Synthesis of rare sugar isomers through site-selective epimerization

Enzyme cascades for high-yield conversion of d-xylose into d-ribose by overcoming equilibrium constraints and enhancing selectivity

d-Ribose is essential for critical cellular functions and the synthesis of antiviral nucleosides. However, traditional chemical synthesis and fermentation methods of d-ribose production suffer from low yields and inefficient resource utilization. Here, we present a highly efficient enzymatic cascade strategy that utilizes selective phosphorylation and dephosphorylation processes, coupled with ATP regeneration to convert d-xylose into d-ribose with high yield. By optimizing this enzyme cascade, we achieved a substantial increase in d-ribose yield from 23.4 % to 93.5 % mol/mol, effectively overcoming the equilibrium limitations of sugar conversion processes. Notably, our approach allows for the selective conversion of d-xylose to d-ribose in lignocellulosic hydrolysates, even in the presence of d-glucose. This work demonstrates the highly efficient enzymatic conversion of d-xylose into d-ribose offering a competitive alternative to existing chemical synthesis methods. Our findings provide a novel approach to cellulosic biomass valorization and represent a significant contribution to the field of biorefinery.

Regioselective O-arylation of 6-hydroxy groups in carbohydrates

We report an efficient method for regioselective O-arylation of the 6-hydroxy group of carbohydrates. The reaction involves a nucleophilic aromatic substitution reaction (SNAr) using electron-deficient arenes under mild conditions. Regioselectively monoarylated carbohydrate derivative was produced smoothly in the reaction by fine-tuning with the base. The pentafluoropyridine (P1) in combination with N, N-diisopropylethylamine (DIPEA) was successfully converted to regioselective O-arylated products of different positional hydroxy group free sugar derivatives with various functional groups.

Generation of Rare Sugars by Electrochemical Oxidation of d-Glucose Using Boron-Doped Diamond Electrode

The electrochemical oxidation of biomass for the production of value-added chemicals represents a promising approach in the field of sustainable chemistry. In this study, we investigated the electrochemical conversion of d-glucose, a biomass-derived compound, using boron-doped diamond (BDD) electrodes under constant applied current (10 mA) or potentials (1.5-3.0 V vs Ag/AgCl). The reaction products were analyzed using high-performance liquid chromatography (HPLC) and liquid chromatography/mass spectrometry (LC/MS) measurements, employing both p-aminobenzoic acid ethyl ester (ABEE) and l-tryptophan amide labeling methods to enable characterization. The results demonstrated that the BDD electrodes achieved 95.9% d-glucose degradation and successfully generated various rare sugars, including d-arabinose (0.126 mmol/L), d-erythrose (0.0544 mmol/L), d-glyceraldehyde, and l-glyceraldehyde (combined 0.148 mmol/L). Under identical conditions, Pt electrodes as a control showed only 10.2% d-glucose degradation with significantly lower rare sugar yields. The applied potential significantly influenced the product distribution, with optimal rare sugar production observed at 2.5 V vs Ag/AgCl, reflecting a balance between glucose oxidation and product degradation. Mechanistic studies suggest that the formation of rare sugars involves a series of oxidation and decarboxylation reactions, facilitated by electrochemically generated active species. The superior performance of the BDD electrodes is attributed to their wide potential window, efficient generation of oxidizing species, and unique surface characteristics. This research provides new insights into the electrochemical transformation of biomass-derived compounds and demonstrates the potential for sustainable production of high-value rare sugars, opening avenues for applications in food science, pharmaceuticals, and green chemistry.

Site-Selective C─H Bond Functionalization of Sugars

Non-typical C-functionalized sugars represent a prominent yet hardly accessible class of biologically-active compounds. The available synthetic methodologies toward such sugar derivatives suffer either from an extensive use of protecting groups, requiring long and laborious synthetic manipulations, or from limited predictability and noncontrollable site-selectivity of the employed C-functionalization reactions. In this work, we disclose an alternative synthetic methodology toward nontypical sugars that allows facile, site-selective, and stereocontrolled C-functionalization of sugars through a traceless tethering approach. The described silyl-based redox-active tethering group appends directly to the unprotected sugar substrate and mediates the C-functionalization reaction through a photochemically-promoted 1,6-hydrogen atom transfer (HAT) mechanism, while transforming into a readily-removable silyl protecting group. The protocol is compatible with a variety of unprotected carbohydrate substrates featuring sensitive aglycons and a diverse set of coupling partners, providing a straightforward and scalable route to pharmaceutically relevant C-functionalized carbohydrate conjugates.

