Bis(1,3,4-Thiadiazolo)-1,3,5-triazinium Halides: Access to Highly Substituted Aromatic Guanidines

Kurt Wermann; Martin Walther; Helmar Görls; Ernst Anders

doi:10.1055/s-2003-40844

LETTER

Bis(1,3,4-Thiadiazolo)-1,3,5-triazinium Halides: Access to Highly Substituted Aromatic Guanidines [1]

Kurt Wermann^a, Martin Walther^a, Helmar Görls^b, Ernst Anders*^a
^a Institut für Organische Chemie und Makromolekulare Chemie der Friedrich-Schiller-Universität, Lessingstrasse 8, 07743 Jena, Germany
^b Institut für Anorganische und Analytische Chemie der Friedrich-Schiller-Universität, Lessingstrasse 8, 07743 Jena, Germany
Fax: +49(3641)948212; e-Mail: Ernst.Anders@uni-jena.de;

Abstract

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Abstract

The reaction of bis(1,3,4-thiadiazolo)-1,3,5-triazinium halides with primary aromatic amines in CHCl₃/Et₃N yields new highly substituted aromatic guanidines (up to 90% yield).

Key words

guanidinylation - nitrogen heterocycles - nucleophilic addition - rearrangements - sulfur heterocycles

Volltext

Referenzen

References

1a

Part 7 in the series ‘Bis(1,3,4-Thiadiazolo)-1,3,5-triazinium Halides’. Part 6, see: Nilson Lill, St. O.; Rauhut, G.; Anders, E. Chem.-Eur. J. 2003, in press.

1b Part 5, see: Wermann K. Walther M. Günther W. Görls H. Anders E. Eur. J. Org. Chem. 2003, 1389

1c Part 4, see: Wermann K. Walther M. Anders E. ARKIVOC 2002, 10: 24

2a Berlinck RGS. Nat. Prod. Rep. 2002, 19: 617

2b Berlinck RGS. Nat. Prod. Rep. 1999, 16: 339

2c Hannon CL. Anslyn EV. Bioorg. Chem. Front. 1993, 3: 193

3 Ishikawa T. Isobe T. Chem.-Eur. J. 2002, 8: 553

4 Costa M. Chiusoli GP. Taffurelli D. Dalmonego G. J. Chem. Soc., Perkin Trans. 1 1998, 1541

5 Heinisch G. Matuszak B. Rakowitz D. Mereiter K. J. Heterocycl. Chem. 2002, 39: 695

6a Isobe T. Fukuda K. Ishikawa T. J. Org. Chem. 2000, 65: 7770

6b Isobe T. Fukuda K. Tokunaga T. Seki H. Yamaguchi K. Ishikawa T. J. Org. Chem. 2000, 65: 7774

6c Isobe T. Fukuda K. Yamaguchi K. Seki H. Tokunaga T. Ishikawa T. J. Org. Chem. 2000, 65: 7779

7a Feichtinger K. Zapf C. Sings HL. Goodman M. J. Org. Chem. 1998, 63: 3804

7b Feichtinger K. Sings HL. Baker TJ. Matthews K. Goodman M. J. Org. Chem. 1998, 63: 8432

8a Katritzky AR. Rogovoy BV. Chassaing C. Vvedensky V. J. Org. Chem. 2000, 65: 8080

8b Kim H.-O. Mathew F. Ogbu C. Synlett 1999, 193

8c Guo Z.-X. Cammidge AN. Horwell DC. Synth. Commun. 2000, 30: 2933

9

See ref. [5] [7a] [8] and literature cited therein.

10a Anders E. Wermann K. Wiedel B. Günther W. Görls H. Eur. J. Org. Chem. 1998, 2923

10b Anders E. Vanden Eynde J.-J. Wermann K. In Advances in Heterocyclic Chemistry Vol. 77: Katritzky AR. Academic Press; New York: 2000. p.183

11a Wermann K. Walther M. Günther W. Görls H. Anders E. J. Org. Chem. 2001, 66: 720

11b Walther M. Wermann K. Görls H. Anders E. Synthesis 2001, 1327

12 Van der Plas HC. In Advances in Heterocyclic Chemistry Vol. 74: Katritzky AR. Academic Press; New York: 1999. p.87

13 Besides 7b, N-(4-methylphenyl)-N-[(1E)-(4-methyl-phenyl) methylene]amine was isolated in almost quantitative yield by the reaction of 2b and 4-methyl benzaldehyde formed in situ, see: Bolognese A. Diumo MV. Mazzoni O. Giordano F. Tetrahedron 1991, 47: 7417

