the basis of geometries fully optimized with mPW1PW91/6-311+G(d,p) with energy calculations with the

Scheme 1. Preparation of IAa from IAg and Its E/Z-Amide Stereochemistry
The Journal of Organic Chemistry pubs.acs.org/joc Article RmPW1PW91/6-311+G (d,p) in the SCRF/IEFPCM model in CHCl 3 . 13 Zero-point energy (ZPE) correction was made on the basis of the frequency calculation with the RmPW1PW91/ 6-311+G (d,p). The results are shown in Table 2. As a representative result obtained in those computational studies, the selected conformers of the E/Z-amide of IAa are illustrated in Figure 4. Others are shown in the Supporting Information.
It was confirmed that N-acyl 5H-dibenzo [b,d]azepin-7(6H)ones exist in the stable relative configuration of a pair of enantiomers [(a 1 R, a 2 R) and (a 1 S, a 2 S)] without the presence of diastereomers [(a 1 R, a 2 S) and (a 1 S, a 2 R)]. Additionally, in each case, the E-amide was preferred, although the energy difference between the E-amide and Z-amide was less than 10.3 kJ/mol. Then we investigated the physicochemical properties of (a 1 R, a 2 R)-and (a 1 S, a 2 S)-axial isomers. As mentioned above, methylene protons (H-6) in these compounds were observed as diastereotopic, meaning the presence of axial chirality. In IAg (R 1 = H), however, the ring inversion via rotation around the axis was too rapid for separation of the axial isomers at rt, and thus IAg was not separated by chiral HPLC. On the other hand, compound IBg with a 4-methyl substituent (R 1 = Me) was conformationally frozen and separable into the stable (a 1 R, a 2 R)-and (a 1 S, a 2 S)-isomers, and the separated isomers showed a high energy barrier to rotation (ΔG ‡ = 124.8 kJ/ mol). 14 Next, N-methoxycarbonyl 5H-dibenzo [b,d]azepin-7(6H)-one (IAh) and its 4-methyl derivative (IBh) were investigated, and similar results were obtained. While each enantiomer of IAh without the 4-methyl substituent (R 1 = H) was not separable at rt, IBh with the 4-methyl substituent (R 1 = Me) was separable into the stable (a 1 R, a 2 R)-and (a 1 S, a 2 S)axial isomers, and the separated isomers showed a high energy barrier to rotation (ΔG ‡ = 116.0 kJ/mol). The physicochemical properties of IBg and IBh are shown in Table 3. As expected, the 4-methyl (R 1 = Me) substituent was helpful to freeze the conformations so that the relatively stable stereoisomers could be separated.
Stereochemistry of N-Sulfonyl-5H-dibenzo [b,d]azepin-7(6H)-ones. The stereochemistry of the N-sulfonyl derivatives (IIA/B) was investigated next, and a general picture of the conformational property is shown in Figure 5. Although the sulfonamide group is an important functional moiety observed in various biologically active compounds, its physicochemical properties are not as well understood as those of the amide group. Similar to N-acyl derivatives (IA/B), N- The Journal of Organic Chemistry pubs.acs.org/joc Article sulfonyl derivatives (IIA/B) should have chirality based on the sp 2 −sp 2 axis arising from the biphenyl (axis 1). In addition, another axial chirality arising from the benzene−sulfonamide bond should exist. Our studies have recently revealed that the atropisomeric property of the sulfonamide group is caused by the Ar−N(SO 2 ) axis (axis 2). 7a, 15 The planarity of the N−SO 2 arises from both the nitrogen atom possessing an sp 2 -like nature and the double-bond character between the S−N bond. As well as N-acyl derivatives (IA/B), it was anticipated that axes 1 and 2 would move together concertedly to form the stable relative configuration of (a 1 R*, a 2 R*). Thus, the configuration of the enantiomers was presumed to be (a 1 R, a 2 R) and (a 1 S, a 2 S), respectively. In order to elucidate how axes 1 and 2 move together concertedly to form the stable relative configuration, N-tosyl 5H-dibenzo [b,d]azepin-7(6H)-one (IIAc) (R 1 = H) and its 4methyl derivative (R 1 = Me) (IIBc) were examined. In the 1 H NMR (CDCl 3 ) spectra of IIAc and IIBc, they exist as a single compound, and each methylene proton (6-H) was observed as a separated sharp peak, which indicates the presence of chirality. The atropisomers of (IIAc) (R 1 = H) were inseparable by chiral HPLC because the ring inversion via rotation around the axis was too rapid for separation at rt. In contrast, those of IIBc (R 1 = Me) were sufficiently stable to be separated and isolated with chiral HPLC at rt; the separated isomers showed a high energy barrier to rotation (ΔG ‡ = 127.5 kJ/mol). Each isomer has opposite [α] D values: that with shorter retention time in HPLC at 96.8% ee showed [α] D −65.8 (c 0.23, CHCl 3 ) and that with a longer retention time in HPLC at 96.1% ee showed [α] D +64.9 (c 0.21, CHCl 3 ), confirming that they are enantiomers. As well as N-tosyl 5Hdibenzo [b,d]azepin-7(6H)-one (IIAc) (R 1 = H), the atropisomers of other N-sulfonyl derivatives (IIAd−f) (R 1 = H) were inseparable by chiral HPLC. On the other hand, the presence of the atropisomers of the corresponding 4-methyl derivative (IIBc−f) (R 1 = Me) was confirmed by isolating each isomer by chiral HPLC. The separated isomers showed a high energy barrier to rotation (IIBd: ΔG ‡ = 126.3 kJ/mol, IIBe: ΔG ‡ = 131.6 kJ/mol, IIBf: ΔG ‡ = 131.6 kJ/mol). The physicochemical properties of IIBc−f are shown in Table 3. It is noteworthy that compounds IIBe and IIBf with the most electron-withdrawing nosyl group showed the highest energy barrier to rotation.
