Tomaymycin (2) and oxotomaymycin (3) are antibiotics isolated from Streptomyces achromogenes var. Tomaymycetics. The structure of tomaymycin (2) was determined by degradation reactions, spectral analysis, and synthesis of the important derivative (8b) containing its basic skeleton, which confirmed the ethylidene group is attached to the C-2 position and the absolute configuration of the C-11a position is the same as that of L-hydroxyproline. Oxotomaymycin (3), a minor product from the same fermentation broth, has a molecular formula of C₁₅H₁₆O₄N₂ (determined by high-resolution mass spectral analysis). Its structure was established by chemical correlation to the derivative (7a) of tomaymycin, as the methyl ether of oxotomaymycin was identical to compound (7a) derived from tomaymycin.