14 Tripathy PB. Rout MR. J. Indian Chem. Soc. 1960, 37: 414

15 Dubenko RG. Benzina IN. Pel’kis PS. Zh. Obshch. Khim. 1963, 33: 274

16

The iminium salt 8a (R¹ = 4-MePh; R² = Me): Identification by HMQC experiments showing a CH correlation between δ = 160.7 and 8.45 ppm. ¹H NMR (400 MHz, CDCl₃): δ = 2.34 (s, 3 H, CH₃), 2.46 (s, 3 H, CH₃), 7.79 (d, 2 H, aryl-CH), 7.25 (d, 2 H, aryl-CH), 7.31 (br s, 1 H, NH), 8.45 (s, 1 H, imine-CH). ¹³C NMR (100.6 MHz, CDCl₃): δ = 17.5, 21.7, 129.2, 129.3, 132.8, 142.6, 150.9, 160.7 (imine-CH, broad), 170.0.

17 Chubb FC. Nissenbaum J. Can. J. Chem. 1959, 37: 1121

18

The hydrolysis products 9 and 10 were unambiguously identified by TLC (EtOAc), GC, derivatization with 2,4-dinitrophenylhydrazine, and ¹H and ¹³C NMR spectroscopy in DMSO-d ₆.

19

General Procedure for the Preparation of Guanidines 6: To a well-stirred mixture of the corresponding bis-(1,3,4-thiadiazolo)-1,3,5-triazinium halide 1 (5 mmol) and amine 2 (5 mmol) in 30 mL CHCl₃, Et₃N (30 mL) was added gradually at r.t. After stirring over a period of 24 h, the resulting suspension was concentrated in vacuo. The residue was washed with cold water and extracted three times with CHCl₃. The combined extracts were dried with Na₂SO₄ and concentrated in vacuo. The residues were purified by column chromatography (silicagel 60; eluting with EtOAc; pentane/EtOAc; Et₂O/EtOAc) or by fractional crystallization from EtOH, MeCN, EtOAc.
All new compounds 6 were fully characterized. Selected analytical data:
5-Methyl-2-(4-methylphenyl)- N ′-(5-methyl-1,3,4-thiadiazol-2-yl)- N -phenyl-1,3,4-thiadiazole-3(2 H )-carboximidamide (6a): Recrystallized from EtOH. ¹H NMR (250 MHz, CDCl₃): δ = 2.00 (s, 3 H, CH₃), 2.26 (s, 3 H, CH₃), 2.44 (s, 3 H, CH₃), 7.19 (s, 1 H, 2-CH), 6.96-7.22 (m, 9 H, CH_ph), 10.04 (s, 1 H, NH). ¹³C NMR (62.9 MHz, CDCl₃): δ = 16.1, 16.6, 21.2, 72.0, 122.4, 124.3, 126.6, 128.6, 129.4, 137.7, 138.6, 139.1, 147.9, 149.0, 158.5, 173.1. MS (DCI/H₂O): m/z (%) = 409 (100) [C₂₀H₂₁N₆S₂]⁺. Anal. Calcd for C₂₀H₂₀N₆S₂ (408.53): C, 58.80; H, 4.93; N, 20.57; S, 15.69. Found: C, 58.54; H, 5.08; N, 20.62; S, 15.53.
