Rapid communication Received: 10 October 2014
Revised: 31 October 2014
Accepted: 31 October 2014
Published online in Wiley Online Library: 22 January 2015
(wileyonlinelibrary.com) DOI 10.1002/mrc.4192
The determination of the absolute configuration of a chiral 2,3′-diindolylarylmethane by NMR spectroscopy Shuai Qi,a,b Chuan-Qing Kanga and Fu-She Hana* We present the determination of the absolute configuration of a chiral 2,3′-diindolylarylmethane 1 by using the combination of NMR spectroscopic and circular dichroism techniques. The results would be useful for the future study of the effect of chirality on the biological activity of 2,3′-diindolylarylmethanes. Copyright © 2015 John Wiley & Sons, Ltd. Keywords: NMR; 1H; absolute configuration; 2,3′-diindolylarylmethane; Mosher’s model; α-methoxyphenylacetic acid (MPA); CD spectrum
Introduction
Magn. Reson. Chem. 2015, 53, 181–187
Results and discussion Synthesis of the diastereomeric amides for NMR study For a safe derivatization, we initially examined the optical stability of chiral 2,3’-diindolylarylmethane 1 (Scheme 2) under a range of acidic and basic conditions in considering that the methine proton of the triarylmethanes (H-8 in structures 2 and 3 in Fig. 1) was relatively acidic, which may cause racemization through the equilibration of deprotonation and protonation process. Our control experiments showed that no racemization was observed by treating the products with acidic silica gel, p-TsOH, Et3N, and pyridine in CH2Cl2, NaOH in THF/H2O (v/v = 10 : 1), and NaH in THF at room temperature, separately. These results indicate that the diindolylarylmethanes exhibit pretty good resistance against a range of acidic and basic conditions and provide useful information for the option of appropriate conditions to derivatize with a chiral reagent. In addition, these observations also preclude the suspicion of racemization for future investigation into the effect of chirality on the biological activity under the conditions similar to a physiological environment because, in general, such environment displays a weaker basicity or acidity than the those employed for control experiments.
* Correspondence to: Fushe Han, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun, Jilin 130022, China. E-mail: [email protected] a Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun, Jilin, 130022, China b The University of Chinese Academy of Sciences, Beijing 100864, China
Copyright © 2015 John Wiley & Sons, Ltd.
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Diindolylarylmethane compounds, originally found in cruciferous vegetables, are an important class of leading compounds in the search of new anticancer agents. They exhibit ample in vivo anticancer activities including growth inhibition, apoptosis, and antiangiogenic activities.[1] Particularly, the diindolylarylmethane analogues display high activity against a broad range of cancer cells such as breast,[2] prostate,[3] colon,[4] pancreatic,[5] acute myelogenous leukemia,[6] bladder,[7] and endometrial cancer cell lines.[8] Furthermore, inhibition of a targeted cell line in a selective manner might also be tuned by means of modifying the indole and/or aryl rings appropriately. Despite of these impressive results, the anticancer activity of chiral diindolylarylmethanes remains not investigated. However, numerous studies have demonstrated that the biological activity of a molecule, including the heterotriarylmethanes,[9] is dramatically influenced by its chirality. It is thus would be greatly interesting to investigate the anticancer property of the related chiral compounds. So far, two protocols for the enantioselective synthesis of 3,3′diindolylarylmethanes have been developed via the Friedel– Crafts-type reaction. The first one was reported in 2010 by You and co-workers through the chiral phosphoric acid-catalyzed reaction of (indol-3-yl aryl)methyl amines and N-methyl indole[10] (Scheme 1a, 1). In a recent report,[11] Jiang and co-workers developed an improved method for accessing chiral 3,3′-heterotriarylmethanes through the chiral imidodiphosphoric acidcatalyzed reaction of (indol-3-yl aryl)methyl silyl ethers with indoles or pyrroles (Scheme 1a, 2). Very recently, we have achieved the asymmetric synthesis of more challenging chiral 2,3′diindolylarylmethanes through a chiral Brønsted acid-catalyzed reaction of indol-2-yl carbinols and indoles[12] (Scheme 1b). Namely, by employing the chiral N-triflylphosphoramides derived from the unsubstituted (R)-BINOL or (S)-BINOL as catalysts, both enantiomers of 2,3′-diindolylarylmethanes could be obtained flexibly in excellent yields of over 90% as well as high enantioselectivity of up to 96% ee. The successfully established protocols offer an opportunity for the investigation of anticancer property of chiral diindolylarylmethanes. To this end, the assignment of the absolute
configuration of the chiral compounds beforehand is of crucial importance. Herein, we present the determination of the absolute configuration of chiral 2,3′-diindolylarylmethanes. The NMR method based on chiral derivatizing reagents (CDAs)[13] and the circular dichroism (CD) spectroscopy,[14] which are well-established methods for the assignment of the absolute stereochemistry of chiral compounds, were employed in our study.
S. Qi, C.-Q. Kang and F.-S. Han
Scheme 1. Protocols for the enantioselective synthesis of 3,3′diindolylarylmethanes and 2,3′-diindolylarylmethanes.
