Streptomyces coelicolor CH999/pJRJ2 harbors a plasmid encoding DEBS(KS1 degrees), a mutant form of 6-deoxyerythronolide B synthase that is blacked in the formation of 6-deoxyerythronolide B (1, 6-dEB) duel to a mutation in the active site of the ketosynthase (KS1) domain that normally-catalyzes the first polyketide chain elongation step of 6-dEB biosynthesis. Administration of (2E,4S,5R)-2,4-dimethyl-5-hydroxy-2-heptenoic acid, N-acetylcysteamine thioester (6) an unsaturated triketide analogue of the natural triketide chain elongation intermediate to cultures-of S. coelicolor CH999/pJRJ2 results in formation-of a 16-membered macrolactone, which is isolated in the hemiketal form 33. The formation of the octaketide 33 indicates that the triketide substrate has been processed by DEBS module 2 as if it were a diketide analogue. The substrate specificity of this novel reaction has been explored by the incubation of three additional analogues of the unsaturated triketide 6, compounds 1.8, 31, and 32, with S. coelicolor CH999/pJRJ2, resulting in the formation of the corresponding macrolactones 34, 35, and 36. By contrast, the unsaturated triketide 10, lacking a methyl group at C-2, did not give rise to any detectable macrolactone product when incubated with S. coelicolor CH999/pJpJ2.