B. Konig, 0. Moller, P. G. Jones, B. Ahrens 1575

Hydrogen Bonding of Benzenesulfonamides in Molecular Recognition Processes Burkhard KonigXa, Oliver Mollera, Peter G. Jonesb and Birte Ahrensb

Institut fur Organische Chemie der Technischen Universitat Braunschweig", Hagenring 30, D-38 106 Braunschweig, Germany

Institut fur Anorganische und Analytische Chemie der Technischen Universitat Braunschweigb, Hagenring 30, D-38106 Braunschweig, Germany

Received March 9. 1995

Key Words: Benzenesulfonamide I Hydrogen bond I Amino acid I Dimerization I Non-covalent bond

Benzenesulfonamides represent a hydrogen donor-acceptor moiety that is changed to a bisacceptor by deprotonation. The dimerization constant of N-tosyl-n-butylamine (7) and N-tosyl-L-alanine (9-H) in CDC13 and the binding constant of

the tetrabutylamrnonium salt 7. NBu4 to urea in DMSO were determined by 'H-NMR titration methods. The X-ray struc- ture analysis of 9-H is reported.

Medicinal chemists have paid considerable attention to amide derivatives of 4-aminobenzenesulfonic acid because of their import- ant pharmaceutical propertied']. Sulfanilamides are antagonists of p-aminobenzoic acid (PAB, l), representing a bacterial growth fac- tor related to the folic acid synthesis[*l. The similar structures of the natural substrate I and sulfanilamides 2 suggest that these com- pounds act as antimetabolites, which are recognized by the enzyme, but block its catalytic activity. In terms of molecular recognition the discrimination of the bacterial enzymes between PAB and sul- fanilamides is not suficient to choose the right substrate. The structure of the amide side chain alters the binding ability of sul- fonamides to proteins and their anti-infective properties. Polar, electron-withdrawing groups lead to highly effective short-term antibiotics, while derivatives with non-polar substituents are useful in the therapy of diabeti~[~I.

H I

1

The sulfonamide group as in 3 is isosteric wil known hydrogen donor-acceptor binding sites such as carboxylic acid 4, amide 5 or 2-aininopyridine moieties 6 . While other hydrogen bonding moiet- ies have been extensively used for the construction of artificial re- ceptors or self-assembling component^[^], examples with sulfonam- ides are rare[5]. However, sulfonamides are distinguished by their facile synthesis and high stability.

A sulfonamide is a self-complementary hydrogen bonding donor- acceptor moiety and therefore dimerization is observed in solu- tion16]. The 'H-NMR spectra of CDCl3 solutions with different concentrations of 7['1, synthesized from p-tosyl chloride and n-bu- tylamine, show a significant shift of the amide 'H-NMR signal (Figure 1). The association constant Ka = 2 (+/-0.5) llmol was determined by a non-linear fitting procedure of the dataLsI. The analogous value for 2-pyrrolidone (3, a carboxylic amide that is

locked in the cis conformation, is of the same order of magnitude (K, = 2-3 1/m01[91).

3 4 5 6

The method of continuous variations['"] (Job's method) was used to determine the binding stoichiomety between 7 and 5. The maxi- mum at X = 0.5 clearly indicates the formation of a 1 : 1 associate (Figure 2). The binding constant can be estimated to be of the order of 2-8 Umol; however, the exact determination of K , by non- linear fitting of the experimental data to theory is dificult, because the dimerization constants Kd of the components are of the same order of magnitude.