On the basis of these preliminary results, we chose methoxyphenylacetic acid (MPA) as the suitable CDA because this compound has been widely exemplified to be one of the most powerful and readily available reagent in the determination of absolute configuration of a rich range of chiral alcohols and amines.[13] However, attempted condensation of (R)-MPA and (S)MPA with chiral 2,3′-diindolylarylmethane 1 (er = 91 : 9, synthesized using the (R)-form of N-triflylphosphoramides[12]) according to the procedure described by Trost (DCC, DMAP, Et3N)[15,16] was unsuccessful (Scheme 2). The reaction did not proceed presumably because of the weak nucleophilicity of indole amine.[17] After some trials, we found that by converting the MPA into its acyl chloride[18,19] followed by in situ trapping the acyl chloride with diindolylarylmethame 1 in the presence of NaH[20] could afford the (R)-MPA and (S)-MPA amides 2 and 3, respectively in ca. 60% yield.
Scheme 2. Preparation of the MPA derivatives 2 and 3.
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Figure 1.
1
H NMR spectra of (R)-MPA amide 2 (a) and (S)-MPA amide 3 (b) in CDCl3.
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Copyright © 2015 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2015, 53, 181–187
The determination of the absolute configuration of novel chiral 2,3′-diindolylarylmethanes is presented
Figure 2.
1
1
H– H NOESY spectra of (R)-MPA amide 2 (a) and (S)-MPA amide 3 (b).
Assignment of the NMR spectra of MPA amides
Magn. Reson. Chem. 2015, 53, 181–187
Assignment of the absolute configuration of 2,3′-diindolylarylmethane 1 To deduce the absolute configuration of 1 based on the information offered by the 1H-NMR data, the chemical shift difference between the two diastereoisomers 2 and 3 is compared. The chemical shifts for 2 (δR ppm), 3 (δS ppm), and the chemical shift difference between 2 and 3 (ΔδRS = δR δS) are listed in Table 1. The data showed that the ΔδRS values for H-4, H-8, H-18, H-2′, H-α, and
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The 1H-NMR and 1H–1H NOESY spectra of 2 and 3 were shown in Figs. 1 and 2 respectively. Most of the protons for 2 and 3 could be easily assigned based on these spectra as labeled in Fig. 1a and 1b, respectively. The chemical shift for proton H-8 (5.75 ppm for 2 and 5.72 ppm for 3) could be assigned by the use of a combination of 1H-NMR, 1H–1H NOESY, and 1H–13C HSQC spectra (Fig. 3). Finally, for the protons of H-10, H-12, H-14, and H-2′ whose chemical shift is somewhat overlapped around 6.80–7.10 ppm could be unambiguously confirmed by a further 1H–13C HMBC spectroscopic analysis. As shown in Fig. 4a, from the correlation of 1H–13C HMBC, we could clearly assign that the peaks that appear at 7.08 and
6.92 ppm belong to H-2′ and H-12 of compound 2, respectively. Similarly, as seen from Fig. 4b, the peaks at 6.95 and 6.92 ppm can be assigned to H-2′ and H-12 of compound 3, respectively.
S. Qi, C.-Q. Kang and F.-S. Han
1
13
Figure 3. Partial H– C HSQC spectra of (R)-MPA amide 2 in CDCl3.
184
H-OMe are positive within a range of from +0.03 to +0.13, suggesting that the chemical shifts of these protons are shifted toward upfield in compound 3 as compared with those of compound 2. The most remarkable effect was observed for H-18 and H-2′, whose ΔδRS value was up to +0.12 and +0.13, respectively. On the other hand, the protons H-15, H-16, and H-17 tend to shift somewhat downfield in 3 (ΔδRS = 0.04, 0.02, and 0.02, respectively), indicating that these protons were less shielded by the phenyl group in 3 than in 2. Based on the chemical shift difference of 2 and 3, the 1H–1H NOESY correlation, and Mosher’s model[13] (i.e. the methoxy and carbonyl groups in MPA and the central indole moiety are assumed to situate approximately in the same plane, and the phenyl ring is almost coplanar to the Cα–H bond), we could establish the optimized conformation of compounds 2 and 3, which are in good agreement with that obtained by the density functional theory (DFT) calculations (E = 1731.1668 au for 2 and 1731.1672 au for 3) at B3LYP/6-31G**/PCM level. As shown in Fig. 5, the methoxy group is syn-periplanar to the carbonyl group, and H-α and H-2′ are arranged approximately in the same side, which are the best arrangement to effectively transmit aromatic shielding to the H-8, H2′, and the indole isopropyl and methyl groups. Consequently, a strong NOE effect was observed between H-α and H-2′ (Fig. 2). In addition, the H-15, H-16, and H-17 of isopropyl in (R)-MPA amide 2 are arranged in the same side with the phenyl group. This explains why the protons are shielded somewhat more significantly by the phenyl group in 2, and ultimately, exhibiting