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Abstract—The reaction of electron-deficient arenesulfonamides 4-XC6H4SO2NH2 (X = Cl, NO2) with hexa-. 1,5-diene in the oxidative system t-BuOCl–NaI ...
ISSN 1070-4280, Russian Journal of Organic Chemistry, 2015, Vol. 51, No. 6, pp. 888–892. © Pleiades Publishing, Ltd., 2015. Original Russian Text © V.V. Astakhova, M.Yu. Moskalik, I.V. Sterkhova, B.A. Shainyan, 2015, published in Zhurnal Organicheskoi Khimii, 2015, Vol. 51, No. 6, pp. 904–908.

Oxidative Cycloaddition of Electron-Deficient Arenesulfonamides to Hexa-1,5-diene V. V. Astakhova, M. Yu. Moskalik, I. V. Sterkhova, and B. A. Shainyan Favorskii Irkutsk Institute of Chemistry, Siberian Branch, Russian Academy of Sciences, ul. Favorskogo 1, Irkutsk, 664033 Russia e-mail: [email protected] Received March 14, 2015

Abstract—The reaction of electron-deficient arenesulfonamides 4-XC6H4SO2NH2 (X = Cl, NO2) with hexa1,5-diene in the oxidative system t-BuOCl–NaI afforded cis- and trans-isomeric 1-(arenesulfonyl)-2,5-bis(iodomethyl)pyrrolidines. The diastereoisomers were separated, and their structure was determined by NMR spectroscopy and X-ray analysis. No further cyclization of these compounds into 3,8-diazabicyclo[3.2.1]octanes was observed.

DOI: 10.1134/S1070428015060123 We have reported previously that the reaction of hexa-1,5-diene with trifluoromethanesulfonamide in the oxidative system t-BuOCl–NaI yields a mixture of trans-2,5-bis(iodomethyl)-1-(trifluoromethanesulfonyl)pyrrolidine and 3,8-bis(trifluoromethanesulfonyl)3,8-diazabicyclo[3.2.1]octane [1]. In contrast, benzenesulfonamide and p-toluenesulfonamide reacted with hexa-1,5-diene to give a mixture of cis- and transisomeric 2,5-bis(iodomethyl)-1-(arenesulfonyl)pyrrolidines but no bicyclic products were detected [1]. Further development of methods for the synthesis of substituted pyrrolidines is worthy taking into account practical importance of these compounds. For example, N-sulfonyl-substituted pyrrolidine ring is a structural fragment of the antimigraine drug almotriptan [2], and 2,5-disubstituted pyrrolidine units are present in molecules of many natural compounds [3], pharmaceuticals [3, 4], transition metal ligands [5–7], and catalysts for asymmetric synthesis [8, 9]. Some procedures for the synthesis of 2,5-disubstituted pyrrolidines [10, 11] and N-sulfonylpyrrolidines are based on tandem S N2 substitution–Michael addition [12] (Scheme 1) or reaction of methylidenecyclopropanes

with p-toluenesulfonamide, catalyzed by Sn(OTf)2 [13] or AgOTf–Au(PPh 3 )Cl [14]; the latter reaction involves opening of the cyclopropane ring and subsequent intramolecular cyclization with participation of the sulfonamide group (Scheme 2). Scheme 2. R1 R2 Au(PPh3)Cl/AgOTf or Sn(OTf)2

I

( )n

EWG

R1 NHTs

R2

N Ts

Data on the synthesis of 2,5-disubstituted pyrrolidines by direct reaction of sulfonamides with alkenes or dienes under experimentally accessible conditions without preliminary activation of the reaction center in the sulfonamide are very few in number. Cyclizations of unconjugated hexa-1,4- and hexa-1,5-dienes with p-toluenesulfonamide in the presence of gold(I) catalyst were reported to produce a mixture of cis- and trans-2,5-dimethylpyrrolidines [15, 16]. However, according to earlier publication [17], analogous reaction catalyzed by Hg(NO3)2 gave only trans-2,5-dimethylpyrrolidine [17]. The reaction of norbornene with p-toluenesulfonamide at a ratio of 2 : 1 under oxidative conditions afforded N-tosylpyrrolidine derivative with two norbornane fragments fused to the C 2 –C 3 and

( )n N Ts

R2 R1

Scheme 1. TsNH2, K2CO3 DMF

+ TsNH2

EWG

n = 1, 2; EWG = COOEt, COMe, COPh, CN.

