Studies on cephem sulfones as inhibitors of human leukocyte elastase (HLE) have been extended to the new class of cephem 4-ketones. tert-Butyl and phenyl ketones were prepared from 4-carboxycephem derivatives, at either the sulfide or sulfone oxidation level, by chemoselective Grignard reaction. Obtained products were functionalized with heterocyclothio and acyloxy substituents at C-3', C-2, or both positions. tert-Butyl ketones of the 7 alpha-chlorocephem series were in general at least as potent as the corresponding esters at inhibiting the enzyme, but improvements in hydrolytic stability were only marginal. On the other hand, tert-butyl ketones of the 7 alpha-methoxycephem series combined potent biochemical activity with acceptable hydrolytic stability, thus overstepping the esters, thiolesters, and amides reported previously. In particular, the tert-butyl ketones possessing a heterocyclothio group at C-3' or C-2 were at least as active as the corresponding tert-butyl esters but 1 order of magnitude more stable in physiologic buffers (pH 7.4, 37 degrees C). Introduction of acyloxy groups at C-2 delivered the most potent HLE inhibitors of the cephem class ever reported, with inhibition parameters often outside the determination limits of our standard protocol (second-order rate constant kon > 2,000,000 M-1 s-1; Ki at steady state < 2 nM). Keto-enol tautomerism was found to depress activity and boost hydrolytic stability. Thus, double substitution with heterocyclic thiols produced compounds with diverging properties, according to the extent of enolate formation at the investigated pH (7.4): the weakly acidic tert-butyl ketones (pKa > or = 5.8) proved to be potent inhibitors (kon over 10(4) M-1 s-1) with reasonable hydrolytic stability (t1/2 = 30-75 h), while the phenyl ketones (pKa < 4) were fair inhibitors (kon over 10(3) M-1 s-1; Ki at steady state approximately 50 nM) with hydrolytic half-lives exceeding 1000 h. Selected compounds efficiently inhibited the degradation of insoluble bovine neck elastin by HLE in a concentration-dependent manner. Intracellular HLE of polymorphonuclear leukocytes was in general unaffected; however, a lipophilic cephem sulfone apparently able to inactivate the enzyme in living cells was identified.

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1,1-Dioxo-7α-methoxy-3-methyl-∆ 3 -cephem-4-yl phenyl ketone, a valuable precursor of potent HLE inhibitors, was obtained in an efficient way starting from 7α-methoxy-3-methyl-∆ 3 -cephem-4-carboxylic acid. By employing the same methodology a variety of 1,1-dioxocephem-4-yl aryl ketones were prepared.Until a decade ago cephem derivatives had long been known only for their antibacterial properties attained through inhibition of bacterial peptidases. 1 During the last decade the capability of modified cephalosporins to inhibit mammalian serine proteinases, and in particular human leukocyte elastase (HLE), has been ascertained. 2 The prospective use of cephems as antiinflammatory drugs in human therapy has fostered renewed efforts to explore the cephem field both from a chemical and a biological point of view.Our investigation 3 has resulted in the disclosure of original 1,1-dioxocephem 4-ketones 4 and tricyclic cephem sulfones. 5 As their synthesis mostly relies on the exploitation of the key-intermediates 1 and 2, we sought for improved methods of preparing both synthons: herein and in the accompanying paper 6 we report our findings on this subject.Scheme 1 outlines the retrosynthetic pathway according to which we had previously obtained cephem 1 starting from 7α-methoxy-3-deacetoxycephalosporanic acid 3. 7 Carbon-carbon bond formation was the critical step, and acid chloride 4, uneventfully obtained from acid 3, was converted into phenyl ketone 5 upon treatment with phenylmagnesium chloride/copper(I) iodide. 4 Scheme 1In developing an alternative synthetic plan we were guided by two main considerations elaborated on the basis of other researchers' and our own experience in the field of cephem chemistry. Firstly, cephem esters had been reported to undergo regioselective acylation at C-4. 8 Secondly we had observed the easy decarboxylation of 4-alkyl-4-carboxy-∆ 2 -cephem sulfones to provide 1,1-dioxo-4-alkyl-∆ 3 -cephems. 9 Both these features turned out to be the keystones of an efficient synthetic pathway, according to which the targeted 4-benzoyl cephem 1 was obtained in five steps from the cheap and commercially available 7β-amino-3-deacetoxycephalosporanic acid (7-ADCA) (Scheme 2).The methoxy-deamination reaction of 7-ADCA to provide 7α-methoxy-3-deacetoxycephalosporanic acid 3 7 was optimised in terms of safety and reproducibility by substituting methanesulfonic acid for perchloric acid in the diazotation of 7-ADCA; crude acid 3 was then protected 10 with p-methoxybenzyl chloride (NaBr, NaHCO 3 , DMF) affording ester 6 11 in satisfactory yield. Upon treatment of 6 with LDA (1.1 mol equiv, THF, -70 °C) and then with benzoyl chloride (1.5 mol equiv, -70 °C) the C-4 acylated ∆ 2 -cephem 7 was produced as a single diastereomer in about 75% yield. It is noteworthy that the delocalized carbanionic species, generated by removal of a cephem C-2 proton, undergoes Scheme 2Downloaded by: University of Liverpool. Copyrighted material. LETTERS SYNLETTacylation with remarkable regio-and stereoselectivity. On mechanistic g...

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