Erythromycin A

This work focuses on the generation of novel fluorinated erythromycin. Analogues of erythromycin in which the macrolide is fluorinated have been generated synthetically, but to date only one example bearing a fluorine in the alkyl side chain has been made. Propionyl coenzyme A (CoA) is the natural starter unit for erythromycin biosynthesis by Saccharopolyspora erythraea and its incorporation leads to the production of erythromycin A 1 (Fig. 1). The loading module of the erythromycin producing polyketide synthase (PKS) 6-deoxyethronolide B synthase (DEBS) is highly selective for propionyl CoA, though it can also accept acetyl CoA. Two different methods of increasing the breadth of starter unit specificity have been explored. The first method, employed by workers in the United States, involves the generation of a strain in which a genetic block is introduced in the first condensation step of erythromycin biosynthesis, the resultant strain is unable to produce polyketide macrolide unless N-acetylcysteamine (NAC) thioesters of a diketide are administered. There has been a recent report of the application of this method to make 15-fluoro-6-deoxyethronolide B.

Erythromycin A is more abundant and therefore easier to isolate than erythromycin B. Presumably because of the success of erythromycin A, erythromycin B has been largely neglected by the pharmaceutical industry. Recently, however, we have high-lighted a number of advantages of erythromycin B over erythromycin A. Firstly, it is much more stable to acid, because it is unable to cyclize in a 12,9-direction; erythromycin A undergoes facile formation of the spiro-6,9:12,9-cyclized anhydroerythromycin A, which has little or no antibacterial activity. The 2’-esters of erythromycin B are also much more stable to acid than those of erythromycin A. This finding is potentially important in paediatric medicine, in which 2’-esters serve as pro-drugs.Finally, we have been able to derivatize erythromycin B to give pro-pro-drugs, for example erythromycin B enol ether ethyl succinate (3), which are almost insoluble in the medicine bottle and therefore unable to hydrolyse to yield the vile-tasting erythromycin free base. Compound 3 is activated in two stages, as shown in Scheme 1, to yield erythromycin B.

Having established 6dEB production from either S. coelicolor or S. lividans, there was still the need to convert this polyketide intermediate to full erythromycin. However, as opposed to building upon 6dEB biosynthesis in CH999 (as would be expected given the incorporation of the ermE resistance gene), purified 6dEB produced from CH999 production cultures was exogenously fed to mutants of S. erythraea, allowing subsequent conversion to full erythromycin. Using this strategy, numerous rationally designed 6dEB derivatives were generated using the Streptomyces strains and these analogs were then converted to erythromycin derivatives via converter S. erythraea strains.