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5-O-Mycarosyltylactone has been isolated as a predominant factor from fermentation broths of a Streptomyces fradiae mutant. The relative configurations of mycarose and tylactone (protylonolide) have been determined by X-ray crystal structure analysis. Hydrolysis of 5-O-mycarosyltylactone yielded (-)-tylactone and L-(-)-mycarose. Taken together, these two experiments establish the absolute configuration of (-)-tylactone. Bioconversion of (-)-tylactone to tylosin by tyl G mutants of S. fradiae proves the absolute configuration of tylosin. Physicochemical data for tylactone and a unique component piece of tylactone are also reported.

A novel low-molecular weight inhibitor of an aminoglycoside-inactivating enzyme, initially isolated from fermentation broths of Streptomyces neyagawaensis, was determined to be 7-hydroxytropolone. Its structure was confirmed by synthesis. In vitro synergy was demonstrated between 7-hydroxytropolone and certain aminoglycosides against bacteria which were resistant to those aminoglycosides by virtue of a 2"-O-adenylyltransferase. The synthesis and characterization of some analogs of 7-hydroxytropolone is also described.

The structure of antibiotic A33853, isolated from the culture broth of Streptomyces sp., NRRL 12068, is reported. The structure was deduced from an X-ray crystallographic study of its tetraacetyl derivative. Tetraacetyl A33853 is unique because it contains an anhydride moiety, an unexpected product from the reaction of A33853 with acetic anhydride and pyridine.

The crystal and molecular structures of the calcium complex of A23187 has been determined by X-ray diffraction studies.