| Summary: | Seven new unstable tetramic acid derivatives, cladosporiumins I-O (<b>1</b>⁻<b>7</b>), together with the known analogue cladodionen (<b>8</b>) were isolated from the extract of the deep-sea-derived fungus <i>Cladosporium sphaerospermum</i> EIODSF 008. Their structures were elucidated by spectroscopic analysis, quantum chemical calculations and ECD spectra. Compound <b>4</b> was a Mg complex of tetramic acid derivative. In acidic solvent, <b>4</b> could change to <b>1</b> and <b>6</b>, and <b>7</b> could change to <b>5</b>. In addition, <b>1</b>, <b>5</b> and <b>8</b> existed as two exchangeable isomers, respectively. The structures of cladosporiumins E-H were reassigned as their Na complexes. The antibacterial and cytotoxic activities of <b>1</b>⁻<b>8</b> were also evaluated. However, because of their instability, all of the isolated compounds did not show significant antibacterial activity as the preliminary EtOAc extracts of the fungal strain.
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