| Summary: | The strain <i>Streptomyces osmaniensis</i> CA-244599 isolated from the Comoros islands was submitted to liquid-state fermentation coupled to in situ solid-phase extraction with amberlite XAD-16 resin. Elution of the trapped compounds on the resin beads by ethyl acetate afforded seven metabolites, osmanicin (<b>1</b>), streptazolin (<b>2</b>), streptazone C (<b>3</b>), streptazone B<sub>1</sub> (<b>4</b>), streptenol C (<b>5</b>), nocardamine (<b>6</b>) and desmethylenylnocardamine (<b>7</b>). Osmanicin (<b>1</b>) is a newly reported unusual scaffold combining streptazolin (<b>2</b>) and streptazone C (<b>3</b>) through a Diels-Alder type reaction. Experimental evidence excluded the spontaneous formation of <b>1</b> from <b>2</b> and <b>3</b>. The isolated compounds were evaluated for their ability to inhibit elastase using normal human diploid fibroblasts. Compound <b>1</b> exhibited the most potent activity with an IC<sub>50</sub> of 3.7 μM.
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