Seven new unstable tetramic acid derivatives, cladosporiumins I-O (1⁻7), together with the known analogue cladodionen (8) were isolated from the extract of the deep-sea-derived fungus Cladosporium sphaerospermum EIODSF 008. Their structures were elucidated by spectroscopic analysis, quantum chemical calculations and ECD spectra. Compound 4 was a Mg complex of tetramic acid derivative. In acidic solvent, 4 could change to 1 and 6, and 7 could change to 5. In addition, 1, 5 and 8 existed as two exchangeable isomers, respectively. The structures of cladosporiumins E-H were reassigned as their Na complexes. The antibacterial and cytotoxic activities of 1⁻8 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.
Keywords: Cladosporium sphaerospermum; cladosporiumins I-O; deep-sea-derived fungus; tetramic acid.