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Fractionation and purification of the ethyl acetate extract of the culture of a sponge-derived actinomycete, <i>Streptomyces</i> species Call-36, resulted in the isolation and identification of a new diketopiperazine, actinozine A (<b>1</b>), cyclo(2-OH-<span style="font-variant: small-caps;">d</span>-Pro-<span style="font-variant: small-caps;">l</span>-Leu) (<b>2</b>), two new nucleosides, thymidine-3-mercaptocarbamic acid (<b>3</b>) and thymidine-3-thioamine (<b>4</b>), together with cyclo(<span style="font-variant: small-caps;">d</span>-Pro-<span style="font-variant: small-caps;">l</span>-Phe) (<b>5</b>) and cyclo(<span style="font-variant: small-caps;">l</span>-Pro-<span style="font-variant: small-caps;">l</span>-Phe) (<b>6</b>). The structure assignments of the compounds were carried out by interpretation of 1D and 2D NMR data and mass spectral determinations. The absolute configurations of <b>1</b> and <b>2</b> were determined by Marfey&#8217;s method and by comparison of the experimental and TDDFT-calculated ECD spectra. Actinozine A possesses an unprecedented hydroperoxy moiety at C-2 of the proline moiety, while <b>3</b> and <b>4</b> possess unusual mercaptocarbamic acid and thiohydroxylamine functionalities at <i>N</i>-3 of the thymine moiety. The isolated compounds displayed variable cytotoxic and antimicrobial activities.