a larger upfield shift (Fig. 5a, ΔδRS < 0 as seen in Table 1). Alternatively, in (S)-MPA amide 3 (Fig. 5b), the protons H-4, H-8, H-18, and H-2′, particularly the H-18 and H-2′, suffer from a stronger shielding effect by the phenyl group of MPA moiety than those in compound 2 (ΔδRS > 0 as seen in Table 1). These observations matched well with the optimized conformation of 3 wherein the mentioned protons and the phenyl group in 3 are arranged in the same side. It should also be mentioned that the H-α, and the protons in OMe of the MPA moiety of compound 3 are slightly shifted upfield (ΔδRS = +0.04 and +0.05, respectively). This is presumably because of the shielding effect of 3,5-dimethylphenyl group as seen from the optimized conformation. Thus, on the basis of the NMR analysis, we could tentatively assign that the absolute configuration of the chiral 2,3′-
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diindolylarylmethanes 1 synthesized by using the (R)-form of Ntriflylphosphoramides as catalyst is R. To further confirm the absolute configuration, we measured the CD spectrum of 1 and compared the experimental CD spectra with those of (R)-1 and (S)-1 obtained from quantum chemical calculations. This approach is also a well-established method for the determination of the absolute stereochemistry of a chiral molecule.[14] As shown in Fig. 6, the experimental CD of chiral compound 1 is in qualitative agreement with the calculated CD of (R)-1. In conclusion, we have determined the absolute configuration of a chiral 2,3′-diindolylarylmethane 1 by using the NMR method based on the CDAs combined with the use of CD analytical method. Namely, the absolute configuration of the product obtained by using the (R)-form and (S)-form of catalyst is R and S, respectively. As mentioned, the 2,3′-diindolylarylmethanes are novel indole derivatives and are anticipated to be interest candidates as new anticancer agents. However, the biological properties of neither the racemic nor the chiral 2,3′-diindolylarylmethanes have been studied. As such, the method for flexibly accessing both R and S enantiomers of chiral 2,3′-diindolylarylmethanes established in our previous study[12] together with the results for the determination of the absolute configuration presented herein would pave the way for a thorough investigation of the anticancer properties of 2,3′-diindolylarylmethane derivatives. On the basis of the results in this work, a further study toward the establishment of a general method for the assignment of the absolute configuration of a range of 2,3′-diindolylarylmethanes by using the NMR technique is underway.
Experimental Preparation of MPA amides To a solution of (R)-α-MPA or (S)-α-MPA (49.8 mg, 0.3 mmol) in dried CH2Cl2 (3 ml) was added oxalyl chloride (26 μl, 0.3 mmol) and N,NDimethylformamide (DMF) (31 μl, 0.4 mmol) at 0 °C. The reactants were mixed with a magnetic stir bar in an ice bath for 1 h to obtain the corresponding methoxyphenylacetyl chloride.
Copyright © 2015 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2015, 53, 181–187
The determination of the absolute configuration of novel chiral 2,3′-diindolylarylmethanes is presented
1
13
Figure 4. Partial H– C HMBC spectra of (R)-MPA amide 2 (a) and (S)-MPA amide 3 (b) in CDCl3.
To a solution of the chiral 2,3′-diindolylarylmethane 1 (81 mg, 0.2 mmol) in dried THF (2 ml) was added NaH (16 mg, 0.4 mmol, 60% in mineral oil) at 0 °C. The slurry thus formed was gradually warmed to room temperature over a 30 min period. The mixture was then cooled to 0 °C, and the previously prepared methoxyphenylacetyl chloride was added dropwise. The mixture was stirred at 0 °C for 4 h. The reaction mixture was poured to ice water and extracted with ethyl ether three times. The organic layer was combined, washed with water, and dried over anhydrous MgSO4. The solvent was removed under vacuum, and residue was purified by chromatography in silica gel to give the desired compound (66.5 mg, 0.12 mmol, 60%). NMR experiments
Magn. Reson. Chem. 2015, 53, 181–187
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All NMR experiments were performed on a Bruker AVANCE HD 500 MHz spectrometer in CDCl3 at 298 K using Bruker TopSpin 3.2
