Cephalosporin methoxylation

Furthermore, the 3D-location of Tyr150 is quite different from that of Glu166 in class-A /3-lactamases. The observed resistance of the methoxylated cephalosporins to class-C /3-lactamases is due to a slow deacylation step [35], This inhibition is the result of the formation of an acyl-enzyme intermediate... 

The finding that the addition of a methoxyl group at the 6-a-position significantly enhances resistance to bacterial beta lactamase actually traces to the cephalosporin... 

Several synthetic procedures for the preparation of C-2-acetoxy- and methoxy-cephalosporins have been reported20). The functionalization at the C-2 position of cephalosporin 15 can be started with the oxidation of the divalent sulfur atom which produces sulfenium cation intermediates, which are precursors of C-2-substituted cephalosporins. The electrochemical conversion of cephalosporins 15 into their C-2-substituted homologs has been realized 2,). For example, the electrochemical acetoxy-lation at the C-2 position of desacetoxycephalosporin 15 is carried out in an AcOH— Ba(OAc)2—(Pt) system to give the C-2-acetoxylated products 24 (R1 = CFL,OPh R2 = Me) in 70% yield (Scheme 2-8). Electromethoxylation at the C-2 position of 15 is performed in a MeOH/CHClj—BuEtjNCl—(Pt) system to give the compounds 25 (R1 = CH2OPh R2 = CH2Ph) in 43% yield. The methoxylated product 25 can lead to the further oxidized product 26 by electrolysis in a H20/CHClj—MgC —(Pt) two-layer system. 

Figure 1 Biosynthetic pathway to DAC. In Cephalosporium acremonium, DAC is ace-tylated by DAC acetyltransferase to cephalosporin C. In Streptomyces clavuligerus, it is carbamoylated by L -carbamoyltransferase followed by methoxylation by C-7 hydroxylase and methyltransferase.

The same procedure can be used for 7a-methoxylation of cephalosporins. Thus (.5) is converted into (6) in 73% yield. 

Cephamyclns - At present, the standard cephamycin is cefoxitin (14). A review lists the isolated natural variants as well as the various methods for the 7a-methoxylation of cephalosporins, SK F 73678 (15),CS-1170 (16) and SQ 14,359 (17) have been studied in the laboratory.- Basically... 

The discovery of cephamycins in streptomycetes fermentation broth and the announcement of their close structural relationship to cephalosporin C spurred efforts in many laboratories to develop chemical methods for methoxylating C-6 penicillins and C-7 cephalosporins. This chemistry engendered yet other methods for introducing a wide variety of substituents at this position. A number of 7-methoxycephalosporins possess interesting and useful biological activity. On the other hand, sporadic attempts to modify cephalosporins at C-2 proved more difficult and have not provided many biologically active compounds. 

While 3-methylenecephams themselves are completely devoid of antimicrobial activity, the exomethylene function is a distinctive feature of these compounds that many workers recognized could be a key intermediate in a chemical route to novel classes of cephalosporins with direct heteroatom substitution at C-3. Whereas 3-methylenecephams were first employed to produce deacetoxycephalosporins, exploration of their chemistry has led to the preparation of 3-chloro-3-cephems and 3-methoxyl-3-cephems that possess marked antimicrobial activity. These compounds represent a new and different generation of cephalosporin antibiotics. Two members of this class of p-lactams have achieved clinical importance. One bears the D-phenylglycyl side chain Lilly compound 99638, with the generic name cefaclor (Ceclor, Lilly). The other bears the D-cyclohexadienylglycyl side chain Ciba-Geigy Compound 9000. Both compounds are potent broad-spectrum p-lactam antibiotics with oral therapeutic efficacy. 

An unusual, although incomplete, sequence for achieving 7a-methoxylation of cephalosporins was executed by Saito and Hiraoka (1977c). The method comprises an intramolecular abstraction of active hydrogen by the anion of the C-7 ylide, concomitant with the elimination of triphenylphosphine (258) to form an imino intermediate. 

In general, none of the 7(6)-alkylated (acylated) cephalosporins or penicillins reported showed enhanced biological activity relative to their unsubstituted parents. These disappointing results, coupled with the concomitant discovery of cephamycins, led researchers to explore the latter more promising area. Among the earliest methods employed for 7(6)-methoxyl introduction was initial -lactam sulfenylation or halogenation,... 

A number of unusual 7(6)-a-methoxylated cephalosporins 527 and 528, and penicillin analogs 529 and 530 have been prepared wherein the 7-amido nitrogen is replaced by sulfur (Sheehan and Commons, 1978). Sheehan s group (1978) has also synthesized new types of 6-spiropenicillins. 

In 1973, Koppel and his colleagues prepared a series of 7a-methoxy-cephalosporins to obtain some idea of structure-activity relationships within the series. The results (Table IX) show a wide variation in activity with modification of the 7 3-side-chain amide, as well as with changes in the group at C-3. In all cases, large differences are seen with the (3-lactamase-producing Serratia marcescens strain against which the non-methoxylated compounds are relatively inactive. 

Substitution of a 7a-methoxyl group on the cephalothin molecule also leads to an improvement in enzyme stability, but not as much as for cefoxitin, indicating that the 3 -carbamoyl contributes to -lactamase resistance. Additionally, the thienylacetyl group promotes -lactamase stability of the antibiotic (Table XII). As shown by their rates of hydrolysis (Table XIII), cefoxitin is more resistant than its naturally occurring precursor, cephamycin C, to a -lactamase from E. cloacae, just as cephalothin is more resistant than its precursor, cephalosporin C. 

The in vitro activities of 2-tetrazolylvinylcephalosporin derivatives are shown in Table XXIV. Introduction of a 7a-methoxy group results in a marked decrease in antibacterial activity for both the thienylacetyl and cyanomethylthioacetyl compounds. These results emphasize the importance of the 3 -position in the overall interrelationship of 7a-methoxylated cephalosporins and their antimicrobial activity. 

A carbon analogue of penicillin V (Ic) was resistant to Bacillus cer-eus B l ctamase and moderately active against gram-positive organisms.6 ) Several 2-acetoxymethyl penicillins (2ja,y6 and 2-spirocyclopropyl cephalosporins (3 )65 vere prepared. Various chemical procedures were developed for the 7(6)a-methoxylation of B-lactam antibiotics.66-72 Conversion of penicillins to cephalosporins was achieved by new synthetic routes.73-76 Some [2,3]-fused tricyclic cephem derivatives (4a-c) were synthesized.77,78 Compound displayed significant antibiotic activity.78 Racemic cephalothin, 7-methoxycephalothin, and cefoxitin were obtained by total synthesis of a novel type.79-81 nuclear analogue of a 7-methylcephalosporin (5), prepared by total synthesis, lacked antibiotic activity,82... 

The cephamycins, a class of cephalosporins obtained from Streptomyces sp., display broad-spectrum antibacterial activity and stability to P-lactamases. The cephamycins contain a methoxyl substituent on the p-lactam ring and the shielding effect makes the ring less susceptible to lactamase attack. A clinically useful semi-synthetic analogue of cephamycin is Cefoxitin (Fig. 22.26). 

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