Protorubradirin, an antibiotic containing a C-nitroso-sugar fragment, is the true secondary metabolite produced by Streptomyces achromogenes var. rubradiris. Rubradirin, described earlier, is its photo-oxidation product.

In an attempt to improve the isolation of the antibiotic rubradirin from fermentations of Streptomyces achromogenes var. rubradiris, the use of preparative reversed-phase chromatography was investigated. The product isolated was a mixture of rubradirin and a new antibiotic named protorubradirin, of extremely similar structure, which is converted into rubradirin on exposure to light and air. Methanolysis of protorubradirin in the dark yields an anomeric mixture of methyl glycosides of a C-nitroso-sugar, converted photo-oxidatively into the methyl rubranitrosides derived from rubradirin. Thus, protorubradirin is the C-nitroso-analogue of rubradirin. It is suggested that the same relationship between protorubradirin and rubradirin may apply to the anthracycline antibiotics viriplanin A and viriplanin D.