TOTAL SYNTHESIS OF BLEOMYCIN GROUP ANTIBIOTICS - TOTAL SYNTHESES OF BLEOMYCIN DEMETHYL A(2), BLEOMYCIN A(2), AND DECARBAMOYL BLEOMYCIN DEMETHYL A(2)

The total syntheses of bleomycin A(2) (1) by two routes are described. The final step in the synthesis of bleomycin A(2) involves methylatio n of bleomycin demethyl A(2) (2). This bleomycin derivative is of inte rest mechanistically, and can also provide access: to other bleomycins via its known chemical conversion to bleomycinic acid. Accordingly, t he synthetic strategy presented represents a particularly versatile ap proach for the elaboration of a wide variety of BLM congeners. Bleomyc in was constructed from five key intermediates, the syntheses of which are described. amoyl)-alpha-D-mannopyranosyl]-beta-L-gulopyranose (3) was converted quantitatively to its disaccharide chloride (4), the la tter of which was condensed with ,N-im-bis(t-Boc)-(S)-erythro-beta-hyd roxyhistidine (7) to provide alpha-O-glycosidated product 16. The subs equent couplings with benzyl valerate 8, threonylbithiazole 9, and N-a lpha-t-Boc-pyrimidoblamic acid (10) afforded access to bleomycin demet hyl A(2) (2) and decarbamoyl bleomycin demethyl A(2) (26). Although it was obtained as a byproduct of the synthesis of BLM, the synthesis of decarbamoyl bleomycin demethyl A(2) nonetheless constitutes the first report of synthetic access to this BLM congener that is of particular importance from a mechanistic perspective. This BLM can be used to re solve the issue of the participation of the carbamoyl group as a metal ligand in metallobleomycins. Also reported is a new route to bleomyci n demethyl A(2) that employed an unprotected carbamoyl group throughou t the synthesis. In conjunction with the use of the uncharged methylth iopropylamide C-substituent, the latter strategy permitted improved fu nctional group manipulation and thereby afforded intermediates that co uld be purified with greater facility. Because the synthesis of bleomy cin is limited by the efficiency of elaboration of the carbohydrate mo iety, a study was carried out to define improved methods for the prepa ration of this constituent of BLM. Three new routes were explored, and a route involving the intermediacy of disaccharide activated as a gly cosyl bromide was found to be particularly efficient and convenient. A lso of importance was the finding that activation of carbohydrate inte rmediates as their glycosyl trichloroacetimidates permitted the requis ite couplings to be carried out conveniently and in good yields. Synth etic BLM demethyl A(2) was shown to have the same potency as a natural ly derived sample in a plasmid DNA relaxation assay. The sequence sele ctivity of DNA cleavage by synthetic and authentic Fe(II).BLMs was als o shown to be the same. Also established for the first time for any sy nthetic bleomycin was the actual chemistry of DNA degradation, which i s the same as that for naturally derived bleomycins.