I 7

The deprotonation of the amido group converts the sulfonamide into a bis(hydrogen bond) acceptor. Derivatives of urea or diols are now suitable binding partners for the stable sulfonamide salt. A 0.05 M solution of the tetrabutylammonium salt of 7 in CDCI3 was titrated with a 0.3 M solution of N,N-dimethylurea (8) and from the shifts of the 'H-NMR resonance of the a-CH2 group and the o-aryl protons a binding constant of K, = 26 (+/-4) l/mol was calculated (Figure 3). By the introduction of both donor or acceptor func-

Liehigs Ann. 1995,1575-1578 0 VCH Verlagsgesellschaft mbH, D-69451 Weinheim, 1995 0947-3440/95/0808-1575 $10.00+.25/0

1576 B. Konig, 0. Moller, I? G. Jones, B. Ahrens

Figure 1. IH-NMR chemical shift of the amide proton of 7 at different concentrations (400 MHz, CDC13)

5 4 0

520

5,OO - E goo I

ro

4,60

4,40

4,20 0,OO 0,20 0,40 0,60 0,80 1,OO

7 [mold]

Figure 2. Job's plot analysis of the binding stoichiometry of 7 and 5

0.16

co 2 0,12 t- - C

0

aJ

.O +d 0.08

2 - 0.04

0 0 0.2 0,4 0,6 0,8 1

mole fraction of 7

tional groups into one molecule, the non-covalent bonding between the components becomes stronger"']. The observed NMR shifts in titration experiments with 2,2-dimethyl-l,3-propanediol and 2- butene-l,4-diol indicate binding of the diols to the tetrabutylam- monium salt of 7; however, the A8 values were too small to be measured accurately for a determination of the binding constants. Two bonding modes are possible for 7 . NBu4: either the nitrogen and oxygen atom or both sulfonyl oxygen atoms can act as binding sites. From the electron distribution of 7 . NBu,, calculated by semi-empirical methods[12], the binding via both sulfonyl oxygen atoms is favored.

Sulfonamides with an additional carboxyl group are readily available by tosylation of amino acids. The 'H-NMR dilution experiment reveals an association constant of N-tosyl-~-alanine (9- H)[I3J in CDC13 of Kd = 200 (+/-50) Vmol (Figure 4). The concen- tration-dependent shift of the N-H 'H-NMR resonance indicates

5

Figure 3. 'H-NMR chemical shift of the a-CH2 ( 0 ) and the o-arene (m) protons of 7 . NBu4 with added urea

0,02

0,oo

- E Y 2 -0,02 ro 4

-0,04

-0,06 0,oo 1 ,oo 2,oo 3,OO

ratio 0 : 7 NBu4

hydrogen bonding of this proton in solution. The much smaller association constant of the analogous methyl ester 9-Me[l4] [K,, = 2 (+/- 1) Ilmol] demonstrates the importance of the free carboxylic acid for the association process. A Job's plot analysis of 9-Me and acetic acid proves the stoichiometry of binding of the carboxylic acid moiety to the sulfonamide (Figure 5).

9-H

Figure 4. Change in 'H-NMR chemical shift of the arnide proton of 9-H (w) and 9-Me ( 0 ) with increasing concentration

0,oo 0,50 1 ,oo c [10-z2mold]

The X-ray crystal structure[151 analysis of 9-H reveals that the sulfonyl oxygen atoms do not participate in hydrogen bonding in the solid state. The chiral molecules are packed in a helical band structure with alternatingly aligned tosyl groups (Figure 6). Each

Liebigs Ann. 1995, 1575-1578

Hydrogen Bonding of Benzenesulfonamides in Molecular Recognition Processes 1577

Figure 5. Job's plot analysis of the stoichiometry of binding 9-Me to acetic acid

0,08 1 0,07

2 0,06 1 0,05

0,04

3 0,03 E

L a

0)

.4-

$ 0,02 0,Ol

0 0 0,2 0,4 0,6 0.8 1

mole fraction 9-Me

carboxyl group forms strong hydrogen bonds (04.-03 267.0 pm) to the neighboring molecules, while a weaker interaction between the amide proton and the carboxyl group (N...03 296.7 pm) was detected.