Dynamic Kinetic Asymmetric Hydroacylation: Racemization by Soft Enolization

We report a dynamic kinetic asymmetric transformation (DyKAT) of racemic aldehydes by Rh-catalyzed hydroacylation of acrylamides. This intermolecular hydroacylation generates 1,4-ketoamides with high enantio- and diastereoselectivity. DFT and experimental studies provide mechanistic insights and reveal an unexpected Rh-catalyzed pathway for aldehyde racemization. Our study represents a pioneering kinetic resolution by intermolecular hydroacylation and contributes to the growing field of stereoconvergent catalysis featuring C-C bond construction.

Photocatalyzed Epimerization of Quaternary Stereocenters

Quaternary stereocenters play a crucial role in shaping both the molecular topology of small molecules and the outcome of stereoselective transformations. While considerable progress has been achieved in constructing highly substituted carbon centers with varied substitution patterns, the stereoselective synthesis of quaternary carbon centers remains a significant challenge. Here we report a protocol for the precise manipulation of quaternary stereocenters through epimerization. The critical design element of our ketone α-epimerization process was developing a photoactive imine, which circumvents the numerous deleterious pathways of carbonyl photochemistry. Excitation of this imine with visible light in the presence of a photocatalyst enables reversible C-C bond cleavage and reformation to vary the stereochemistry of the quaternary center. This approach allows us to override intrinsic stereochemical outcomes of C-C bond construction, therefore providing novel tactics for retrosynthetic planning. The broad utility of this protocol was demonstrated by the topological alteration of various classes of carbocyclic scaffolds bearing diverse functional groups.

Direct α-C-H alkylation of alcohols via photoinduced hydrogen atom transfer

A general approach for the α-C-H alkylation of alcohols has been established, offering access to highly functionalized alcohols via visible light irradiation with TBAP as a co-catalyst. The methodology exhibits excellent functional group tolerance and a wide substrate scope. Its practicality is underscored by successful gram-scale synthesis and a sunlight-driven reaction demonstration.

Stereochemical Editing at sp3-Hybridized Carbon Centers by Reversible, Photochemically Triggered Hydrogen Atom Transfer

ConspectusMillions of chiral compounds contain a stereogenic sp3-hybridized carbon center with a hydrogen atom as one of the four different substituents. The stereogenic center can be edited in an increasing number of cases by selective hydrogen atom transfer (HAT) to and from a photocatalyst. This Account describes the development of photochemical deracemization reactions using chiral oxazole-annulated benzophenones with a bonding motif that allows them to recognize chiral lactam substrates by two-point hydrogen bonding. The backbone of the catalysts consists of a chiral azabicyclo[3.3.1]nonan-2-one with a U-shaped geometry, which enables substrate recognition to occur parallel to the benzoxazole part of the aromatic ketones. The photocatalysts facilitate a catalytic photochemical deracemization of several compound classes including hydantoins, N-carboxyanhydrides, oxindoles, 2,5-diketopiperazines, and 4,7-diaza-1-isoindolinones. In addition, if more than one stereogenic center is present, the editing delivers a distinct diastereoisomer upon the appropriate selection of the respective photocatalyst enantiomer. The chiral photocatalysts operate via the benzophenone triplet that selectively abstracts a properly positioned hydrogen atom in exclusively one of the two substrate enantiomers. The photochemical step creates a planar carbon-centered radical and erases the absolute configuration at this position. While returning HAT to the same position would likely recreate the stereogenic center with the same absolute configuration, spectroscopic and quantum chemical studies suggest that the hydrogen atom is delivered from the photocatalyst to a heteroatom that is in conjugation to the radical center. Two scenarios can be distinguished for the hydrogen atom shuttling process. For hydantoins, N-carboxyanhydrides, and 4,7-diaza-1-isoindolinones, the back HAT occurs to a carbonyl oxygen atom or an imine-type nitrogen atom which is not involved in binding to the catalyst. For oxindoles and 2,5-diketopiperazines, a single lactam carbonyl group in the substrate is available to accept the hydrogen atom. It is currently assumed that back HAT occurs to this group, although the carbonyl oxygen atom is involved in hydrogen bonding to the catalyst. In comparison to the former reaction pathway, the latter process appears to be less efficient and more prone to side reactions. For both cases, an achiral enol or enamine is formed, which delivers upon dissociation from the catalyst statistically either one of the two stereoisomers of the substrate. Since only one substrate enantiomer (or diastereoisomer) is processed, a high enantioselectivity (or diastereoselectivity) results. Even though the editing is a contra-thermodynamic process, the described decoupling of a photochemical and a thermal step allows the usage of a single catalyst in loadings that vary between 2.5 and 10 mol % depending on the specific mode of action.