PL6 software with a 5 mm smart probe equipped with a z-gradient coil. The 1D 1H NMR spectra were measured with a spectral width of 10 kHz on a data size of 32 K with pulse length of 13.0 μs. The 13C NMR spectra were measured with a spectral width of 32.5 kHz on a data size of 128 K with pulse length of 7.5 μs. The acquisition times were 5 s in 1H NMR spectra and 3 s in 13C NMR spectra. Chemical shifts were recorded relative to internal standard TMS (δ 0.0 ppm) for 1H NMR and the central peak of CDCl3 (δ 77.0 ppm) for 13C NMR. All 2D spectra were measured with Bruker standard TopSpin 3.2 PL6 program noesygpphpp for 1H–1H NOESY, hsqcedetgpsp.3 for 1 H–13C HSQC, and shmbcctetgpl2nd for 1H–13C HMBC. The 1H–1H NOESY spectra were obtained with a spectral width of 6000 Hz in a 2048 × 128 data matrix with four scans per t1 increment. The 1 H–13C HSQC spectra were collected with 2048 × 256 data points in a spectral width of 6000 Hz for F2 (1H) and 100 000 Hz for F1 (13C). The 1H–13C HMBC spectra were collected with 2048 × 256 data points in a spectral width of 6000 Hz for F2 (1H) and
S. Qi, C.-Q. Kang and F.-S. Han 1
RS
Table 1. H NMR data and Δδ values (in ppm) for (R)-MPA and (S)MPA amides of 2 and 3 in CDCl3 1
H chemical shifts, δ (ppm)
Proton signal H-4 H-7 H-8 H-10, H-14 H-12 H-15 H-16 H-17 H-18 H-19, H-20 H-α H-OMe H-2′ H-7′
2
3
7.54 7.46 5.75 6.77 6.92 4.53 1.15 1.33 1.85 2.26 5.21 3.47 7.08 8.53
7.51 7.46 5.72 6.77 6.92 4.57 1.17 1.35 1.73 2.26 5.17 3.42 6.95 8.54
RS
Δδ (ppm) R
δ
S
δ
+0.03 0.00 +0.03 0.00 0.00 0.04 0.02 0.02 +0.12 0.00 +0.04 +0.05 +0.13 0.01
13
37 500 Hz for F1 ( C). The data were processed using QSINE for weighting in both dimensions before Fourier transformation. Measurement of CD spectrum of 1 The CD spectra were recorded on a Bio-Logic MOS-450 spectropolarimeter using a quartz cell of 1-cm optical path length. The scanning speed was 150 nm min 1 with a response time of 0.1 s and over a wavelength range of 220–400 nm. The concentration of chiral 2,3′-diindolylarylmethane 1 was 1 × 10 4 M in methanol, and the CD spectra was representative of three averaged scans taken at 25 °C. Computational details All calculations were performed in Gaussian 09 program package.[21] The geometry of compounds 2 and 3 at ground state
Figure 6. The experimental and simulated electronic circular dichroism spectra of 1 at B3LYP/6-31G**/PCM (methanol) level.
(S0) was optimized using DFT together with the D3 version of Grimme’s dispersion (D3) with Becke–Johnson damping (BJ).[22] The used functional and basis set is B3LYP and 6-31G**, respectively. The frequency analysis was also performed at the same level to check the stationary nature of optimized geometry. In addition, the optimized geometry of compounds (R)-1 and (S)-1 in methanol solvent were obtained using the same theoretical level mentioned in the preceding texts. Then, based on the S0 geometry, linear response timedependent DFT (TDDFT) calculations with B3LYP functional and 6-31G** basis set were performed to obtain the excitation energy and rotatory strengths of compounds (R)-1 and (S)-1, which can be convoluted into electronic circular dichroism (ECD) spectrum in multiwfn 3.3.5[23] using Gaussian model. The solvation effect of methanol solvent, in which experimental data of ECD spectrum were measured, was considered during the optimization and TDDFT calculations by means of the polarizable continuum model (PCM).
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Figure 5. The optimized geometry of compounds 2 and 3 at B3LYP/6-31G**/PCM level.
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Magn. Reson. Chem. 2015, 53, 181–187
The determination of the absolute configuration of novel chiral 2,3′-diindolylarylmethanes is presented Acknowledgements Financial support from NSFC (21272225) is acknowledged. The authors gratefully acknowledge computational support from Dr. HaiBin Li and Prof. Zhong-Min Su at Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University.
References [1] For a recent review, see: S. Safe, S. Papineni, S. Chintharlapalli. Cancer Lett. 2008, 269, 326–338. [2] C. Qin, D. Morrow, J. Stewart, K. Spencer, W. Porter, R. Smith III, T. Phillips, M. Abdelrahim, I. J. Samudio, S. Safe. Mol. Cancer Ther. 2004, 3, 247–260. [3] S. Chintharlapalli, S. Papineni, S. Safe. Mol. Pharmacol. 2007, 71, 558–569. [4] J. Guo, S. Chintharlapalli, S. Lee, S. D. Cho, P. Lei, S. Papineni, S. Safe. Cancer Chemother. Pharmacol. 2010, 66, 141–150. [5] M. Abdelrahim, K. Newman, K. Vanderlaag, I. Samudio, S. Safe. Carcinogenesis 2006, 27, 717–728. [6] R. Contractor, I. J. Samudio, Z. Estrov, D. Harris, J. A. McCubrey, S. Safe, M. Andreeff, M. Konopleva. Cancer Res. 2005, 65, 2890–2898. [7] W. Kassouf, S. Chintharlapalli, M. Abdelrahim, G. Nelkin, S. Safe, A. M. Kamat. Cancer Res. 2006, 66, 412–418. [8] J. Hong, I. Samudio, S. Chintharlapalli, S. Safe. Mol. Carcinog. 2008, 47, 492–507. [9] For an example, see: P. M. Wood, L. L. Woo, J. R. Labrosse, M. N. Trusselle, S. Abbate, G. Longhi, E. Castiglioni, F. Lebon, A. Purohit, M. J. Reed. J. Med. Chem. 2008, 51, 4226–4238. [10] F. L. Sun, X. J. Zheng, Q. Gu, Q. L. He, S. L. You. Eur. J. Org.Chem. 2010, 47–50.