Figure 6. Packing diagram of 9-H in the crystal

Y 1

C

We have shown that the non-covalent binding of benzenesulfona- mides can be detected by NMR methods. The stability and the facile synthesis of sulfonamides favor their application in the design of artificial receptor molecules. However, compared with carboxylic acids and amides they are much weaker binding sites.

B. K. thanks the Fonds der Chemischen Industrie for a Liebig stipend and Prof. Dr. H. Hopf for his support. Generous gifts of chemicals from the Degussa AG and grants from the Fonds der Chemischen Industrie, Deutsche Forschungsgemeinschaft and the Volkswagen-Stiftung are gratefully acknowledged.

Experimental 'H NMR: Bruker AM 400 and AC 200; 6 = 0 for tetramethylsil-

ane as internal standard, 6 = 7.26 for chloroform. - IR: Nicolet 320 FT IR. - UVNis: Hewlett Packard 8452A.

X-Ray Structure Determination of Compound 9-H (Figure 7)[151: CI0Hl3NO4S (243.27), monoclinic, space group P21, a = 774.72(12), b = 682.6(2), c = 1098.9(3) pm, p = 97.92(2)", V = 0.5756(2) nm3, Z = 2, h(Mo-K,) = 71.073 pm, p = 0.28 mm-', D, = 1.404 Mg/m3, F(OO0) = 256, T = 143 K. A clear prism with dimensions 0.9 X 0.5 X 0.25 mm was mounted on a glass fiber in inert oil and transferred to the cold gas stream of a Stoe STADI- 4 diffractometer fitted with a Siemens LT-2 low-temperature at- tachment. A total of 2831 intensities (2529 unique, Rint = 0.0228) were measured to 2 0 55". The structure was solved by direct meth- ods and refined anisotropically on F2 (program SHELXL-93, G. M. Sheldrick, University of Gottingen). The final w R ( p ) for all

reflections was 0.081, with a conventional R ( Q of 0.031 for 152 parameters.

Figure 7. The molecule of 9-H in the crystal

General Procedure for NMR Titrations and Determination of As- sociation Constants. - A ) Titration Method: For a specific example the analysis of the binding of 7 . NBu4 to 8 is described in the following. 7 . NBu, (234 mg, 0.5 mmol) was dissolved in [D6]DMS0 (10 ml, 0.05 M) and 90 mg (1.5 mmol) of 8 in 5 ml of [D6]DMS0 (0.3 M). Then 10 pl up to 300 p1 of ahquots of the solution of 8 were added to 600 fl of the solution of 7 . NBu4 and the 'H-NMR spectra were recorded. The change in concentration with increasing volume was considered for data analysis. The prob- ability of bindingp, as defined by Weber[I61, is in the range 0.2-0.8 for most of the data points. The average from three measurements was calculated and the standard deviation was determined to give K, and error limits.

B ) Dilution Method: For a specific example the analysis of the di- merization of 7 is described in the following. 7 (2.27 g, 10 mmol) was dissolved in 10 ml of CDCI,. Then 500 pi samples with decreasing concentrations were prepared by means of p1 syringes or an Eppendorf pipette and their 'H-NMR spectra recorded: 1) 7 (500 pl), 2) 7 (375 PI) + CDC13 (125 PI), 3) 7 (250 PI) + CDC13 (250 pl), 4) 7 (175 pl) + CDC13 (325 PI), 5) 7 (125 pl) + CDC13 (375 pl), 6) 7 (50 pl) + CDC13 (450 pl), 7) 7 (25 pl) + CDC13 (475 pl), 8) 7 (5 pl) + CDCI3 (495 pl). By removing of 250 pl from these samples after measurement and replacing it by CDCl, an ad- ditional set of samples with half concentrations were obtained.