Fortunately, a single crystal for the X-ray crystal structure analysis of IIBc (racemate) was obtained, in which IIBc possessed the stable relative configuration of (aR, aR) and (aS, aS) as expected in a unit cell ( Figure 6). It was revealed that axial chirality caused by the Ar−N(SO 2 ) axis (axis 2), which showed a rather high energy barrier (ΔG ‡ = 126.3−131.6 kJ/ mol), moves concertedly with the axis at the biphenyl (axis 1) to form the stable relative configuration of (aR*, aR*) without the presence of diastereomers (aR*, aS*). Such a high energy barrier might be due to the planarity of the nitrogen atom. 16 The sum of angles around the nitrogen atom in the >N−SO 2 moiety is 359.2°, indicating the sp 2 -like nature of the nitrogen atom. In addition, the bond length between N−S (0.16 nm) suggests the double-bond character of the N−S bond. While these data imply that the > N−S moiety forms a plane, it was found that the SO 2 moiety locates so as to interweave with the N−SO 2 axis: dihedral angles ∠O 1 −S−N−C6 and ∠O 2 −S− N−C4a were −44.90°and +12.64°, respectively. The important point to note is that the sulfonyl (SO) bond is not on the >N−S plane. Considering that the carbonyl (C O) is on the amide (>N−CO) plane and the N−C bond has a double-bond character due to the resonance, the doublebond character of the N−S bond without the planarity of sulfonamide (>N−SO) is interesting. It was also found that the seven-membered ring exists in a boat-like form, the benzene ring of the tosyl moiety locates over the benzene ring of biphenyl (folded form), and they are nearly parallel to each other.
Blockade of Potassium Channel Kv1.3. We next conducted a preliminary investigation of the blockade of the potassium channel. Considering the level of activity on Kv1.3 of IIAc (IC 50 5.8 μM), 4 blocking activity on the voltage-gated potassium channel Kv1.3 with 4-aminopyridine as a positive control was tested for IIBc using patch-clamp technology ( Table 4). IIBc in racemic form showed a moderate level of inhibitory activity of 63% at 10 μM when the channel was in the closed state. Hence, the separated enantiomers of (+)-IIBc and (−)-IIBc were subjected to the binding assay to examine the difference in potency between the enantiomers. Although the enantiomers and the racemate exhibited similar levels of affinity (within a 4.5-fold difference), (−)-IIBc showed more potent affinity than (+)-IIBc.

■ EXPERIMENTAL SECTION
General Information. All reagents were purchased from commercial suppliers and used as received. Reaction mixtures were stirred magnetically, and the reactions were monitored by thin-layer chromatography (TLC) on precoated silica gel plates. For the reactions that require heating, an oil bath was used. Column chromatography was performed using silica gel (45−60 μm). For recrystallizaton, crude products were dissolved in AcOEt/diisopropyl ether/hexane, and the precipitated crystals were collected. Extracted solutions were dried over anhydrous Na 2 SO 4 . Solvents were evaporated under reduced pressure. NMR spectra were recorded on a spectrometer at 600 MHz for 1 H NMR and at 150 MHz for 13 C NMR at 296 K unless otherwise stated. Tetramethylsilane (TMS) (δ 0.00) or residual internal CHCl 3 (δ 7.26) was used as an internal reference for the 1 H spectroscopy measurements of samples in CDCl 3 . TMS (δ 0.00) or residual internal CHCl 3 (δ 77.16) was used as an internal reference for the 13 C spectroscopy measurements of samples in CDCl 3 . Coupling constants (J) are reported in hertz (Hz). Splitting patterns are abbreviated as follows: singlet (s); doublet (d); triplet (t); quartet (q); multiplet (m); and broad (br). The highresolution mass spectra (HRMS) were recorded using an ESI/TOF, APCI/TOF, or EI-MS mass spectrometer. IR spectra were recorded on an FT-IR spectrometer equipped with ATR (Diamond). Melting points were recorded on a melting point apparatus and are uncorrected. The chemical structures of S1−S10 were shown in the Supporting Information.