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Syntheses of New Cephalosporins as Potent Antibacterial Agents

Jih Ru Hwu,*,§,¶ Shwu-Chen Tsay,§ Shu-Mei Liao,§ and Shahram Hakimelahi§

§Organosilicon and Synthesis Laboratory, Institute of Chemistry, Academia Sinica, Nankang, Taipei, Taiwan 11529, Republic of China, and Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan 30043, Republic of China

New isodethiaazacephems (±)-2, (±)-3, and (±)-10 were synthesized and found to possess biological activity against S. aureus FDA 209P in vitro. The mesylate and the triflate functionalities in isodethiaazacephems (±)-2 and (±)-3 acted as electron sinks and remarkably enhanced the biological activity in comparison with the parent 3-hydroxy-isodethiaazacephem (±)-10.
b-Lactam antibiotics can acylate a serine residue of transpeptidases,1 which is responsible for cross-linking of peptidoglycans.2 For enhancement of the antibacterial activity resulting from b-lactam cephalosporins (1), a nucleofuge may be attached to their C-3' position.3 Upon reaction with bacterial enzymes, the b-lactam ring therein is opened readily to result in liberation of the nucleofuge as shown in Scheme 1.4-7 Accordingly, we designed and synthesized isodethiaazacephems (±)-2 and (±)-3 . We believe that the sulfone moieties at the O-3' position of 2 and 3 could act as nucleofuges and thus enhance the antibacterial activity in comparison with that of the parent isodethiaazacephem (±)-10.
Scheme 1

For the synthesis of isodethiaazacephem (±)-2, we treated racemic b-lactam mesylate 58 with NaN3 in DMF to give azido b-lactam 6 in 90% yield (Scheme 2). Subsequently, azido b-lactam 6 was chlorinated with CF3SO2Cl and Et3N to give 7

Scheme 2

in 90% yield.9 Reaction of 7 with H2S in Et3N produced a mixture of isodethiaazapenam 8 (40%) and isodethiaazacephem 9 (15%). Catalytic reduction of 7 by use of Pd/C and H2 (30-35 psi) in EtOAc gave the desired compound 9 (87%) exclusively. On the other hand, catalytic hydrogenation of 6 by use of Pd/C and H2 (30-35 psi) in EtOAc resulted in the reduction of the azide moiety and spontaneous formation of isodethiaazacephem 9 in 94% yield. Debenzylation of 9 to 10 (50%) was accomplished by use of H2 (60 psi) and PdCl2 in EtOH.
We attached the sulfonyl group to the cephem nucleus by mesylation of 9 with MeSO2Cl in pyridine. The resultant 3-mesyloxy b-lactam 11 was then hydrogenated with PdCl2 in EtOH at 60 psi of H2 to give the desired isodethiaazacephem (±)-2 in 35% yield.
Moreover, we treated b-lactam 9 with CF3SO2Cl in pyridine to afford a 3:1 mixture of trifluoromethanesulfonates 12 and 13 in 40% overall yield (Scheme 3). Catalytic reduction of 12 with PdCl2 in EtOH at 60 psi of H2 gave the target isodethiaazacephem (±)-3 in 30% yield.
Scheme 3