General Procedure for the Determination of Binding Stoichi- ometry: For a specific example the Job's plot analysis of the binding of 7 and 5 is described in the following. 340 mg (1.5 mmol) of 7 or 109 mg (1.5 mmol) of 5 was dissolved in 5 ml of CDCI3. From these 0.3 molar solutions the following mixtures were prepared by means of p1 syringes or Eppendorf pipettes and their 'H-NMR spectra were recorded: 1) 7 (500 pl), 2) 7 (400 pI) + 5 (100 pl), 3) 7 (300 pI) + 5 (200 pl), 4) 7 (250 pl) + 5 (250 pl), 5) 7 (200 pl) + 5 (300 pl), 6) 7 (100 pl) + 5 (400 pl), 7) 7 (50 pl) + 5 (450 pl).

Preparation of the Tetrabutylammonium Salt oJ I: To a stirred solution of 7 (1.0 g , 4.4 mmol) in 20 ml of methanol was added 5.7 ml (4.4 mmol) of tetrabutylammonium hydroxide (20% in water) and the resulting mixture was stirred at room temp. for 2 h. The solvent was evaporated in vacuo and the solid residue further dried for 24 h high vacuo to yield 2.1 g of 7 . NBu4 as a soft white solid. An analytical sample was crystallized from CH3CN; m.p. 85 "C. - IR (neat): 0 = 2960 cm-', 1474, 1381, 1071, 554. - 'H NMR (400

Hz, 12H), 1.22 (m, 4H), 1.30 (m, SH), 1.56 (m, 8H), 2.27 (s, 3H), 2.58 (t, ' J = 6.7 Hz, 2H), 3.18 (t, 3 J = 8.5 Hz, 8H), 7.08 (d, 3 J = 8.0 Hz, 2H), 7.45 (d, ' J = 8.0 Hz, 2H). - "C NMR (100 MHz,

MHz, [D6]DMSO): 6 = 0.76 (t, ' J = 7.0 Hz, 3H), 0.92 (t, 3J = 7.3

Liebigs Ann. 1995, 1575-1578

1578 B. Konig, 0. Moller, P. G. Jones, B. Ahrens

[D,]DMSO): 6 = 13.49 (+), 14.04 (+), 19.23 (-), 20.14 (-), 20.77 (+), 23.15 (-), 34.86 (-), 45.61 (-), 56.57 (-), 126.17 (+), 127.95 (+), 137.13 (Cquat), 145.84 (Cqnar). - MS (FAB+, NBA); ?7dZ (%): 242 (100); (FAB-) 226 (100). - C27H52NZ02S . H20 (486.7): calcd. C 66.62, H 11.18, N 5.75; found C 66.60, H 10.99, N 5.41. - Mol. mass 468 (MS).

G. Kuschinsky, H. Liillmann, Pharmakologie, 9th ed., G. Thie- me, Stuttgart, 1981, p. 326ff. T. M. Devlin, Textbook of'BiochenuWy, 3rd ed., J . Wiley, New York, 1992, p. 159. [3al The association of sulfonamides with proteins has been in- vestigated in detail. For examples see: T. Fujii, K. Nakamura, H. Furukawa, M. Watanahe, J. Kuwashima, Arzneim. Forsch. 1983, 33, 1535-1537. - [3b] H. Ueda, K. Higashiyama, T. Na- gai, Chem. Pharm. Bull. 1980, 28, 1016-1021. - [jC1 M. H. Abraham, H. S. Chadha, G. S. Whiting, R. C. Mitchell, J Pharm. Sci. 1994, 83, 1085-1100. - [3d] N. Cohen, G. Weber, B. L. Banner, R. Lopresti, B. Schaer, .I Med. Chem. 1989, 32, 1842-1860. - [3el J.-H. Fuhrhop, Bio-orgunische Chemie, G. Thieme Verlag, Stuttgart, 1982, p. 269. [4a] For reviews of hydrogen binding in molecular recognition see: F. Diederich, Cyclophanes, Royal Society of Chemistry, London, 1991. - [4b1 A. D. Hamilton, Advances in Supramolecu- lar Chemistry (Ed.: G. Gokel), Jai Press, Greenwich, CT, 1990. - [4c1 J. Rebek, Acc. Chem. Res. 1990, 23, 399-404. - [4d1 For a review on self-assembly see: J. S. Lindsey, New J Chem. 1991, 15, 152-180. C. Raposo, M. Almaraz, M. Crego, M. L. Mussons, N. Perez, M. C. Caballero, J. R. Moran, Tetrahedron Lett. 1994, 35,