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OXIDATIVE CYCLOADDITION OF ELECTRON-DEFICIENT ARENESULFONAMIDES

C4–C5 bonds and oriented trans with respect to each other (cis arrangement of these fragment is hampered for steric reasons) [18]. α,ω-Disubstituted octa-2,6-diene reacted with p-nitrobenzenesulfonamide in the presence of iridium complex and chiral phosphoramidite ligands with very high enantioselectivity (>99%) and moderate diastereoselectivity [19] (Scheme 3). Scheme 3. X

X

[Ir(COD)Cl]2, TBD, Cat* 4-O2NC6H4SO2NH2, Et3N, THF

H 2C

CH2 N SO2C6H4NO2-4

X = OCOOMe; Cat* stands for chiral phosphoramidites.

trans-2,5-Dialkyl-3-iodo-N-tosylpyrrolidines were obtained with high stereoselectivity by heterocyclization of enantiomerically pure homoallylic sulfonamides [20] (Scheme 4). Scheme 4. Ts

R

R'

R

Scheme 5. O S

O Bu-t

S

Pd(TFA)2

Bu-t

N

Nu

R

R OH Me

K2OsO2(OH)4, Sc(OTf)3 4-nitropyridine N-oxide CH2

NHTs

HO

Me

2, 3

I I

Stereoselective formation of cis-substituted N-sulfinyl- [21] and N-sulfonyl-2,5-disubstituted pyrrolidines [22] was observed when the initial sulfinyl(sulfonyl)amino alkene possessed a chiral center in the γ- or β-position with respect to the double bond (Scheme 5).

N H

+ 4-XC 6H4SO2NH2

N

Ts

Nu

CH2

t-BuOCl, NaI MeCN, –10°C, 24 h

R'

N

Scheme 6. 1

I2/K2CO3

NH

saturated substrate. Moreover, catalysis by heavy metal salts is necessary for many reactions. In the present work we examined reactions of hexa1,5-diene (1) with electron-deficient arenesulfonamides 4-XC6H4SO2NH2 [X = NO2 (2), Cl (3)] in the oxidative system t-BuOCl–NaI with a view to comparing the results with our previous data for the reactions of diene 1 with trifluoromethanesulfonamide, p-toluenesulfonamide, and benzenesulfonamide under analogous conditions [1]. In particular, it was interesting to find out whether most electron-deficient arenesulfonamide 2 would react like trifluoromethanesulfonamide, i.e., with formation of a bicyclic product, disubstituted 3,8-diazabicyclo[3.2.1]octane. The reaction of hexa-1,5-diene (1) with 4-nitrobenzenesulfonamide (2) gave a mixture of cis- and transisomeric 2,5-bis(iodomethyl)pyrrolidines 4 and 5 at a ratio of 1 : 3. 4-Chlorobenzenesulfonamide (3) reacted in a similar way, but the ratio of cis and trans isomers 6 and 7 was 1 : 1 (Scheme 6).

H 2C

I

N Ts

OH

All the above listed methods for the synthesis of pyrrolidines require functionalization of the sulfonamide component or preliminary activation of the un-

889

SO2C6H4X-4

I +

N I

4, 6

SO2C6H4X-4 5, 7

2, 4, 5, X = NO2; 3, 6, 7, X = Cl.

After removal of tarry products by silica gel column chromatography, diastereoisomers 4 and 5 were separated due to their different solubilities in propan-2-ol, and diastereoisomers 6 and 7 were isolated by fractional crystallization from ethyl acetate. The structure of 4–7 was determined by comparing their 1H NMR spectra with the spectrum of 2,5-bis(iodomethyl)-1-(trifluoromethanesulfonyl)pyrrolidine whose trans configuration was determined by X-ray analysis. The spectrum of the latter was similar to those of diastereoisomers 5 and 7; therefore, they were assigned trans configuration, and diastereoisomers 4 and 6, cis (Fig. 1). This assignment was confirmed by the X-ray diffraction data for compound 7 (Fig. 2). Unlike cis-2,5bis(iodomethyl)-1-tosylpyrrolidine studied previously [1], the iodomethyl substituents in molecule 7 are arranged trans. The N1S1C3 angle is 107.05°, which is close to the corresponding angle in analogous struc-

