Putting the "benz" in indolizinones: A cycloisomerization approach to benz[g]indolizinones and benz[e]indolizinones provides the first general route to these unique azacycles (see example). The utility of the benzindolizinone products was demonstrated by the application of this method to the total synthesis of the Erythrina alkaloids 3-demethoxyerythratidinone and cocculidine.

Highly regioselective remote hydroxylation of a natural product scaffold is demonstrated by exploiting the anchoring mechanism of the biosynthetic P450 monooxygenase PikCD50N-RhFRED. Previous studies have revealed structural and biochemical evidence for the role of a salt bridge between the desosamine N,N-dimethylamino functionality of the natural substrate YC-17 and carboxylate residues within the active site of the enzyme, and selectivity in subsequent C-H bond functionalization. In the present study, a substrate-engineering approach was conducted that involves replacing desosamine with varied synthetic N,N-dimethylamino anchoring groups. We then determined their ability to mediate enzymatic total turnover numbers approaching or exceeding that of the natural sugar, while enabling ready introduction and removal of these amino anchoring groups from the substrate. The data establish that the size, stereochemistry, and rigidity of the anchoring group influence the regioselectivity of enzymatic hydroxylation. The natural anchoring group desosamine affords a 1:1 mixture of regioisomers, while synthetic anchors shift YC-17 analogue C-10/C-12 hydroxylation from 20:1 to 1:4. The work demonstrates the utility of substrate engineering as an orthogonal approach to protein engineering for modulation of regioselective C-H functionalization in biocatalysis.

The diversification of late stage synthetic intermediates provides significant advantages in efficiency in comparison to conventional linear approaches. Despite these advantages, accessing varying ring scaffolds and functional group patterns from a common intermediate poses considerable challenges using existing methods. The combination of regiodivergent nickel-catalyzed C-C couplings and site-selective biocatalytic C-H oxidations using the cytochrome P450 enzyme PikC addresses this problem by enabling a single late-stage linear intermediate to be converted to macrolactones of differing ring size and with diverse patterns of oxidation. The approach is made possible by a novel strategy for site-selective biocatalytic oxidation using a single biocatalyst, with site selectivity being governed by a temporarily installed directing group. Site selectivities of C-H oxidation by this directed approach can overcome positional bias due to C-H bond strength, acidity, inductive influences, steric accessibility, or immediate proximity to the directing group, thus providing complementarity to existing approaches.