[11] M. H. Zhuo, Y. J. Jiang, Y. S. Fan, Y. Gao, S. Liu, S. Q. Zhang. Org. Lett. 2014, 16, 1096–1100. [12] S. Qi, C. Y. Liu, J. Y. Ding, F. S. Han. Chem. Commun. 2014, 50, 8605–8608. [13] (a) J. M. Seco, E. Quiñoá, R. Riguera. Chem. Rev. 2004, 104, 17–117; (b) J. M. Seco, E. Quiñoá, R. Riguera. Chem. Rev. 2012, 112, 4603–4641. [14] N. Berova, L. D. Bari, G. Pescitelli. Chem. Soc. Rev. 2007, 36, 914–931. [15] B. M. Trost, R. C. Bunt, S. R. Pulley. J. Org. Chem. 1994, 59, 4202–4205. [16] M. Lee, L. W. Lown, S. R. Pulley. J. Org. Chem. 1987, 52, 5717–5721. [17] J. Magolan, C. A. Carson, M. A. Kerr. Org. Lett. 2008, 10, 1437–1440. [18] P. A. Stadler. Helu Chim. Acta. 1978, 61, 1675–1681. [19] Y. Dai, L. Harinantenaina, P. J. Brodie, M. W. Callmander, R. Randrianaivo, S. Rakotonandrasana, E. Rakotobe, V. E. Rasamison, Y. Shen, K. TenDyke, E. M. Suh, D. G. I. Kingston. J. Nat. Prod. 2012, 75, 479–483. [20] X. Mu, S. J. Chen, X. L. Zhen, G. S. Liu. Chem. Eur. J. 2011, 17, 6039–6042. [21] Gaussian 09, (Revision D.01), M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox. Gaussian, Inc., Wallingford CT, 2009. [22] S. Grimme, S. Ehrlich, L. Goerigk. J. Comput. Chem. 2011, 32, 1456–1465. [23] T. Lu, F. W. Chen. J. Comput. Chem. 2012, 33, 580–592.
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Revised: 31 October 2014
Accepted: 31 October 2014
Published online in Wiley Online Library: 22 January 2015
(wileyonlinelibrary.com) DOI 10.1002/mrc.4192
The determination of the absolute configuration of a chiral 2,3′-diindolylarylmethane by NMR spectroscopy Shuai Qi,a,b Chuan-Qing Kanga and Fu-She Hana* We present the determination of the absolute configuration of a chiral 2,3′-diindolylarylmethane 1 by using the combination of NMR spectroscopic and circular dichroism techniques. The results would be useful for the future study of the effect of chirality on the biological activity of 2,3′-diindolylarylmethanes. Copyright © 2015 John Wiley & Sons, Ltd. Keywords: NMR; 1H; absolute configuration; 2,3′-diindolylarylmethane; Mosher’s model; α-methoxyphenylacetic acid (MPA); CD spectrum
Introduction
Magn. Reson. Chem. 2015, 53, 181–187
Results and discussion Synthesis of the diastereomeric amides for NMR study For a safe derivatization, we initially examined the optical stability of chiral 2,3’-diindolylarylmethane 1 (Scheme 2) under a range of acidic and basic conditions in considering that the methine proton of the triarylmethanes (H-8 in structures 2 and 3 in Fig. 1) was relatively acidic, which may cause racemization through the equilibration of deprotonation and protonation process. Our control experiments showed that no racemization was observed by treating the products with acidic silica gel, p-TsOH, Et3N, and pyridine in CH2Cl2, NaOH in THF/H2O (v/v = 10 : 1), and NaH in THF at room temperature, separately. These results indicate that the diindolylarylmethanes exhibit pretty good resistance against a range of acidic and basic conditions and provide useful information for the option of appropriate conditions to derivatize with a chiral reagent. In addition, these observations also preclude the suspicion of racemization for future investigation into the effect of chirality on the biological activity under the conditions similar to a physiological environment because, in general, such environment displays a weaker basicity or acidity than the those employed for control experiments.