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ICH2

CH2I

N

SO2CF3

ICH2

CH2I

N

SO2C6H4NO2-4 5

ICH2

CH2I

N

SO2C6H4NO2-4 4

ICH2

CH2I

N

SO2C6H4Cl-4 6

ICH2

CH2I

N

SO2C6H4Cl-4 7

4.4

4.0

3.6

3.2

2.8

2.4

2.0

δ, ppm

1

Fig. 1. H NMR spectra of diastereoisomeric 2,5-bis(iodomethyl)-N-sulfonylpyrrolidines. The spectra of 4–6 contain signals of the second diastereoisomer.

tures [23–25]. The unit cell of 7 contains 4 molecules. Shortened contacts (3.09–3.16 Å) are observed between the iodine atoms and hydrogen atoms of CH2 groups of the neighboring molecules. The bond lengths and bond and torsion angles in molecule 7 are available from the authors by e-mail. I2 O2

C4

C5 C6

C12

C9

Cl1

C3

S1

C7 1

N

10

C

C11

C

O1

C2

8

I1 C1

Fig. 2. Structure of the molecule of trans-1-(4-chlorobenzenesulfonyl)-2,5-bis(iodomethyl)pyrrolidine (7) according to the X-ray diffraction data.

No bicyclic products analogous to 3,8-bis(trifluoromethanesulfonyl)-3,8-diazabicyclo[3.2.1]octane (which was formed together with the corresponding trans-pyrrolidine in the reaction of trifluoromethanesulfonamide hexa-1,5-diene [1]) were detected in the reaction mixtures obtained from arenesulfonamides 2 and 3. We also found that cis isomers 4 and 6 whose steric structure is favorable for the cyclization into 3,8-diazabicyclo[3.2.1]octanes do not react with trifluoromethanesulfonamide under the examined conditions. Analysis of the 1H NMR spectra of the reaction mixture showed the presence of unreacted cis isomers 4 and 6 at the same ratio to trans isomers 5 and 7 as in the initial isomer mixture. Insofar as the difference in the electron-withdrawing powers of the 4-nitrobenzenesulfonyl and trifluoromethanesulfonyl groups is not large (σ I 0.61 and 0.71, respectively [26]), our results confirm the assumption that trifluoromethanesulfonamide exhibits a specific reactivity [27], which differs from the reactivity of even its closest analog, electron-deficient 4-nitrobenzenesulfonamide.

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OXIDATIVE CYCLOADDITION OF ELECTRON-DEFICIENT ARENESULFONAMIDES

EXPERIMENTAL The IR spectra were recorded in KBr on Varian 3100 FT-IR and Bruker Vertex 70 spectrometers. The NMR spectra were obtained on a Bruker DPX-400 instrument at 400 MHz for 1H and 100 MHz for 13C using CDCl3 as solvent; the chemical shifts are given relative to tetramethylsilane (1H, 13C) or CCl3F (19F). Elemental analysis was performed on a ThermoFinnigan Flash EA analyzer. The progress of reactions was monitored by TLC on silica gel 60 F-254 plates using hexane–diethyl ether (1 : 1) or hexane–diethyl ether–acetone (2 : 3 : 1) as eluent. The products were isolated by column chromatography on coarse (0.060–0.200 mm, Acros Organics) or fine silica gel (0.040–0.063 mm, Fluka). The melting points were determined on a Boetius (VEB Analytik) melting point apparatus. The X-ray diffraction data for compound 7 were obtained at 100 K from a 0.50 × 0.35 × 0.30-mm colorless single crystal on a Bruker D8 Venture diffractometer (Photon 100 detector; MoK α radiation, λ = 0.71073 Å; θ/2θ scanning in the range from 2.73 to 30.08°). A correction for absorption was applied by the multiscan technique. Monoclinic crystal system, space group P21/c; C12H14ClI2NO2S; unit cell parameters: a = 7.9157(12), b = 26.866(4), c = 7.8509(12) Å; β = 109.423(4); V = 1574.6(4) Å3; Z = 4; dcalc = 2.217 g× cm–3; μ = 4.294 mm–1. The structure was solved by the direct method using Bruker SAINT and SHELXS-13 [28] and was refined by the full-matrix least-squares procedure in anisotropic approximation for non-hydrogen atoms and isotropic approximation for hydrogens; weight sch e me w = 1 / [σ 2 (F o2 ) + (0.0340 P ) 2 + 3.5343P], where P = (Fo2 + 2Fc2)/3. The positions of hydrogen atoms were determined from the difference electron density maps. Final divergence factor R = 0.0338 for 4100 reflections with I > 2σ(I) (total reflection number 48 781). The CIF file containing the complete set of crystallographic data for compound 7 was deposited to the Cambridge Crystallographic Data Centre (entry no. CCDC 1 046 103) and is available at www.ccdc.cam.ac.uk/data_request/cif. 2,5-Bis(iodomethyl)-1-[(4-nitrobenzene)sulfonyl]pyrrolidines 4 and 5. A solution of 2 g (9.9 mmol) of 4-nitrobenzenesulfonamide, 4.46 g (30 mmol) of NaI, and 1.2 mL (9.9 mmol) of hexa-1,5-diene in 80 mL of acetonitrile was cooled to –10°C, 3.4 mL (30 mmol) of tert-butyl hypochlorite was added dropwise under stirring in an argon atmosphere with protection from light, and the mixture was kept for 24 h. The solvent was distilled off under reduced pressure, the residue