5-Methyl-2-(4-methylphenyl)- N ′-(5-methyl-1,3,4-thiadiazol-2-yl)- N -(4-methylphenyl)-1,3,4-thiadiazole-3(2 H )-carboximidamide (6b): Recrystallized from MeCN. ¹H NMR (250 MHz, CDCl₃): δ = 2.12 (s, 3 H, CH₃), 2.26 (s, 3 H, CH₃), 2.32 (s, 3 H, CH₃), 2.49 (s, 3 H, CH₃), 7.12 (s, 1 H, 2-CH), 6.95-7.26 (m, 8 H, CH_ph), 10.03 (s, 1 H, NH). ¹³C NMR (62.9 MHz, CDCl₃): δ = 16.8, 17.0, 21.3, 21.6, 72.3, 122.8, 126.9, 129.6, 129.7, 134.4, 136.8, 138.2, 139.0, 148.1, 148.7, 155.2, 173.5. MS (DCI/H₂O): m/z (%) = 423(100) [C₂₁H₂₃N₆S₂]⁺. Anal. Calcd for C₂₁H₂₂N₆S₂ (422.56): C, 59.69; H, 5.25; N, 19.89; S, 15.17. Found: C, 59.58; H, 5.33; N, 20.28; S, 15.05.
5-Methyl-2-(4-methylphenyl)- N ′-(5-methyl-1,3,4-thiadiazol-2-yl)- N -(3-nitrophenyl)-1,3,4-thiadiazole-3(2 H )-carboximidamide (6c): Purified by column chromatography. ¹H NMR (250 MHz, CDCl₃): δ = 1.99 (s, 3 H, CH₃), 2.23 (s, 3 H, CH₃), 2.49 (s, 3 H, CH₃), 6.96 (s, 1 H, 2-CH), 7.19-7.87 (m, 8 H, CH_ph), 10.59 (s, 1 H, NH). ¹³C NMR (62.9 MHz, CDCl₃): δ = 16.5, 16.9, 21.6, 72.2, 117.4, 118.9, 125.5, 127.7, 127.9, 129.5, 137.6, 138.4, 141.0, 148.4, 148.6, 149.5, 159.7, 173.7. MS (DCI/H₂O): m/z (%) = 454 (81) [C₂₀H₂₀N₇O₂S₂]⁺. Anal. Calcd for C₂₀H₁₉N₇O₂S₂ (453.53): C, 52.97; H, 4.22; N, 21.62; O, 7.06; S, 14.14. Found: C, 52.80; H, 4.38; N, 21.57; S, 13.83.
5-Methyl-2-(4-methylphenyl)- N ′-(5-methyl-1,3,4-thiadiazol-2-yl)- N -(4- t -butylphenyl)-1,3,4-thiadiazole-3(2 H )-carboximidamide (6d): Purified by column chromatography and recrystallized from Et₂O. ¹H NMR (250 MHz, CDCl₃): δ = 1.32 (s, 9 H, CH₃), 2.11 (s, 3 H, CH₃), 2.36 (s, 3 H, CH₃), 2.53 (s, 3 H, CH₃), 7.01-7.16 (AA′/XX′, 4 H, CH_ph), 7.13 (s, 1 H, 2-CH), 7.27-7.29 (AA′/XX′, 4 H, CH_ph), 10.02 (br s, 1 H, NH). ¹³C NMR (62.9 MHz, CDCl₃): δ = 16.1, 16.6, 21.2, 31.3, 34.4, 72.0, 122.1, 125.5, 126.4, 129.4, 136.3, 137.8, 138.6, 147.4, 147.8, 149.2, 158.7, 173.0. MS (DCI/H₂O): m/z (%) = 465(100) [C₂₄H₂₉N₆S₂]⁺. Anal. Calcd for C₂₄H₂₈N₆S₂ (464.65): C, 62.04; H, 6.07; N, 18.09; S, 13.80. Found: C, 61.98; H, 6.47; N, 18.40; S, 13.93.
5-Methyl-2-(4-methylphenyl)- N ′-(5-methyl-1,3,4-thiadiazol-2-yl)- N -(4-methoxyphenyl)-1,3,4-thiadiazole-3(2 H )-carboximidamide (6e): Recrystallized from EtOAc. ¹H NMR (250 MHz, CDCl₃): δ = 2.10 (s, 3 H, CH₃), 2.32 (s, 3 H, CH₃), 2.54 (s, 3 H, CH₃), 3.81 (s, 3 H, OCH₃), 6.78-7.00 (AA′/XX′, 4 H, CH_ph), 7.11 (s, 1 H, 2-CH), 7.15-7.26 (AA′/XX′, 4 H, CH), 10.08 (br s, 1 H, NH). ¹³C NMR (62.9 MHz, CDCl₃): δ = 16.5, 17.0, 21.5, 55.8, 72.4, 114.2, 124.9, 126.9, 129.7, 132.5, 138.3, 139.0, 147.9, 150.0, 157.1, 159.1, 173.7. MS (DCI/H₂O): m/z (%) = 439 (439) [C₂₁H₂₃N₆OS₂]⁺. Anal. Calcd for C₂₁H₂₂N₆OS₂ (438.56): C, 57.51; H, 5.06; N, 19.16; O, 3.65; S, 14.12. Found: C, 57.47; H, 5.31; N, 19.10; S, 14.24.