* Correspondence to: Fushe Han, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun, Jilin 130022, China. E-mail: [email protected] a Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun, Jilin, 130022, China b The University of Chinese Academy of Sciences, Beijing 100864, China
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181
Diindolylarylmethane compounds, originally found in cruciferous vegetables, are an important class of leading compounds in the search of new anticancer agents. They exhibit ample in vivo anticancer activities including growth inhibition, apoptosis, and antiangiogenic activities.[1] Particularly, the diindolylarylmethane analogues display high activity against a broad range of cancer cells such as breast,[2] prostate,[3] colon,[4] pancreatic,[5] acute myelogenous leukemia,[6] bladder,[7] and endometrial cancer cell lines.[8] Furthermore, inhibition of a targeted cell line in a selective manner might also be tuned by means of modifying the indole and/or aryl rings appropriately. Despite of these impressive results, the anticancer activity of chiral diindolylarylmethanes remains not investigated. However, numerous studies have demonstrated that the biological activity of a molecule, including the heterotriarylmethanes,[9] is dramatically influenced by its chirality. It is thus would be greatly interesting to investigate the anticancer property of the related chiral compounds. So far, two protocols for the enantioselective synthesis of 3,3′diindolylarylmethanes have been developed via the Friedel– Crafts-type reaction. The first one was reported in 2010 by You and co-workers through the chiral phosphoric acid-catalyzed reaction of (indol-3-yl aryl)methyl amines and N-methyl indole[10] (Scheme 1a, 1). In a recent report,[11] Jiang and co-workers developed an improved method for accessing chiral 3,3′-heterotriarylmethanes through the chiral imidodiphosphoric acidcatalyzed reaction of (indol-3-yl aryl)methyl silyl ethers with indoles or pyrroles (Scheme 1a, 2). Very recently, we have achieved the asymmetric synthesis of more challenging chiral 2,3′diindolylarylmethanes through a chiral Brønsted acid-catalyzed reaction of indol-2-yl carbinols and indoles[12] (Scheme 1b). Namely, by employing the chiral N-triflylphosphoramides derived from the unsubstituted (R)-BINOL or (S)-BINOL as catalysts, both enantiomers of 2,3′-diindolylarylmethanes could be obtained flexibly in excellent yields of over 90% as well as high enantioselectivity of up to 96% ee. The successfully established protocols offer an opportunity for the investigation of anticancer property of chiral diindolylarylmethanes. To this end, the assignment of the absolute
configuration of the chiral compounds beforehand is of crucial importance. Herein, we present the determination of the absolute configuration of chiral 2,3′-diindolylarylmethanes. The NMR method based on chiral derivatizing reagents (CDAs)[13] and the circular dichroism (CD) spectroscopy,[14] which are well-established methods for the assignment of the absolute stereochemistry of chiral compounds, were employed in our study.
S. Qi, C.-Q. Kang and F.-S. Han
Scheme 1. Protocols for the enantioselective synthesis of 3,3′diindolylarylmethanes and 2,3′-diindolylarylmethanes.
On the basis of these preliminary results, we chose methoxyphenylacetic acid (MPA) as the suitable CDA because this compound has been widely exemplified to be one of the most powerful and readily available reagent in the determination of absolute configuration of a rich range of chiral alcohols and amines.[13] However, attempted condensation of (R)-MPA and (S)MPA with chiral 2,3′-diindolylarylmethane 1 (er = 91 : 9, synthesized using the (R)-form of N-triflylphosphoramides[12]) according to the procedure described by Trost (DCC, DMAP, Et3N)[15,16] was unsuccessful (Scheme 2). The reaction did not proceed presumably because of the weak nucleophilicity of indole amine.[17] After some trials, we found that by converting the MPA into its acyl chloride[18,19] followed by in situ trapping the acyl chloride with diindolylarylmethame 1 in the presence of NaH[20] could afford the (R)-MPA and (S)-MPA amides 2 and 3, respectively in ca. 60% yield.
Scheme 2. Preparation of the MPA derivatives 2 and 3.
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Figure 1.
1
H NMR spectra of (R)-MPA amide 2 (a) and (S)-MPA amide 3 (b) in CDCl3.
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Magn. Reson. Chem. 2015, 53, 181–187
The determination of the absolute configuration of novel chiral 2,3′-diindolylarylmethanes is presented
Figure 2.
1
1
H– H NOESY spectra of (R)-MPA amide 2 (a) and (S)-MPA amide 3 (b).
Assignment of the NMR spectra of MPA amides
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Assignment of the absolute configuration of 2,3′-diindolylarylmethane 1 To deduce the absolute configuration of 1 based on the information offered by the 1H-NMR data, the chemical shift difference between the two diastereoisomers 2 and 3 is compared. The chemical shifts for 2 (δR ppm), 3 (δS ppm), and the chemical shift difference between 2 and 3 (ΔδRS = δR δS) are listed in Table 1. The data showed that the ΔδRS values for H-4, H-8, H-18, H-2′, H-α, and
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The 1H-NMR and 1H–1H NOESY spectra of 2 and 3 were shown in Figs. 1 and 2 respectively. Most of the protons for 2 and 3 could be easily assigned based on these spectra as labeled in Fig. 1a and 1b, respectively. The chemical shift for proton H-8 (5.75 ppm for 2 and 5.72 ppm for 3) could be assigned by the use of a combination of 1H-NMR, 1H–1H NOESY, and 1H–13C HSQC spectra (Fig. 3). Finally, for the protons of H-10, H-12, H-14, and H-2′ whose chemical shift is somewhat overlapped around 6.80–7.10 ppm could be unambiguously confirmed by a further 1H–13C HMBC spectroscopic analysis. As shown in Fig. 4a, from the correlation of 1H–13C HMBC, we could clearly assign that the peaks that appear at 7.08 and
6.92 ppm belong to H-2′ and H-12 of compound 2, respectively. Similarly, as seen from Fig. 4b, the peaks at 6.95 and 6.92 ppm can be assigned to H-2′ and H-12 of compound 3, respectively.