Ihd1 For previous reports on the association of benzenesulfon- aniides based on cryoscopically measured molecular weight see: L. Hunter, H. 0. Chaplin, J Chenz. Soc, 1937, 1114-1118. - 16b] L. Hunter, N. G. Reynolds, J Chem. SOC. 1950, 2857-2864. - [6c] IR spectroscopy: R. Konig, G. Malewski, Spectrochim. Actu, Part A 1968,24,219-230, in micelles: H. Ihara, H. Hach-

7065 - 7068.

isako, C. Hirayama, K. Yamada, J Chem. Soc., Chem. Com- mun. 1992, 1244- 1245.

"1 [7a1 E. J. Sakellarios. Helv. Chim. Acta 1946. 29, 1675-1684. - [7b1 E. H. White, C: P. Lewis, M. A. Ribi,'T. J. Ryan, J. Org. Chem. 1981, 46, 552-558. - I7=1 H. Suzuki, T. Koji, Chem. Lett.

C. S. Wilcox, M. D. Cowart, Tetrahedron Lett. 1986, 27, 5563-5566. - rxb1 We thank Prof. Craig Wilcox, who made the

L91 gal S. E. Krikorian, .I Phys. Chem. 1982, 86, 1875-1881. - [9b1 For comparison we have determined the dirnerization con-

1984, 1733-1736.

rogram HOSTEST5 available to us.

stant of methyl (S)-2-pyrrolidone-5-carboxylate in CDC13 by di- lution experiments and analysis of the proton-NMR shifts by non-linear fitting. A value of Ka = 5 I/mol was obtained. [leal K. A. Connors, Binding Constunts, J. Wiley, New York, 1987, p. 24. - [lob] For a recent example of Job's plot analysis in non-covalent binding see: M. T. Blanda, J. H. Horner, M. Newcomb, J Org. Chem. 1989, 54, 4626-4636.

E. Fan, S. A. Van Arman, S. Kincaid, A. D. Hamilton, .I Am. Chem. Soc. 1993, 115, 369-370. - [ ' I b ] W. L. Jorgensen J. Pranata, J. Am. Chem. Soc. 1990, 112, 2008-2010. - ['Ic1 S C. Zimmerman, T. J. Murray, J Am. Chem. Soc. 1992, 114, 4010-401 1. - For a detailed study of the binding affinity of carboxylate and its isosteres see: T. R. Kelly, M. H. Kim, J Am. Chem. Soc. 1994, 116, 7072-7080. The electron distribution was calculated with the program SPARTAN by using the AM-1 method.

E. Fischer, W. Lipschitz, BeK Dtsch. Chem. Ges. 1915, 48, 360-378. - A dimerization constant of 9-H of the same order of magnitude (Kd = 160 l/mol) was determined from the concentration dependent shift of the 'H-NMR resonance of the carboxylic acid proton. T. Kataoka, M. Yoshimatsu, Y. Noda, T. Sato. H. Shimizu, M. Hori, .I Chem. Soc., Perkin Truns. I , 1993, 121-130. Full crystallographic details for 9-H are available on request from the Fachinformationszentrum Karlsruhe, D-76344 Eg- genstein-Leopoldshafen, Germany, referring to No. CSD- 401 588 and the full literature citation. C. S. Wilcox in Frontiers in Supramolecular Organic Chemistry and Photochemistry (Ed.: H.-J. Schneider, H. Diirr), VCH, Weinheim, 1991, p. 123 and ref. 36.

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Liebigs Ann. 1995, 1575- 1578