891

was dissolved in 80 mL of ethyl acetate, the solution was treated with a solution of Na2S2O3, and the organic phase was separated and dried over CaCl2. The solvent was removed under reduced pressure, and the dark viscous residue [~3.45 g (65%)] was purified from tars by passing through a silica gel column using hexane, chloroform–hexane (2 : 1), and chloroform as eluents. We thus isolated a mixture of cis and trans isomers 4 and 5 at a ratio of 1 : 3. The isomer mixture was dissolved in propan-2-ol, and the undissolved material (trans isomer 5) was filtered off, washed with propan2-ol, and dried. The filtrate was cooled, and the precipitate of cis isomer 4 was filtered off and dried. cis-2,5-Bis(iodomethyl)-1-(4-nitrobenzenesulfonyl)pyrrolidine (4). Yield 0.80 g (15%), light yellow crystals, mp 200°C (decomp.). IR spectrum, ν, cm–1: 3109, 3037, 2962, 2922, 2865, 1966, 1948, 1930, 1816, 1698, 1606, 1527, 1476, 1448, 1426, 1401, 1348, 1314, 1304, 1290, 1209, 1163, 1094, 1049, 1029, 1006, 973, 954, 858, 837, 825, 779, 747, 737, 686, 674, 624, 598, 577, 496, 463. 1H NMR spectrum, δ, ppm: 1.78 m and 1.96 m (2H each, CH2), 3.34 t (2H, CHBI, J = 9.9 Hz), 3.59 d.d (2H, CHBI, J = 9.9, 3.0 Hz), 3.76 m (2H, CH), 8.05 d (2H, o-H, J = 8.6 Hz), 8.43 d (2H, m-H, J = 8.8 Hz). 13C NMR spectrum, δC, ppm: 10.5 (CH2I), 30.4 (CH2), 63.7 (CH), 125 (Co), 129.1 (Cm), 147.4 (Ci), 150.9 (Cp). Found, %: C 26.38; H 2.47; N 5.20. C12H14I2N2O4S. Calculated, %: C 26.88; H 2.63; N 5.23. trans-2,5-Bis(iodomethyl)-1-(4-nitrobenzenesulfonyl)pyrrolidine (5). Yield 2.50 g (47%), light yellow crystals, mp 162°C. The IR spectrum was identical to that of 4. 1H NMR spectrum, δ, ppm: 2.06 m and 2.27 m (2H each, CH2), 3.08 t (2H, CHBI, J = 9.8 Hz), 3.65 d.d (2H, CHAI, J = 9.9, 2.0 Hz), 4.21 m (2H, CH), 8.07 d (2H, m-H, J = 8.9 Hz), 8.39 d (2H, o-H, J = 8.7 Hz). 13C NMR spectrum, δC, ppm: 8.4 (CH2I), 29.7 (CH2), 62.6 (CH), 125 (Co), 128.5 (Cm), 147.4 (Ci), 150.5 (Cp). Found, %: C 26.38; H 2.47; N 5.20. C12H14I2N2O4S. Calculated, %: C 26.88; H 2.63; N 5.23. 1-(4-Chlorobenzenesulfonyl]-2,5-bis(iodomethyl)pyrrolidines 6 and 7. A solution of 4 g (21 mmol) of 4-chlorobenzenesulfonamide, 9.45 g (63 mmol) of NaI, and 2.47 mL (21 mmol) of hexa-1,5-diene in 120 mL of acetonitrile was cooled to –10°C, 7.15 mL (63 mmol) of tert-butyl hypochlorite was added dropwise under stirring in an argon atmosphere with protection from light, and the mixture was kept for 24 h. The solvent was distilled off under reduced pressure, the residue was dissolved in 80 mL of ethyl acetate, the solution was treated with a solution of Na2S2O3, and the organic phase was separated and dried over CaCl2.

