| Summary: | Four new thiodiketopiperazine alkaloids, namely, 5’-hydroxy-6’-ene-epicoccin G (<b>1</b>), 7-methoxy-7’-hydroxyepicoccin G (<b>2</b>), 8’-acetoxyepicoccin D (<b>3</b>), and 7’-demethoxyrostratin C (<b>4</b>), as well as a pair of new enantiomeric diketopiperazines, (±)-5-hydroxydiphenylalazine A (<b>5</b>), along with five known analogues (<b>6</b>−<b>10</b>), were isolated and identified from the culture extract of <i>Epicoccum nigrum</i> SD-388, a fungus obtained from deep-sea sediments (−4500 m). Their structures were established on the basis of detailed interpretation of the NMR spectroscopic and mass spectrometric data. X-ray crystallographic analysis confirmed the structures and established the absolute configurations of compounds <b>1</b>−<b>3</b>, while the absolute configurations for compounds <b>4</b> and <b>5</b> were determined by ECD calculations. Compounds <b>4</b> and <b>10</b> showed potent activity against Huh7.5 liver tumor cells, which were comparable to that of the positive control, sorafenib, and the disulfide bridge at C-2/C-2’ is likely essential for the activity.
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