Enzymatic Total Synthesis of Defucogilvocarcin M and Its Implications for Gilvocarcin Biosynthesis

Gilvocarcin V (GV, 4) is the major metabolite of Streptomyces griseoflavus Go 3592 and various other Streptomyces species. GV is usually produced along with its minor congeners, gilvocarcin M (3) and gilvocarcin E (5), that vary with respect to their side chain at the C8-position.[1] Several analogues of GV (for example, 6–8, Scheme 1), which are collectively called the gilvocarcin group of natural products, have been isolated from different Streptomyces species. All of these analogues contain the characteristic, polyketide-derived benzo[d]naphtho[1,2-b]pyran-6-one chromophore, but different C-glycosidically linked sugar units Scheme 1).[2] Members of this group of natural products are well-known for strong antitumor activities,[3] a unique mode of action,[4] and remarkably low toxicities.[2c, 5] However, the inherent poor solubility of these molecules appears to be a major obstacle in their development as therapeutics. The chemical syntheses that have been developed so far are unsuitable for generating a library of analogues,[6] whereas combinatorial biosynthetic efforts have shown more promise.[7] The continued advancement and successful implementation of such combinatorial biosynthetic and mutasynthetic approaches requires an in-depth knowledge of the biosynthetic pathway. Incorporation studies with isotope-labeled precursors[8] and genetic experiments[1a, 8d, 9] have revealed that the benzo[d]naphtho[1,2-b]pyran-6-one chromophore of the gilvocarcins is produced from a polyketide-derived angucyclinone intermediate through a complex oxidative rearrangement process. However, the details of the exact sequence of biosynthetic events and the enzymes that are involved have remained elusive. In this context, we herein report a complete, one-pot, enzymatic total synthesis of defucogilvocarcin M(1), a model compound that contains the unique chromophore common to all members of the gilvocarcin group of natural products.[10] The reconstitution of this pathway then enabled further investigation into the details of the oxidative rearrangement process of GV biosynthesis by systematic variation of the enzyme mixture used. For this approach we suggest the term “combinatorial biosynthetic enzymology”.