Recent Advances in Organic Photocatalyst-Promoted Carbohydrate Synthesis and Modification under Light Irradiation

Photoredox catalysis has been developed as a sustainable and eco-friendly catalytic strategy, which might provide innovative solutions to solve the current synthetic challenges and barriers in carbohydrate chemistry. During the last few decades, the study of organic photocatalyst-promoted carbohydrate synthesis and modification has received significant attention, which provides an excellent and inexpensive metal-free alternative to photoredox catalysis as well as introduces a new fastest-growing era to access complex carbohydrates simply. In this review, we aim to provide an overview of organic photocatalyst-promoted carbohydrate synthesis and modification under light irradiation, which is expected to provide new directions for further investigation.

Enhanced Synthesis of Rare d-Allose from d-Glucose by Positively Pulling and Forcing Reversible Epimerization in Engineered Escherichia coli

d-Allose has great potential for application in the food and pharmaceutical industries owing to its remarkable physiological properties. Most studies on d-allose production have primarily focused on enzyme catalysis using the Izumoring strategy, which typically requires the use of expensive d-allulose as a substrate. Herein, a metabolically engineered strain of Escherichia coli was developed to synthesize d-allose directly from inexpensive d-glucose. The synthesis pathway was systematically optimized through a modular metabolic engineering. The functionality of the isomerases involved in the conversion of d-allulose to d-allose was confirmed in vivo, while the byproduct and transporter pathways were blocked to positively pull the reversible epimerization. Gene knockouts were employed to weaken glycolytic pathways, redirecting the carbon flux toward product synthesis. Additionally, the nonphosphorylated transport of d-glucose was introduced to enhance substrate utilization. In fed-batch fermentation, the engineered strain achieved a d-allose titer of 4.17 g/L, with a yield of 0.103 g/g from d-glucose. Our achievements are expected to advance the industrial production of d-allose, and this strategy is also applicable for producing other rare sugars.

Photocatalyzed Dehydration of 1-Aryl-1,2-Ethanediols to Methyl Ketones Driven by Eosin Y Fragmentation Products

Herein, we report a mild photocatalytic redox-neutral dehydration of aryl-1,2-ethanediols forming the respective methyl ketones. In the proposed mechanistic cycle an initial hydrogen atom abstraction (HAT) is followed by a 1,2-spin center shift (SCS) as key steps. Interestingly, Eosin Y was found to act as a pre-catalyst dissociating into a catalytically active mixture under irradiation. To the best of our knowledge, this exemplifies the first synthetic utilization of Eosin Y degradation products. As a result, our reaction can be realized with a single organic photocatalyst and releases water as a sole by-product.

Ni-Catalyzed Stereodivergent Synthesis of N-Glycosides

Herein, we describe a stereoselective Ni-catalyzed N-glycosylation of glycals. The reaction is enabled by addition of an in situ generated nickel hydride across an olefin prior to C-N bond-formation. Stereodivergence can be accomplished on kinetic or thermodynamic grounds, thus giving access to either α- or β-N-glycosides with equal ease. The protocol is distinguished by its operational simplicity, generality and exquisite selectivity, thus offering a new gateway to expedite the synthesis of N-glycosides.