S. Qi, C.-Q. Kang and F.-S. Han
1
13
Figure 3. Partial H– C HSQC spectra of (R)-MPA amide 2 in CDCl3.
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H-OMe are positive within a range of from +0.03 to +0.13, suggesting that the chemical shifts of these protons are shifted toward upfield in compound 3 as compared with those of compound 2. The most remarkable effect was observed for H-18 and H-2′, whose ΔδRS value was up to +0.12 and +0.13, respectively. On the other hand, the protons H-15, H-16, and H-17 tend to shift somewhat downfield in 3 (ΔδRS = 0.04, 0.02, and 0.02, respectively), indicating that these protons were less shielded by the phenyl group in 3 than in 2. Based on the chemical shift difference of 2 and 3, the 1H–1H NOESY correlation, and Mosher’s model[13] (i.e. the methoxy and carbonyl groups in MPA and the central indole moiety are assumed to situate approximately in the same plane, and the phenyl ring is almost coplanar to the Cα–H bond), we could establish the optimized conformation of compounds 2 and 3, which are in good agreement with that obtained by the density functional theory (DFT) calculations (E = 1731.1668 au for 2 and 1731.1672 au for 3) at B3LYP/6-31G**/PCM level. As shown in Fig. 5, the methoxy group is syn-periplanar to the carbonyl group, and H-α and H-2′ are arranged approximately in the same side, which are the best arrangement to effectively transmit aromatic shielding to the H-8, H2′, and the indole isopropyl and methyl groups. Consequently, a strong NOE effect was observed between H-α and H-2′ (Fig. 2). In addition, the H-15, H-16, and H-17 of isopropyl in (R)-MPA amide 2 are arranged in the same side with the phenyl group. This explains why the protons are shielded somewhat more significantly by the phenyl group in 2, and ultimately, exhibiting a larger upfield shift (Fig. 5a, ΔδRS < 0 as seen in Table 1). Alternatively, in (S)-MPA amide 3 (Fig. 5b), the protons H-4, H-8, H-18, and H-2′, particularly the H-18 and H-2′, suffer from a stronger shielding effect by the phenyl group of MPA moiety than those in compound 2 (ΔδRS > 0 as seen in Table 1). These observations matched well with the optimized conformation of 3 wherein the mentioned protons and the phenyl group in 3 are arranged in the same side. It should also be mentioned that the H-α, and the protons in OMe of the MPA moiety of compound 3 are slightly shifted upfield (ΔδRS = +0.04 and +0.05, respectively). This is presumably because of the shielding effect of 3,5-dimethylphenyl group as seen from the optimized conformation. Thus, on the basis of the NMR analysis, we could tentatively assign that the absolute configuration of the chiral 2,3′-
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diindolylarylmethanes 1 synthesized by using the (R)-form of Ntriflylphosphoramides as catalyst is R. To further confirm the absolute configuration, we measured the CD spectrum of 1 and compared the experimental CD spectra with those of (R)-1 and (S)-1 obtained from quantum chemical calculations. This approach is also a well-established method for the determination of the absolute stereochemistry of a chiral molecule.[14] As shown in Fig. 6, the experimental CD of chiral compound 1 is in qualitative agreement with the calculated CD of (R)-1. In conclusion, we have determined the absolute configuration of a chiral 2,3′-diindolylarylmethane 1 by using the NMR method based on the CDAs combined with the use of CD analytical method. Namely, the absolute configuration of the product obtained by using the (R)-form and (S)-form of catalyst is R and S, respectively. As mentioned, the 2,3′-diindolylarylmethanes are novel indole derivatives and are anticipated to be interest candidates as new anticancer agents. However, the biological properties of neither the racemic nor the chiral 2,3′-diindolylarylmethanes have been studied. As such, the method for flexibly accessing both R and S enantiomers of chiral 2,3′-diindolylarylmethanes established in our previous study[12] together with the results for the determination of the absolute configuration presented herein would pave the way for a thorough investigation of the anticancer properties of 2,3′-diindolylarylmethane derivatives. On the basis of the results in this work, a further study toward the establishment of a general method for the assignment of the absolute configuration of a range of 2,3′-diindolylarylmethanes by using the NMR technique is underway.
Experimental Preparation of MPA amides To a solution of (R)-α-MPA or (S)-α-MPA (49.8 mg, 0.3 mmol) in dried CH2Cl2 (3 ml) was added oxalyl chloride (26 μl, 0.3 mmol) and N,NDimethylformamide (DMF) (31 μl, 0.4 mmol) at 0 °C. The reactants were mixed with a magnetic stir bar in an ice bath for 1 h to obtain the corresponding methoxyphenylacetyl chloride.
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Magn. Reson. Chem. 2015, 53, 181–187
The determination of the absolute configuration of novel chiral 2,3′-diindolylarylmethanes is presented
1
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Figure 4. Partial H– C HMBC spectra of (R)-MPA amide 2 (a) and (S)-MPA amide 3 (b) in CDCl3.
