Igidamycins A–C, Rare Sugar-Bearing Anthracycline Glycosides from a Marine Actinomycete, Streptomyces sp.

Genome-guided mining of marine Streptomyces sp. GCU-142 led to the identification of three unprecedented sugar-bearing anthracycline glycosides, igidamycins A–C (1–3). The structures of 1–3 were assigned by spectroscopic analysis including NMR, HR-MS, and UV spectroscopy. Their configurations were determined by ROESY NMR data, the modified Mosher’s method, l-cysteine methyl ester derivatization, and TD-DFT ECD calculations. Bioinformatic analysis suggested a plausible biosynthetic pathway of igidamycins, integrating type II polyketide assembly, extensive sugar tailoring, and late-stage nitration. Notably, igidamycin A (1), bearing a nitro group at C-6, exhibited potent antibacterial activity against various Gram-positive pathogens. In contrast, the non-nitrated congeners (2 and 3) were inactive, highlighting a key role of the nitro functionality in its antibacterial potency.