5-Methyl-2-(4-methylphenyl)- N ′-(5-methyl-1,3,4-thiadiazol-2-yl)- N -(2-methoxyphenyl)-1,3,4-thiadiazole-3(2 H )-carboximidamide (6f): Recrystallized from EtOAc. ¹H NMR (250 MHz, CDCl₃): δ = 2.12 (s, 3 H, CH₃), 2.38 (s, 3 H, CH₃), 2.53 (s, 3 H, CH₃), 3.88 (s, 3 H, OCH₃), 6.76-7.29 (m, 8 H, CH_ph, 1 H, 2-CH), 9.71 (br s, 1 H, NH). ¹³C NMR (62.9 MHz, CDCl₃): δ = 16.1, 16.5, 21.5, 56.1, 72.2, 110.9, 120.6, 122.9, 125.0, 126.7, 128.5, 129.7, 138.3, 138.9, 148.3, 149.2, 151.2, 159.2, 172.8. MS (DCI/H₂O): m/z (%) = 439 (100) [C₂₁H₂₃N₆OS₂]⁺. Anal. Calcd for C₂₁H₂₂N₆OS₂ (438.56): C, 57.51; H, 5.06; N, 19.16; S, 14.12. Found: C, 56.91; H, 5.32; N, 18.67; S, 14.04.
2-Butyl-5-methyl-2- N ′-(5-methyl-1,3,4-thiadiazol-2-yl)- N -phenyl-1,3,4-thiadiazole-3(2 H )-carboximidamide (6g): Purified by column chromatography. ¹H NMR (250 MHz, CDCl₃): δ = 0.91 (t, 3 H, CH₃), 1.21-1.40 (m, 4 H, CH₂), 1.89-2.21 (m, 2 H, CH₂), 2.06 (s, 3 H, CH₃), 2.57 (s, 3 H, CH₃), 6.18-6.22 (t, 1 H, 2-CH), 7.02-7.29 (m, 5 H, CH_ph), 9.86 (s, 1 H, NH). ¹³C NMR (62.9 MHz, CDCl₃): δ = 14.3, 16.5, 17.2, 22.4, 27.1, 36.7, 71.7, 122.6, 124.7, 129.0, 139.5, 149.5, 150.1, 159.1, 173.4. MS (DCI/H₂O): m/z (%) = 375 (100) [C₁₇H₂₃N₆S₂]⁺. Anal. Calcd for C₁₇H₂₂N₆S₂ (374.52): C, 54.52; H, 5.92; N, 22.44; S, 17.12. Found: C, 54.59; H, 6.31; N 22.60; S, 16.75.
2-Butyl-5-methyl-2- N ′-(5-methyl-1,3,4-thiadiazol-2-yl)- N -(2-hydroxyphenyl)-1,3,4-thiadiazole-3(2 H )-carboximidamide (6h): Purified by column chromatography. ¹H NMR (250 MHz, CDCl₃): δ = 0.91 (t, 3 H, CH₃), 1.28-1.40 (m, 4 H, CH₂), 2.07-2.21 (m, 2 H, CH₂), 2.16 (s, 3 H, CH₃), 2.56 (s, 3 H, CH₃), 6.75-6.81 (t, 1 H, 2-CH), 6.93-7.07 (m, 4 H, CH_ph), 6.17 (s, 1 H, NH), 8.94 (s, 1 H, OH). ¹³C NMR (62.9 MHz, CDCl₃): δ = 14.1, 16.5, 17.1, 22.7, 26.9, 36.4, 71.3, 117.6, 120.3, 125.3, 126.9, 127.5, 150.8, 151.2, 151.3, 159.6, 172.7. MS (DCI/H₂O): m/z (%) = 391(100) [C₁₇H₂₃N₆OS₂]⁺. Anal. Calcd for C₁₇H₂₂N₆OS₂ (390.52): C, 52.29; H, 5.68; N, 21.52; O, 4.10; S, 16.42. Found: C, 51.76; H, 5.88; N, 21.53; S, 16.29.
5-Methyl-2-naphthyl- N ′-(5-methyl-1,3,4-thiadiazol-2-yl)- N -phenyl-1,3,4-thiadiazole-3(2 H )-carboximidamide (6i): Purified by column chromatography. ¹H NMR (250 MHz, CDCl₃): δ = 2.10
