The geometric and electronic structures of high-spin ferrous complexes of bleomycin (FeIIBLM) and a series of systematically perturbed BLM derivatives have been investigated by optical absorption, circular dichroism (CD), and magnetic circular dichroism (MCD) spectroscopies. The active site of the unmodified drug complex is six-coordinate with the coordination sphere completed by at least five endogenous ligands including the pyrimidine, imidazole, deprotonated amide, and secondary and primary amine functionalities with either the 3-O-carbamoyl substituent of the mannose sugar or solvent bound at the sixth site. This weak sixth ligand is the exchangeable site of exogenous small molecule binding. Perturbing the carbamoyl substituent alters the coordination environment of the metal and decreases the azide binding affinities of the perturbed complexes. This is correlated with altered DNA cleaving capabilities. Additionally, altering the binding of the axial primary amine significantly affects the iron coordination sphere as evidenced by reduced π-back-bonding interactions specifically with the pyrimidine ligand. This pyrimidine π-back-bonding appears to play a key role in mediating the electron density localized on the ferrous center, which contributes to the unique oxygen chemistry and reactivity exhibited by FeIIBLM relative to other non-heme iron sites. Oxygen binding to derivatives in which the β-aminoalanine fragment has been removed leads to a high-spin ferric complex and no observed DNA strand scission, in contrast to the long-lived low-spin activated BLM intermediate that precedes DNA degradation.

No abstract

The total syntheses of bleomycin A2 (1) by two routes are described. The final step in the synthesis of bleomycin A2 involves methylation of bleomycin demethyl A2 (2). This bleomycin derivative is of interest mechanistically, and can also provide access to other bleomycins via its known chemical conversion to bleomycinic acid. Accordingly, the synthetic strategy presented represents a particularly versatile approach for the elaboration of a wide variety of BLM congeners. Bleomycin was constructed from five key intermediates, the syntheses of which are described. 1,6-Di-O-acetyl-3,4-di-O-benzyl-2-O-[2,4,6-tri-O-acetyl-3-O-(N-acetylcarbamoyl)-α-d-mannopyranosyl]-β-l-gulopyranose (3) was converted quantitatively to its disaccharide chloride (4), the latter of which was condensed with N α,N im-bis(t-Boc)-(S)-erythro-β-hydroxyhistidine (7) to provide α-O-glycosidated product 16. The subsequent couplings with benzyl valerate 8, threonylbithiazole 9, and N α -t-Boc-pyrimidoblamic acid (10) afforded access to bleomycin demethyl A2 (2) and decarbamoyl bleomycin demethyl A2 (26). Although it was obtained as a byproduct of the synthesis of BLM, the synthesis of decarbamoyl bleomycin demethyl A2 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 resolve the issue of the participation of the carbamoyl group as a metal ligand in metallobleomycins. Also reported is a new route to bleomycin demethyl A2 that employed an unprotected carbamoyl group throughout the synthesis. In conjunction with the use of the uncharged methylthiopropylamide C-substituent, the latter strategy permitted improved functional group manipulation and thereby afforded intermediates that could be purified with greater facility. Because the synthesis of bleomycin is limited by the efficiency of elaboration of the carbohydrate moiety, a study was carried out to define improved methods for the preparation of this constituent of BLM. Three new routes were explored, and a route involving the intermediacy of disaccharide activated as a glycosyl bromide was found to be particularly efficient and convenient. Also of importance was the finding that activation of carbohydrate intermediates as their glycosyl trichloroacetimidates permitted the requisite couplings to be carried out conveniently and in good yields. Synthetic BLM demethyl A2 was shown to have the same potency as a naturally derived sample in a plasmid DNA relaxation assay. The sequence selectivity of DNA cleavage by synthetic and authentic Fe(II)·BLMs was also shown to be the same. Also established for the first time for any synthetic bleomycin was the actual chemistry of DNA degradation, which is the same as that for naturally derived bleomycins.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.