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The solvent was removed under reduced pressure, and the dark viscous residue [~9.51 g (87%)] was purified from tars by passing through a silica gel column using hexane, chloroform–hexane (1 : 1), and chloroform as eluents. We thus isolated a mixture of cis and trans isomers 6 and 7, which was dissolved in hot ethyl acetate. After cooling, the precipitate of trans isomer 7 was filtered off, and crystals of cis isomer 6 separated from the mother liquor on the next day. cis-1-(4-Chlorobenzenesulfonyl)-2,5-bis(iodomethyl)pyrrolidine (6). Yield 5.27 g (48%), colorless crystals, mp 171°C. IR spectrum, ν, cm–1: 3089, 3061, 3042, 3023, 2964, 2889, 1929, 1636, 1587, 1572, 1477, 1450, 1426, 1395, 1343, 1280, 1222, 1208, 1160, 1117, 1094, 1055, 1034, 1009, 971, 939, 913, 884, 865, 831, 767, 752, 702, 638, 598, 577, 538, 503, 482, 440. 1 H NMR spectrum, δ, ppm: 1.75 m and 1.92 m (2H each, CH2), 3.31 t (2H, CHBI, J = 9.9 Hz), 3.58 d.d (2H, CHAI, J = 9.9, 3.0 Hz), 3.71 m (2H, CH), 7.55 d (2H, o-H, J = 8.7 Hz), 7.79 d (2H, m-H, J = 8.7 Hz). 13C NMR spectrum, δC, ppm: 8.2 (CH2I), 29.1 (CH2), 62.1 (CH), 128.3 (Co), 129.7 (Cm), 139.9 (C i ), 140.3 (C p ). Foun d, %: C 27 .56; H 2 .4 8; N 2.66. C 12 H 14 ClI 2 NO 2 S. Calculated, %: C 27.42; H 2.68; N 2.67. trans-1-(4-Chlorobenzenesulfonyl)-2,5-bis(iodomethyl)pyrrolidine (7). Yield 4 g (36%), colorless crystals, mp 165°C. The IR spectrum was identical to that of 6. 1 H NMR spectrum, δ, ppm: 2.08 m and 2.21 m (2H each, CH2), 3.02 t (2H, CHBI, J = 10 Hz), 3.68 d.d (2H, CHAI, J = 9.7, 2.1 Hz), 4.17 m (2H, CH), 7.51 d (2H, m-H, J = 8.4 Hz), 7.80 d (2H, o-H, J = 8.6 Hz). 13 C NMR spectrum, δ C, ppm: 8.5 (CH 2 I), 29.5 (CH2), 62.5 (CH), 128.7 (Co), 130 (Cm), 139.9 (C i ), 14 0.3 (C p ). Fou nd, %: C 2 7 .56; H 2 . 48; N 2.66. C 12 H 14 ClI 2 NO 2 S. Calculated, %: C 27.42; H 2.68; N 2.67. REFERENCES 1. Shainyan, B.A., Moskalik, M.Yu., Astakhova, V.V., and Schilde, U., Tetrahedron, 2014, vol. 70, p. 4547. 2. Keam, S.J., Goa, K.L., and Figgitt, D.P., Drugs, 2002, vol. 62, p. 387. 3. O’Hagan, D., Nat. Prod. Rep., 2000, vol. 17, p. 435. 4. Wright, S.W., Ammirati, M.J., Andrews, K.M., Brodeur, A.M., Danley, D.E., Doran, S.D., Lillquist, J.S., McClure, L.D., McPherson, R.K., Orena, S.J., Parker, J.C., Polivkova, J., Qiu, X., Soeller, W.C., Soglia, C.B., Treadway, J.L., van Volkenburg, M.A., Wang, H., Wilder, D.C., and Olson, T.V., J. Med. Chem., 2006, vol. 49, p. 3068.

