Isolation and selective glycosylation of antisalmonellal anthraquinones from the stem bark of Morinda lucida Benth. (Rubiaceae)

In this work we report the isolation, identification and antibacterial activity of two anthraquinones, 2-hydroxy-1-methoxyanthraquinone (1) and 2,5-dihydroxy-1-methoxy-6-methoxymethylanthraquinone (2), from the stem bark of Morinda lucida . These two natural products were selectively converted into two new glycosylated de- rivatives, 2-hydroxy-1-methoxyanthraquinone-4'-O-methyl-2-O-beta-D-glucopyranoside (3) and 2,5-dihydroxy-1-methoxy-6-methoxymethylanthraquinone-4'-O-methyl-2-O-beta- D-glucopyranoside (4) by the filamentous fungus Beauveria bassiana ATCC 7159. Structure elucidation was accomplished based on the 1D and 2D NMR, IR and mass spectra. The glycosylated compounds 3 and 4 showed higher in vitro antibacterial activity against Salmonella enterica subsp. enterica serovars Typhimurim (MIC of 8 mu g/mL each) than the corresponding aglycons 1 and 2 (MIC of 16 mu g/mL and 32 mu g/mL, respectively). These results indicated that microbial glycosylation is an effective approach to modify natural products for enhanced biological activities.