Biosynthesis-based artificial evolution of microbial natural products

Dissection of the Enzymatic Process for Forming a Central Imidazopiperidine Heterocycle in the Biosynthesis of a Series c Thiopeptide Antibiotic

Thiopeptide antibiotics are a family of ribosomally synthesized and posttranslationally modified peptide natural products of significant interest in anti-infective agent development. These antibiotics are classified into five subfamilies according to differences in the central 6-membered heterocycle of the thiopeptide framework. The mechanism through which imidazopiperidine, the most heavily functionalized central domain characteristic of a series c thiopeptide, is formed remains unclear. Based on mining and characterization of the genes specifically involved in the biosynthesis of Sch40832, we here report an enzymatic process for transforming a series b thiopeptide into a series c product through a series a intermediate. This process starts with F₄₂₀-dependent hydrogenation of the central dehydropiperidine unit to a saturated piperidine unit. With the activity of a cytochrome P450 monooxygenase, the piperidine-thiazole motif of the intermediate undergoes an unusual oxygenation-mediated rearrangement to provide an imidazopiperidine heterocycle subjected to further S-methylation and aldehyde reduction. This study represents the first biochemical reconstitution of the pathway forming a stable series c thiopeptide.

Oxidative Indole Dearomatization for Asymmetric Furoindoline Synthesis by a Flavin-Dependent Monooxygenase Involved in the Biosynthesis of Bicyclic Thiopeptide Thiostrepton

The interest in indole dearomatization, which serves as a useful tool in the total synthesis of related alkaloid natural products, has recently been renewed with the intention of developing new methods efficient in both yield and stereoselective control. Here, we report an enzymatic approach for the oxidative dearomatization of indoles in the asymmetric synthesis of a variety of furoindolines with a vicinal quaternary carbon stereogenic center. This approach depends on the activity of a flavin-dependent monooxygenase, TsrE, which is involved in the biosynthesis of bicyclic thiopeptide antibiotic thiostrepton. TsrE catalyzes 2,3-epoxidation and subsequent epoxide opening in a highly enantioselective manner during the conversion of 2-methyl-indole-3-acetic acid or 2-methyl-tryptophol to furoindoline, with up to >99 % conversion and >99 % ee under mild reaction conditions. Complementing current chemical methods for oxidative indole dearomatization, the TsrE activity-based approach enriches the toolbox in the asymmetric synthesis of products possessing a furoindoline skeleton.

Discovery of caerulomycin/collismycin-type 2,2'-bipyridine natural products in the genomic era

2,2'-Bipyridine (2,2'-BP) is the unique molecular scaffold of the bioactive natural products represented by caerulomycins (CAEs) and collismycins (COLs). CAEs and COLs are highly similar in the chemical structures in which their 2,2'-BP cores typically contain a di- or tri-substituted ring A and an unmodified ring B. Here, we summarize the CAE and COL-type 2,2'-BP natural products known or hypothesized to date: (1) isolated using methods traditional for natural product characterization, (2) created by engineering the biosynthetic pathways of CAEs or COLs, and (3) predicted upon bioinformatics-guided genome mining. The identification of these CAE and COL-type 2,2'-BP natural products not only demonstrates the development of research techniques and methods in the field of natural product chemistry but also reflects the general interest in the discovery of CAE and COL-type 2,2'-BP natural products.

Rapid Generation of Molecular Complexity by Chemical Synthesis: Highly Efficient Total Synthesis of Hexacyclic Alkaloid (-)-Chaetominine and Its Biosynthetic Implications

The efficiency becomes a key issue in today's natural product total synthesis. While biomimetic synthesis is one of the most elegant strategies to achieve synthetic efficiency and thus to approach the ideal synthesis, most biogenetic pathways are unknown or unconfirmed. In this account, we demonstrate, through the shortest and also the most efficient asymmetric total syntheses of the hexacyclic alkaloid (-)-chaetominine to date, that on the basis of biogenetic thinking, one can develop quite efficient bio-inspired total synthesis, which in turn serves to suggest and chemically validate plausible biosynthetic routes for the natural product. The synthetic strategy thus developed is also inspiring for the development of other synthetic methods and efficient total synthesis of other natural products.