Iso‐maleimycin, a Constitutional Isomer of Maleimycin, fromStreptomyces sp. QL37

Iso-maleimycin, a previously unknown constitutional isomer of the antibiotic maleimycin, has been detected in an extract ofStreptomycessp. QL37. Chemical synthesis of both maleimycin (20 % yield over seven steps) andiso-maleimycin, (15 % yield over six steps) allowed access to reference materials for identification. Gas Chromatography coupled Mass Spectrometry (GC-MS) analysis demonstrated that of the two isomers, only iso-maleimycin was present in the extract. This finding supports our hypothesis thatiso-maleimycin is a biosynthetic intermediate of lugdunomycin.Iso-maleimycin displays low antibiotic activity, with a Minimum Inhibitory Concentration (MIC) value on bothE. coliandB. subtilisof 250 mu g/mL.