We tested the antibacterial activity of the synthesized b-Lactams (±)-2, (±)-3, and (±)-10 as well as reference compound benzyl pencillin10 in vitro against S. aureus FDA 209P. The doses were used up to a level as high as 128 mg/mL.
Results from biological tests revealed the interesting antimicrobial activity of the enol sulfonate b-lactams (±)-2 (0.070 mg/mL) and (±)-3 (0.010 mg/mL). In contrast, enol b-lactam (±)-10 exhibited much lower activity (29.50 mg/mL). Benzyl penicillin was active at the level of 0.40 mg/mL against S. aureus FDA 209P.
The antibacterial activity of cephalosporins against sensitive pathogens is greater by possessing a potential nucleofuge at the C-3' position.3,4,12-16 Trifluoromethanesulfone unit in (±)-3 possesses stronger leaving capability than the methanesulfone unit in (±)-2. Thus, antibacterial activity is more potent for (±)-3 than (±)-2. This is in agreement with our hypothesis on their mode of action in biological systems as shown in Scheme 1.
The preparation of cephalosporin 4 having an excellent leaving group at C-3' position is in progress. The complete biological screening experiments against different strains of five pathogenic microorganisms will be described elsewhere.
Acknowledgment. For financial support, we thank the National Science Council of Republic of China and Academia Sinica.
(1) Waxman, D. J.; Strominger, J. L. Sequence of Active Site Peptides from the Penicillin-sensitive D-Alanine Carboxypeptidase of Bacillus Subtilis. J. Biol. Chem. 1980, 255, 3964-3976.
(2) Frère, J. M.; Nguyen-Distèche, M.; Coyette, J.; Joris, B. Mode of Action: Interaction with the Penicillin Binding Proteins. In The Chemistry of b-Lactam; Page, M. I., Ed.; Blackie Academic & Professional: New York, 1992; pp 148-197.
(3) Boyd, D. B. Elucidating the Leaving Group Effect in the b-Lactam Ring Opening Mechanism of Cephalosporins. J. Org. Chem. 1985, 50, 886-888.
(4) Boyd, D. B.; Lunn, W. H. W. Electronic Structures of Cephalosporins and Penicillins. 9. Departure of a Leaving Group in Cephalosporins. J. Med. Chem. 1979, 22, 778-784.
(5) Faraci, W. S.; Pratt, R. F. Elimination of a Good Leaving Group from the 3'-Position of a Cephalosporin Need Not Be Concerted with b-Lactam Ring Opening. J. Am. Chem. Soc. 1984, 106, 1489-1490.
(6) Page, M. L.; Proctor, P. Mechanism of b-Lactam Opening in Cephalosporins. J. Am. Chem. Soc. 1984, 106, 3820-3825.
(7) Grabowski, E. J. J.; Douglas, A. W.; Smith, G. B. Ammonolysis of Cephalosporins: 13C NMR Characterization of the Intermediates from b-Lactam Ring Cleavage Prior to Loss of the 3'-Group. J. Am. Chem. Soc. 1985, 107, 267-268.
(8) Hakimelahi, G. H.; Just, G.; Ugolini, A. The Synthesis of an O-2-Isooxacephem. Helv. Chim. Acta 1982, 65, 1368-1373.
(9) Hakimelahi, G. H.; Tsay, S.-C.; Ramezani, Z.; Hwu, J. R. Syntheses of New Isocephems and Isodethiaoxacephems As Antimicrobial Agents. Helv. Chim. Acta 1996, 79, 813-819.
(10) Morris, J. J.; Page, M. I. Intra- and Intermolecular Catalysis in the Aminolysis of Benzylpenicillin. J. Chem. Soc., Perkin Trans 2 1980, 212-219.
(11) Pursiano, T. A.; Misiek, M.; Leitner, F.; Price, K. E. Effect of Assay Medium on the Antibacterial Activity of Certain Penicillins and Cephalosporins. Antimicrob. Agents Chemother. 1973, 3, 33-39.
(12) Boyd, D. B.; Hermann, R. B.; Presti, D. E.; Marsh, M. M. Electronic Structures of Cephalosporins and Penicillins. 4. Modeling Acylation by the b-Lactam Ring. J. Med. Chem. 1975, 18, 408-417.
(13) Boyd, D. B.; Herron, D. K.; Lunn, W. H. W.; Spitzer, W. A. Parabolic Relationships Between Antibacterial Activity of Cephalosporins and b-Lactam Reactivity Predicted from Molecular Orbital Calculations. J. Am. Chem. Soc. 1980, 102, 1812-1814.
(14) Boyd, D. B. In Chemistry and Biology of b-Lactam Antibiotics; Morin, R. B.; Gorman, M., Eds.; Academic Press: New York, 1982; Vol. 1, pp 437-545.
(15) Boyd, D. B. Electronic Structures of Cephalosporins and Penicillins. 15. Inductive Effect of the 3-Position Side Chain in Cephalosporins. J. Med. Chem. 1984, 27, 63-66.
(16) Nishikawa, J.; Tori, K. 3-Substituent Effect and 3-Methylene Substituent Effect on the Structure-Reactivity Relationship of 7b-(Acylamino)-3-cephem-4-carboxylic Acid Derivatives Studied By Carbon-13 and IR Spectroscopies. J. Med. Chem. 1984, 27, 1657-1663.