Benanomicin B

In the search for novel antibiotics, natural products continue to represent a valid source of bioactive molecules. During a program aimed at identifying previously unreported taxa of actinomycetes as potential source of novel compounds, we isolated hundreds of different representatives of a new group, initially designated as 'Alpha' and independently described as Actinoallomurus. We report on a PCR-specific method for the detection of this taxon, on appropriate growth conditions and on a pilot-screening program on 78 strains. The strains produce antibacterial or antifungal compounds at a relatively high frequency. Four strains were characterized in further detail: one produced the aromatic polyketide benanomicin B and its dexylosyl derivative; a second strain produced N-butylbenzenesulfonamide; a third strain was an efficient converter of soymeal isoflavonoids from soymeal constituents; and a fourth strain produced several coumermycin-related aminocoumarins, with coumermycin A2 as the major peak, and with some new congeners as minor components of the complex. These data suggest that Actinoallomurus strains possess several pathways for secondary metabolism and represent an attractive source in the search for novel antibiotics.

An antifungal antibiotic, benanomicin A, binds in the presence of Ca2+ to susceptible fungi and some bacteria, but not to antibiotic-resistant bacteria and mammalian cells. With the susceptible yeast Saccharomyces cerevisiae, benanomicin A binds similarly to whole cells and to protoplasts. Studies using benanomicin A and three structurally related derivatives suggested that a carboxylic acid in the D-alanine moiety and a sugar moiety in the benanomicin A molecule are essential for both binding and antifungal activities against growing S. cerevisiae. An amino substituent on the sugar moiety can be replaced with a hydroxyl group without the loss of activities. Benanomicin A binds to various yeast mannans which differ in glycosidic linkages. These results indicate that binding of benanomicin A to the mannan portion of fungal cells is essential for exertion of the antifungal activity.

A general approach to the regio- and stereoselective total synthesis of the benanomicin-pradimicin antibiotics (BpAs) is described. Construction of the aglycon has been achieved by 1) the diastereoselective ring-opening of a biaryl lactone by using (R)-valinol as a chiral nucleophile and 2) the stereocontrolled semi-pinacol cyclization of the aldehyde acetal by using SmI(2) in the presence of BF(3)OEt(2) and a proton source to afford the ABCD tetracyclic monoprotected diol. This strategy enabled us to control the two stereogenic sites in the B ring (C-5 and C-6) and the regioselective introduction of the carbohydrate moiety. The ABCD tetracycle could serve as an ideal platform for the divergent access to various BpAs. The amino acid (D-alanine) was introduced onto the ABCD tetracycle. Glycosylation was promoted by the combination of Cp(2)HfCl(2) and AgOTf (1:2 ratio). Construction of the E ring followed by deprotection completed the first total synthesis of benanomicin A (2 a), benanomicin B (2 b), and pradimicin A (1 a). The route is flexible enough to allow the synthesis of other congeners differing in their amino acid and carbohydrate moieties.