Directed Evolution of a Ketone Synthase for Efficient and Highly Selective Functionalization of Internal Alkenes by Accessing Reactive Carbocation Intermediates

The direct regioselective oxidation of internal alkenes to ketones could simplify synthetic routes and solve a longstanding challenge in synthesis. This reaction is of particular importance because ketones are predominant moieties in valuable products as well as crucial intermediates in synthesis. Here we report the directed evolution of a ketone synthase that oxidizes internal alkenes directly to ketones with several thousand turnovers. The evolved ketone synthase benefits from more than a dozen crucial mutations, most of them distal to the active site. Computational analysis reveals that all these mutations collaborate to facilitate the formation of a highly reactive carbocation intermediate by generating a confined, rigid and preorganized active site through an enhanced dynamical network. The evolved ketone synthase fully exploits a catalytic cycle that has largely eluded small molecule catalysis and consequently enables various challenging functionalization reactions of internal alkenes. This includes the first catalytic, enantioselective oxidation of internal alkenes to ketones, as well as the formal asymmetric hydration and hydroamination of unactivated internal alkenes in combination with other biocatalysts.

Copper(ii)-catalyzed and acid-promoted highly regioselective oxidation of tautomerizable C(sp3)–H bonds adjacent to 3,4-dihydroisoquinolines using air (O2) as a clean oxidant

A novel Cu(ii)-catalyzed and acid-promoted highly regioselective oxidation of tautomerizable C(sp3)–H bonds adjacent to 1-Bn-DHIQs was developed. This method was successfully applied in the first total synthesis of canelillinoxine.

In situ formation of photoactive B-ring reduced chlorophyll isomer in photosynthetic protein LH2

Natural chlorophylls have a D-ring reduced chlorin π-system; however, no naturally occurring photosynthetically active B-ring reduced chlorins have been reported. Here we report a B-ring reduced chlorin, 17,18-didehydro-bacteriochlorophyll (BChl) a, produced by in situ oxidation of B800 bacteriochlorophyll (BChl) a in a light-harvesting protein LH2 from a purple photosynthetic bacterium Phaeospirillum molischianum. The regioselective oxidation of the B-ring of B800 BChl a is rationalized by its molecular orientation in the protein matrix. The formation of 17,18-didehydro-BChl a produced no change in the local structures and circular arrangement of the LH2 protein. The B-ring reduced 17,18-didehydro-BChl a functions as an energy donor in the LH2 protein. The photoactive B-ring reduced Chl isomer in LH2 will be helpful for understanding the photofunction and evolution of photosynthetic cyclic tetrapyrrole pigments.

Synthesis and Mechanistic Interrogation of Ginkgo biloba Chemical Space en route to (–)-Bilobalide

Here we interrogate the structurally dense (1.63 mcbits/ų) GABA_A receptor antagonist bilobalide, intermediates en route to its synthesis and related mechanistic questions. ¹³C isotope labeling identified an unexpected bromine migration en route to an α-selective, catalytic asymmetric Reformatsky reaction, ruling out an asymmetric allylation pathway. Experiment and computation converge on the driving forces behind two surprising observations. First, an oxetane acetal is shown to persist in concentrated mineral acid (1.5 M DCl in THF-d₈/D₂O), and its longevity is correlated to destabilizing steric clashes between substituents. Second, a regioselective oxidation of <i>des</i>-hydroxybilobalide is found to rely on lactone acidification through lone-pair delocalization, which leads to extremely rapid intermolecular enolate equilibration. In addition, we describe multiple pitfalls, puzzles and unexpected reactions that ultimately uncovered a concise total synthesis. These problems arose from the high information density of bilobalide that distinguishes it from other scaffolds and may characterize natural product (NP) space more generally. Therefore, we also include a Python script to quickly (<i>ca.</i> 132,000 molecules/ minute) calculate information content (Böttcher scores), which may be helpful to identify important features of NP space.

Synthesis and Mechanistic Interrogation of Ginkgo biloba Chemical Space en route to (–)-Bilobalide

Here we interrogate the structurally dense (1.63 mcbits/ų) GABA_A receptor antagonist bilobalide, intermediates en route to its synthesis and related mechanistic questions. ¹³C isotope labeling identified an unexpected bromine migration en route to an α-selective, catalytic asymmetric Reformatsky reaction, ruling out an asymmetric allylation pathway. Experiment and computation converge on the driving forces behind two surprising observations. First, an oxetane acetal is shown to persist in concentrated mineral acid (1.5 M DCl in THF-d₈/D₂O), and its longevity is correlated to destabilizing steric clashes between substituents. Second, a regioselective oxidation of <i>des</i>-hydroxybilobalide is found to rely on lactone acidification through lone-pair delocalization, which leads to extremely rapid intermolecular enolate equilibration. In addition, we describe multiple pitfalls, puzzles and unexpected reactions that ultimately uncovered a concise total synthesis. These problems arose from the high information density of bilobalide that distinguishes it from other scaffolds and may characterize natural product (NP) space more generally. Therefore, we also include a Python script to quickly (<i>ca.</i> 132,000 molecules/ minute) calculate information content (Böttcher scores), which may be helpful to identify important features of NP space.

