Involved in the biosynthesis of the aromatic polyketide antibiotic nogalamycin. Catalyzes the formation of nogalaviketone from nogalonic acid methyl ester (NAME), the last ring-closure step in the biosynthesis of nogalamycin. nogalaviketone = methyl nogalonate Antibiotic biosynthesis. Homotetramer (PubMed:15159574, PubMed:15071504). Dimer of dimers (PubMed:15071504). Belongs to the polyketide cyclase DnrD family.