Chem. Pharm. Bull. 55(7) 1097—1098 (2007)

been proven to be a rich source of new biologically active secondary metabolites. In our search for new natural products from marine-derived organisms, here we report the isolation and structure elucidation of a new antimicrobial anthraquinone, monodictyquinone A (1), along with three known compounds, pachybasin (2), chrysophanol (3), and emodin (4). The marine-derived fungus Monodictys sp. was isolated from the sea urchin Anthocidaris crassispina, collected in Toyama Bay in the Sea of Japan. The fungus was grown on agar plates composed of 50% seawater with nutrients. The culture plates were extracted with EtOH. The extract was partitioned between EtOAc and H2O. The EtOAc-soluble fraction (15 g) was further partitioned between hexane and 90% MeOH–H2O. The aqueous MeOH layer was subjected to normal and reverse-phase silica gel column chromatography to furnish 1 (0.67 mg) as a yellow solid. Compound 1 has a molecular formula of C16H12O5 as established by HR-EI-MS, and the molecular formula requires eleven degrees of unsaturation. The H-NMR spectrum in CDCl3 (Table 1) revealed two methyl groups at d 2.45 (s, 6Me) and 4.00 (s, 2-OMe), two doublet [d 7.15 (1H, d, J 8.4 Hz, H-3) and 7.85 (1H, d, J 8.4 Hz, H-4)] and two singlet resonances [d 7.06 (1H, s, H-7) and 7.64 (1H, s, H5)] in the aromatic region, and two D-exchangeable protons at d 11.97 (s, 8-OH) and 12.51 (s, 1-OH). The C-NMR spectrum in CDCl3 (Table 1) showed two methyl, four methine, and ten quaternary carbons including two carbonyl carbons assignable to C-9 (d 192.9) and C-10 (d 181.1); these data strongly suggested the characteristic feature of anthraquinone derivative. The positions of substituted groups were determined by heteronuclear multiple-bond correlation (HMBC) experiments using the optimized J value of 8.3 or 5.0 Hz (Fig. 1); plain arrows showed correlations with the optimized J value of 8.3 Hz, while dashed arrows revealed correlations with the optimized J value of 5.0 Hz. The experiments indicated that two hydroxy (d 11.97, 12.51), methyl (d 2.45), and methoxy (d 4.00) groups are attached to C-8, C-1, C-6, and C-2, respectively; thereby the structure of 1 was established (Fig. 1). Several biological activities have been hitherto reported for anthraquinones isolated from the marine environment; protein kinase inhibitory activity for 1,3,6,8-tetrahydroxyanthraquinone congeners isolated from the sponge-associated fungus Microsphaeropsis sp., antibacterial activity for 1,8dihydroxy-4-methylanthraquinone isolated from the cyanoMonodictyquinone A: a New Antimicrobial Anthraquinone from a Sea Urchin-Derived Fungus Monodictys sp.