The benz[a]anthraquinone antibiotic PD 116740 is formed from the regular cyclization of a decaketide intermediate folded in a manner to generate the angular tetracyclic skeleton. The 6-deoxybenz[a] anthraquinone tetrangulol is an intermediate, indicating that 6-deoxygenation occurs ata prearomatic stage in the biosynthesis. This was consistent with the lack of incorporation of acetate-derived oxygen at this site. Labelling of the C-5 hydroxyl by molecular oxygen indicates that enzymatic epoxidation of the K-region double bond, followed by action of an epoxide hydrolase, generates the 5,6-trans-diol moiety.