a, Molecular structures of marmycin A (1) and B (2), jadomycin B (3), hedamycin (4), the clinically approved anthracycline doxorubicin (DXR, 5) and landomycin E (6). b, Retrosynthetic analysis towards marmycin A precursors featuring a stepwise fusion of the aromatic and chiral sugar moieties and a regioselective cycloaddition.