Angucyclinones are a subclass of angucyclines, which are defined as microbial quinone natural products related to tetracyclines and anthracyclines and bearing, as characteristic structural feature, a tetracyclic benz[a]anthracene ring system assembled in an angular manner.1 This structural moiety is biosynthetically derived from a decaketide chain formed via the polyketide biosynthetic pathway. The term ‘angucycline’ includes molecules with hydrolysable sugar moieties, whereas ‘angucyclinone’ refers to a sugarless compound or a compound with a C-glycosidic linked sugar moiety.1, 2 Angucyclines are isolated from the fermentation broth of actinomycetes, mostly from the genus Streptomyces, but also from Actinomadura, Nocardia and Streptosporangium genera.1 Angucyclines show a broad spectrum of biological activities including antitumor,1, 2 antibacterial, antifungal, antiviral,3, 4 enzyme inhibitory5, 6 and platelet aggregation inhibitory properties.7, 8 In a continuous search for new bioactive compounds from actinomycetes other than the genus Streptomyces, several Saharan soil samples collected in arid ecosystems were explored.9, 10, 11 Among the isolates, we were interested by a new actinomycete strain belonging to the genus Streptosporangium and producing a new compound (R2) identified as a new angucyclinone. This paper describes the isolation, structure elucidation and antimicrobial activities of compound R2.

Details on the isolation and the taxonomy of the producing organism Streptosporangium sp. Sg3 were described in a previous paper.11 To isolate compound R2, Streptosporangium sp. Sg3 was cultivated at 30 °C for 9 days in Erlenmeyer flasks (500 ml) containing 100 ml of ISP2 broth (yeast extract 4 g, malt extract 10 g, glucose 4 g, in 1 l distilled water, pH 7.2) on a rotary shaker (250 r.p.m.). The cultures (8 l) were centrifuged and filtered to remove mycelium. The culture filtrate was extracted with an equal volume of n-butanol to generate a crude extract (1.9 g). The latter was fractionated by size exclusion chromatography on Sephadex LH-20 (75% MeOH in H2O), resulting in five fractions, I–V. Fraction IV including R2, was of red color and exhibited an antibacterial activity. It was subjected to semipreparative reversed-phase HPLC using an Interchim UP5ODB column (250 × 7.8 mm) (Interchim, Montluçon, France) and developed using a continuous grade from 20 to 75% MeOH in H2O (UV detection at 220 nm), yielding three active fractions (1, 2, 3). The major peak 2 with the main antibacterial activity was subjected to repeated HPLC to yield 3.5 mg of compound R2.

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