Convergent Asymmetric Synthesis of (+)-Aureothin Employing an Oxygenase-Mediated Resolution Step

Enzymatic Kinetic Resolution by Addition of Oxygen

Kinetic resolution using biocatalysis has proven to be an excellent complementary technique to traditional asymmetric catalysis for the production of enantioenriched compounds. Resolution using oxidative enzymes produces valuable oxygenated structures for use in synthetic route development. This Minireview focuses on enzymes which catalyse the insertion of an oxygen atom into the substrate and, in so doing, can achieve oxidative kinetic resolution. The Baeyer-Villiger rearrangement, epoxidation, and hydroxylation are included, and biological advancements in enzyme development, and applications of these key enantioenriched intermediates in natural product synthesis are discussed.

Photocatalytic Reductive Radical-Polar Crossover for a Base-Free Corey-Seebach Reaction

A metal-free generation of carbanion nucleophiles is of prime importance in organic synthesis. Herein we report a photocatalytic approach to the Corey-Seebach reaction. The presented method operates under mild redox-neutral and base-free conditions giving the desired product with high functional group tolerance. The reaction is enabled by the combination of photo- and hydrogen atom transfer (HAT) catalysis. This catalytic merger allows a C-H to carbanion activation by the abstraction of a hydrogen atom followed by radical reduction. The generated nucleophilic intermediate is then capable of adding to carbonyl electrophiles. The obtained dithiane can be easily converted to the valuable α-hydroxy carbonyl in a subsequent step. The proposed reaction mechanism is supported by emission quenching, radical-radical homocoupling and deuterium labeling studies as well as by calculated redox-potentials and bond strengths.

P450 Monooxygenases Enable Rapid Late-Stage Diversification of Natural Products via C-H Bond Activation

The biological potency of natural products has been exploited for decades. Their inherent structural complexity and natural diversity might hold the key to efficiently address the urgent need for the development of novel pharmaceuticals. At the same time, it is that very complexity, which impedes necessary chemical modifications such as structural diversification, to improve the effectiveness of the drug. For this purpose, Cytochrome P450 enzymes, which possess unique abilities to activate inert sp3-hybridised C-H bonds in a late-stage fashion, offer an attractive synthetic tool. In this review the potential of cytochrome P450 enzymes in chemoenzymatic lead diversification is illustrated discussing studies reporting late-stage functionalisations of natural products and other high-value compounds. These enzymes were proven to extend the synthetic toolbox significantly by adding to the flexibility and efficacy of synthetic strategies of natural product chemists, and scientists of other related disciplines.

Rational design of modular polyketide synthases: morphing the aureothin pathway into a luteoreticulin assembly line

The unusual nitro-substituted polyketides aureothin, neoaureothin (spectinabilin), and luteoreticulin, which are produced by diverse Streptomyces species, point to a joint evolution. Through rational genetic recombination and domain exchanges we have successfully reprogrammed the modular (type I) aur polyketide synthase (PKS) into a synthase that generates luteoreticulin. This is the first rational transformation of a modular PKS to produce a complex polyketide that was initially isolated from a different bacterium. A unique aspect of this synthetic biology approach is that we exclusively used genes from a single biosynthesis gene cluster to design the artificial pathway, an avenue that likely emulates natural evolutionary processes. Furthermore, an unexpected, context-dependent switch in the regiospecificity of a pyrone methyl transferase was observed. We also describe an unprecedented scenario where an AT domain iteratively loads an extender unit onto the cognate ACP and the downstream ACP. This aberrant function is a novel case of non-colinear behavior of PKS domains.