5. Stranne, R. and Moberg, C., Eur. J. Org. Chem., 2001, p. 2191. 6. Hoen, R., van den Berg, M., Bernsmann, H., Minnaard, A.J., de Vries, J.G., and Feringa, B.L., Org. Lett., 2004, vol. 6, p. 1433. 7. Chen, H., Sweet, J.A., Lam, K.-C., Rheingold, A.L., and McGrath, D.V., Tetrahedron: Asymmetry, 2009, vol. 20, p. 1672. 8. Halland, N., Braunton, A., Bachmann, S., Marigo, M., and Jørgensen, K.A., J. Am. Chem. Soc., 2004, vol. 126, p. 4790. 9. Simonini, V., Benaglia, M., Pignataro, L., Guizzetti, S., and Celentano, G., Synlett, 2008, p. 1061. 10. Katritzky, A.R., Cui, X.-L., Yang, B., and Steel, P.J., Tetrahedron Lett., 1998, vol. 39, p. 1697. 11. Yus, M., Soler, T., and Foubelo, F., J. Org. Chem., 2001, vol. 66, p. 6207. 12. Bunce, R.A. and Allison, J.C., Synth. Commun., 1999, vol. 29, p. 2175. 13. Chen, Y. and Shi, M., J. Org. Chem., 2004, vol. 69, p. 426. 14. Shi, M., Liu, L.-P., and Tang, J., Org. Lett., 2006, vol. 8, p. 4043. 15. Zhang, J., Yang, C.-G., and He, C., J. Am. Chem. Soc., 2006, vol. 128, p. 1798. 16. Giner, X., Nájera, C., Kovács, G., Lledós, A., and Ujaque, G., Adv. Synth. Catal., 2011, vol. 353, p. 3451. 17. Barluenga, J., Jimenez, C., Najera, C., and Yus, M., J. Chem. Soc. Perkin Trans. 1, 1984, p. 721. 18. Brice, J.L., Harang, J.E., Timokhin, V.I., Anastasi, N.R., and Stahl, S.S., J. Am. Chem. Soc., 2005, vol. 127, p. 2868. 19. Weihofen, R., Dahnz, A., Tverskoy, O., and Helmchen, G., Chem. Commun., 2005, p. 3541. 20. Davis, F.A., Song, M., and Augustine, A., J. Org. Chem., 2006, vol. 71, p. 2779. 21. Redford, J.E., McDonald, R.I., Rigsby, M.L., Wiensch, J.D., and Stahl, S.S., Org. Lett., 2012, vol. 14, p. 242. 22. Donohoe, T.J., Lindsay-Scott, P.J., Parker, J.S., and Callens, C.K.A., Org. Lett., 2010, vol. 12, p. 1060. 23. Nair, V., Mohanan, K., Suja, T.D., and Suresh, E., Tetrahedron Lett., 2006, vol. 47, p. 2803. 24. Castellano, S., Fiji, H.D.G., Kinderman, S.S., Watanabe, M., de Leon, P., Tamanoi, F., and Kwon, O., J. Am. Chem. Soc., 2007, vol. 129, p. 5843. 25. Li, Q., Jiang, X., Fu, C., and Ma, S., Org. Lett., 2011, vol. 13, p. 466. 26. Hansch, C., Leo, A., and Taft, R.W., Chem. Rev., 1991, vol. 91, p. 165. 27. Shainyan, B.A. and Tolstikova, L.L., Chem. Rev., 2013, vol. 113, p. 699. 28. Sheldrick, G.M., Acta Crystallogr., Sect. A, 2008, vol. 64, p. 112.

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