(s, 3 H, CH₃), 2.44 (s, 3 H, CH₃), 7.09-7.95 (m, 12 H, CH_ph;naphthyl), 7.95 (s, 1 H, 2-CH), 10.03 (s, 1 H, NH). ¹³C NMR (62.9 MHz, CDCl₃): δ = 16.4, 17.1, 69.8, 123.0, 123.1, 124.2, 124.8, 125.9, 126.3, 126.9, 129.0, 129.5, 129.6, 129.8, 134.4, 136.1, 139.5, 149.3, 149.8, 159.4, 173.3. MS (DCI/H₂O): m/z (%) = 445 (100) [C₂₃H₂₁N₆S₂]⁺. Anal. Calcd for C₂₃H₂₀N₆S₂ (444.57): C, 62.14; H, 4.53; N, 18.90; S, 14.42. Found: C, 61.90; H, 4.60; N, 19.25; S, 14.32.
5-Methyl-2-naphthyl- N ′-(5-methyl-1,3,4-thiadiazol-2-yl)- N -(4- t -butylphenyl)-1,3,4-thiadiazole-3(2 H )-carboximidamide (6j): Purified by column chromatography. ¹H NMR (250 MHz, CDCl₃): δ = 1.33 (s, 9 H, CH₃), 2.09 (s, 3 H, CH₃), 2.42 (s, 3 H, CH₃), 7.08-7.29 (AA′/XX′, 4 H, CH_ph), 7.48-7.93 (m, 7 H, CH_naphthyl), 7.91 (s, 1 H, 2-CH), 9.98 (s, 1 H, NH). ¹³C NMR (62.9 MHz, CDCl₃): δ = 16.2, 17.0, 31.8, 34.8, 69.7, 122.4, 122.7, 123.5, 125.5, 125.6, 125.9, 126.3, 126.8, 129.1, 129.2, 134.4, 136.1, 136.6, 147.9, 149.5, 149.8, 159.7, 173.2. MS (DCI/H₂O): m/z (%) = 501 (100) [C₂₇H₂₉N₆S₂]⁺. Anal. Calcd for C₂₇H₂₈N₆S₂ (500.67): C, 64.77; H, 5.64; N, 16.79; S, 12.81. Found: C, 64.53; H, 6.14; N, 16.51; S, 12.39.
5-Methyl-2-naphthyl- N ′-(5-methyl-1,3,4-thiadiazol-2-yl)- N -(4-methoxyphenyl)-1,3,4-thiadiazole-3(2 H )-carboximidamide (6k): Recrystallized from EtOH. ¹H NMR (250 MHz, CDCl₃): δ = 2.10 (s, 3 H, CH₃), 2.44 (s, 3 H, CH₃), 3.83 (s, 3 H, OCH₃), 6.81-7.08 (AA′/XX′, 4 H, CH_ph), 7.48-7.93 (m, 7 H, CH_naphthyl), 7.89 (s, 1 H, 2-CH). ¹³C NMR (62.9 MHz, CDCl₃): δ = 16.3, 17.1, 55.9, 69.8, 114.2, 123.1, 124.2, 125.2, 126.0, 126.3, 126.8, 129.4, 129.5, 129.6, 132.4, 134.4, 136.3, 149.3, 150.0, 157.3, 159.2, 173.5. MS (DCI/H₂O): m/z (%) = 475 (100) [C₂₄H₂₃N₆OS₂]⁺. Anal. Calcd for C₂₄H₂₂N₆OS₂ (474.57): C, 60.74; H, 4.67; N, 17.71; O, 3.37; S, 13.51. Found: C, 60.57; H, 4.87; N, 17.90; S, 13.20.
5-Methyl-2-naphthyl- N ′-(5-methyl-1,3,4-thiadiazol-2-yl)- N -(2-hydroxyphenyl)-1,3,4-thiadiazole-3(2 H )-carboximidamide (6l): Recrystallized from EtOAc. ¹H NMR (250 MHz, CDCl₃): δ = 2.14 (s, 3 H, CH₃), 2.28 (s, 3 H, CH₃), 6.79-7.08 (m, 4 H, CH_ph), 7.49 (s, 1 H, NH), 7.38-7.83 (m, 7 H, CH_naphthyl), 7.95 (s, 1 H, 2-CH), 9.13 (s, 1 H, OH). ¹³C NMR (62.9 MHz, CDCl₃): δ = 16.2, 16.9, 69.1, 118.4, 120.4, 122.7, 123.7, 125.7, 126.1, 126.3, 126.9, 127.0, 128.0, 129.4, 129.9, 134.3, 2 × 135.3, 150.4, 150.6, 151.7, 160.0, 171.5. MS (DCI/H₂O): m/z (%) = 461(94) [C₂₃H₂₁N₆OS₂]⁺. Anal. Calcd for C₁₇H₂₂N₆OS₂ (460.57): C, 59.98; H, 4.38; N, 18.25; O, 3.47; S, 13.92. Found: C, 59.57; H, 4.70; N, 18.09; S, 13.71.