To a solution of the chiral 2,3′-diindolylarylmethane 1 (81 mg, 0.2 mmol) in dried THF (2 ml) was added NaH (16 mg, 0.4 mmol, 60% in mineral oil) at 0 °C. The slurry thus formed was gradually warmed to room temperature over a 30 min period. The mixture was then cooled to 0 °C, and the previously prepared methoxyphenylacetyl chloride was added dropwise. The mixture was stirred at 0 °C for 4 h. The reaction mixture was poured to ice water and extracted with ethyl ether three times. The organic layer was combined, washed with water, and dried over anhydrous MgSO4. The solvent was removed under vacuum, and residue was purified by chromatography in silica gel to give the desired compound (66.5 mg, 0.12 mmol, 60%). NMR experiments
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All NMR experiments were performed on a Bruker AVANCE HD 500 MHz spectrometer in CDCl3 at 298 K using Bruker TopSpin 3.2
PL6 software with a 5 mm smart probe equipped with a z-gradient coil. The 1D 1H NMR spectra were measured with a spectral width of 10 kHz on a data size of 32 K with pulse length of 13.0 μs. The 13C NMR spectra were measured with a spectral width of 32.5 kHz on a data size of 128 K with pulse length of 7.5 μs. The acquisition times were 5 s in 1H NMR spectra and 3 s in 13C NMR spectra. Chemical shifts were recorded relative to internal standard TMS (δ 0.0 ppm) for 1H NMR and the central peak of CDCl3 (δ 77.0 ppm) for 13C NMR. All 2D spectra were measured with Bruker standard TopSpin 3.2 PL6 program noesygpphpp for 1H–1H NOESY, hsqcedetgpsp.3 for 1 H–13C HSQC, and shmbcctetgpl2nd for 1H–13C HMBC. The 1H–1H NOESY spectra were obtained with a spectral width of 6000 Hz in a 2048 × 128 data matrix with four scans per t1 increment. The 1 H–13C HSQC spectra were collected with 2048 × 256 data points in a spectral width of 6000 Hz for F2 (1H) and 100 000 Hz for F1 (13C). The 1H–13C HMBC spectra were collected with 2048 × 256 data points in a spectral width of 6000 Hz for F2 (1H) and
S. Qi, C.-Q. Kang and F.-S. Han 1
RS
Table 1. H NMR data and Δδ values (in ppm) for (R)-MPA and (S)MPA amides of 2 and 3 in CDCl3 1
H chemical shifts, δ (ppm)
Proton signal H-4 H-7 H-8 H-10, H-14 H-12 H-15 H-16 H-17 H-18 H-19, H-20 H-α H-OMe H-2′ H-7′
2
3
7.54 7.46 5.75 6.77 6.92 4.53 1.15 1.33 1.85 2.26 5.21 3.47 7.08 8.53
7.51 7.46 5.72 6.77 6.92 4.57 1.17 1.35 1.73 2.26 5.17 3.42 6.95 8.54
RS
Δδ (ppm) R
δ
S
δ
+0.03 0.00 +0.03 0.00 0.00 0.04 0.02 0.02 +0.12 0.00 +0.04 +0.05 +0.13 0.01
13
37 500 Hz for F1 ( C). The data were processed using QSINE for weighting in both dimensions before Fourier transformation. Measurement of CD spectrum of 1 The CD spectra were recorded on a Bio-Logic MOS-450 spectropolarimeter using a quartz cell of 1-cm optical path length. The scanning speed was 150 nm min 1 with a response time of 0.1 s and over a wavelength range of 220–400 nm. The concentration of chiral 2,3′-diindolylarylmethane 1 was 1 × 10 4 M in methanol, and the CD spectra was representative of three averaged scans taken at 25 °C. Computational details All calculations were performed in Gaussian 09 program package.[21] The geometry of compounds 2 and 3 at ground state
Figure 6. The experimental and simulated electronic circular dichroism spectra of 1 at B3LYP/6-31G**/PCM (methanol) level.
(S0) was optimized using DFT together with the D3 version of Grimme’s dispersion (D3) with Becke–Johnson damping (BJ).[22] The used functional and basis set is B3LYP and 6-31G**, respectively. The frequency analysis was also performed at the same level to check the stationary nature of optimized geometry. In addition, the optimized geometry of compounds (R)-1 and (S)-1 in methanol solvent were obtained using the same theoretical level mentioned in the preceding texts. Then, based on the S0 geometry, linear response timedependent DFT (TDDFT) calculations with B3LYP functional and 6-31G** basis set were performed to obtain the excitation energy and rotatory strengths of compounds (R)-1 and (S)-1, which can be convoluted into electronic circular dichroism (ECD) spectrum in multiwfn 3.3.5[23] using Gaussian model. The solvation effect of methanol solvent, in which experimental data of ECD spectrum were measured, was considered during the optimization and TDDFT calculations by means of the polarizable continuum model (PCM).
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Figure 5. The optimized geometry of compounds 2 and 3 at B3LYP/6-31G**/PCM level.
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The determination of the absolute configuration of novel chiral 2,3′-diindolylarylmethanes is presented Acknowledgements Financial support from NSFC (21272225) is acknowledged. The authors gratefully acknowledge computational support from Dr. HaiBin Li and Prof. Zhong-Min Su at Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University.
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