Synthesis and Mechanistic Interrogation of Ginkgo biloba Chemical Space en route to (–)-Bilobalide

Here we interrogate the structurally dense (1.63 mcbits/ų) GABA_A receptor antagonist bilobalide, intermediates en route to its synthesis and related mechanistic questions. ¹³C isotope labeling identified an unexpected bromine migration en route to an α-selective, catalytic asymmetric Reformatsky reaction, ruling out an asymmetric allylation pathway. Experiment and computation converge on the driving forces behind two surprising observations. First, an oxetane acetal is shown to persist in concentrated mineral acid (1.5 M DCl in THF-d₈/D₂O), and its longevity is correlated to destabilizing steric clashes between substituents. Second, a regioselective oxidation of <i>des</i>-hydroxybilobalide is found to rely on lactone acidification through lone-pair delocalization, which leads to extremely rapid intermolecular enolate equilibration. In addition, we describe multiple pitfalls, puzzles and unexpected reactions that ultimately uncovered a concise total synthesis. These problems arose from the high information density of bilobalide that distinguishes it from other scaffolds and may characterize natural product (NP) space more generally. Therefore, we also include a Python script to quickly (<i>ca.</i> 132,000 molecules/ minute) calculate information content (Böttcher scores), which may be helpful to identify important features of NP space.

Selective Modification of Streptozotocin at the C3 Position to Improve Its Bioactivity as Antibiotic and Reduce Its Cytotoxicity towards Insulin-Producing β Cells

With the increasing resistance of bacteria to current antibiotics, novel compounds are urgently needed to treat bacterial infections. Streptozotocin (STZ) is a natural product that has broad-spectrum antibiotic activity, albeit with limited use because of its toxicity to pancreatic β cells. In an attempt to derivatize STZ through structural modification at the C3 position, we performed the synthesis of three novel STZ analogues by making use of our recently developed regioselective oxidation protocol. Keto-STZ (2) shows the highest inhibition of bacterial growth (minimum inhibitory concentration (MIC) and viability assays), but is also the most cytotoxic compound. Pre-sensitizing the bacteria with GlcNAc increased the antimicrobial effect, but did not result in complete killing. Interestingly, allo-STZ (3) revealed moderate concentration-dependent antimicrobial activity and no cytotoxicity towards β cells, and deoxy-STZ (4) showed no activity at all.

Chemistry of Hetera-buckybowl Trichalcogenasumanenes

Buckybowls attract significant attention in chemistry and materials science owing to their unique features related to both geometric and electronic aspects. Doping the π-skeleton of buckybowls with the main group elements results in hetera-buckybowls, and accordingly has a large influence on the chemical and physical properties. This account summarizes our research progress on hetera-buckybowl trichalcogenasumanenes (TCSs), including their synthesis, regioselective oxidations, transformation into various hetero polycycles (chiral π-systems, molecular spoons, etc.), and their application as optoelectronic materials.1 Introduction2 Synthesis of TCSs3 Structural and Electronic Features of TCSs4 Regioselective Oxidation of TCSs4.1 Cleavage of the Edged Benzene Rings4.2 Oxidation of the Thiophene Rings4.3 Transformation of Butoxy Groups into an ortho-Quinone4.4 Intermolecular Charge Transfer4.5 Influence of Chalcogen Atoms on Oxidation Reactions5 Synthesis of Hetero Polycycles from TCSs6 Optoelectronic Properties of TCSs and Their Derivatives7 Conclusion

Selective Modification of Streptozotocin at the C3 Position to Improve Its Bioactivity as Antibiotic and Reduce Its Cytotoxicity towards Insulin-Producing β Cells

With the increasing resistance of bacteria to current antibiotics, novel compounds are urgently needed to treat bacterial infections. Streptozotocin (STZ) is a natural product that has broad-spectrum antibiotic activity, albeit with limited use because of its toxicity to pancreatic &beta; cells. In an attempt to derivatize STZ through structural modification at the C3 position, we performed the synthesis of three novel STZ analogues by making use of our recently developed regioselective oxidation protocol. Keto-STZ (2) shows the highest inhibition of bacterial growth (MIC and viability assays), but is also the most cytotoxic compound. Pre-sensitizing the bacteria with GlcNAc increased the antimicrobial effect, but did not result in complete killing. Interestingly, allo-STZ (3) revealed moderate concentration-dependent antimicrobial activity and no cytotoxicity towards &beta; cells, and deoxy-STZ (4) showed no activity at all.