5- t -Butyl-2-(2-hydroxyphenyl- N ′-(5- t -butyl-1,3,4-thiadiazol-2-yl)- N -(4- t -butylphenyl)-1,3,4-thiadiazole-3(2 H )-carboximidamide (6m): Recrystallized from Et₂O. ¹H NMR (250 MHz, CDCl₃): δ = 0.99 (s, 9 H, CH₃), 1.28 (s, 9 H, CH₃), 1.42 (s, 9 H, CH₃), 6.84-7.41 (m, 9 H, CH_ph, 2-CH), 11.0 (s, br, 1 H, OH), 11.16 (br s, 1 H, NH). ¹³C NMR (62.9 MHz, CDCl₃): δ = 28.8, 31.1, 31.7, 34.8, 36.6, 36.7, 66.4, 119.5, 121.3, 123.8, 125.7, 126.4, 129.4, 131.3, 137.0, 148.4, 150.8, 154.9, 163.8, 172.4, 174.3. MS (DCI/H₂O): m/z (%) = 551(20) [C₂₉H₃₉N₆OS₂]⁺. Anal. Calcd for C₂₉H₃₈N₆OS₂ (550.78): C, 63.24; H, 6.95; N, 15.26; S, 11.64. Found: C, 63.03; H, 7.43; N, 15.37; S, 11.43.
General Procedure for the Formation of 2-(Het)arylamino-5-methyl-1,3,4-thiadiazoles 7: The suspension of 1 (5 mmol) and amine 2 (10.1 mmol) in pyridine (60 mL) was stirred at r.t. for 24-60 h. [11a] The reaction mixture, changing to a nearly clear solution, was concentrated to dryness in vacuo. The solid residue was washed off with water and recrystallized as described in the literature. [14] [16] The novel compound 7e was purified by column chromatography and fractional crystallization from CHCl₃/EtOAc to prepare crystalline material suitable for X-ray investigation.
N -(5-methyl-1,3,4-thiadiazol-2-yl)pyridin-2-amine (7e): Purified by column chromatography and crystallized from EtOAc. ¹H NMR (250 MHz, DMSO-d ₆): δ = 2.56 (s, 3 H, CH₃), 6.94 (t, 1 H, pyridine-5 H), 7.05 (d, 1 H, pyridine-3 H), 7.68 (dd, 1 H, pyridine-4 H), 8.24 (d, 1 H, pyridine-6 H), 11.44 (s, 1 H, NH). ¹³C NMR (62.9 MHz, DMSO-d ₆): δ = 14.6 (CH₃), 111.0 (pyridine-5C), 116.5 (pyridine-4C), 138.3 (pyridine-3C), 146.5 (pyridine-6C), 150.9 (pyridine-2C), 156.9 (thiadiazole-5C), 159.9 (thiadiazole-2C). MS (DEI): m/z (%) = 192(100) [ C₈H₈N₄S]⁺. Anal. Calcd for C₈H₈N₄S (192.2): C, 49.98; H, 4.19; N, 29.14; S, 16.68. Found: C, 50.22; H, 4.18; N, 28.83; S, 16.47.

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CCDC 210984 and 210985 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44(1223)336033; or deposit@ccdc.cam.ac.uk).