A dendritic cell-recruiting, antimicrobial blood clot hydrogel for melanoma recurrence prevention and infected wound management

Surgical resection, the mainstay for melanoma treatment, faces challenges due to high tumor recurrence rates and complex postoperative wound healing. Chronic inflammation from residual disease and the risk of secondary infections impede healing. We introduce an innovative, injectable hydrogel system that integrates a multifaceted therapeutic approach. The hydrogel, crosslinked by calcium ions with sodium alginate, encapsulates a blood clot rich in dendritic cells (DCs) chemoattractants and melanoma cell-derived nanovesicles (NVs), functioning as a potent immunostimulant. This in situ recruitment strategy overcomes the limitations of subcutaneous tumor vaccine injections and more effectively achieves antitumor immunity. Additionally, the hydrogel incorporates Chlorella extracts, enhancing its antimicrobial properties to prevent wound infections and promote healing. One of the key findings of our research is the dual functionality of Chlorella extracts; they not only expedite the healing process of infected wounds but also increase the hydrogel's ability to stimulate an antitumor immune response. Given the patient-specific nature of the blood clot and NVs, our hydrogel system offers customizable solutions for individual postoperative requirements. This personalized approach is highlighted by our study, which demonstrates the synergistic impact of the composite hydrogel on preventing melanoma recurrence and hastening wound healing, potentially transforming postsurgical melanoma management.

Synergistic Boronic Acid and Photoredox Catalysis: Synthesis of C-Branched Saccharides via Selective Alkylation of Unprotected Saccharides

Here we present a regio- and stereoselective alkylation approach for unprotected saccharides using synergistic boronic acid and photoredox catalysis. Targeting the equatorial C-H bond of the cis-1,2-diol motif, this method employs MeB(OH)2 as a catalyst. Mechanistic investigations indicate that the formation of a tetracoordinate boron species, resulting from the interaction between the cyclic boronic diol ester and a free hydroxyl group in the saccharide, is critical to this transformation. Notably, this method enables efficient late-stage modification of complex carbohydrates, such as raffinose and the drug digoxin, expanding opportunities for carbohydrate functionalization.

Establishing Thermodynamic Graphs of Nitrogenous Radical Cations Abstracting Hydrogen Atoms and Their Applications in Photoredox Reactions

Nitrogenous compounds have been extensively utilized as hydrogen atom transfer (HAT) catalysts in photoredox reactions, with nitrogenous radical cations being the actual hydrogen atom abstractors. Building upon our previous work, 120 thermodynamic graphs of nitrogenous radical cations abstracting hydrogen atoms, which encompass seven vital thermodynamic parameters, are designed and established to elucidate their redox characteristics. Furthermore, the applications of thermodynamic graphs to select appropriate photocatalysts, assess the feasibility of the HAT process, and diagnose the possible activation mechanism were discussed, which would enable the utilities of nitrogenous compounds as HAT catalysts or nitrogenous radical cations as hydrogen atom abstractors in photoredox reactions.

Photochemical Deracemization of 4,7-Diaza-1-isoindolinones by Unidirectional Hydrogen Atom Shuttling

By coupling a photochemical and a thermal step, a single chiral catalyst can establish a photostationary state in which the enantiopure form of a chiral compound is favored over its racemate. Following this strategy, 3-substituted 4,7-diaza-1-isoindolones were successfully deracemized (74-98% yield, 86-99% ee) employing 2.5 mol % of a photocatalyst. Key to the success of the reaction is the fact that a chiral benzophenone recruits selectively one enantiomer of the substrate for a photoinduced hydrogen atom transfer. A combination of computational and experimental studies suggests that the hydrogen atom is shuttled via the oxygen atom of the catalyst to the 4-nitrogen atom of the substrate.

Photochemical Deracemization of Lactams with Deuteration Enabled by Dual Hydrogen Atom Transfer

Photochemical deracemization has emerged as one of the most straightforward approaches to access highly enantioenriched compounds in recent years. While excited-state events such as energy transfer, single electron transfer, and ligand-to-metal charge transfer have been leveraged to promote stereoablation, approaches relying on hydrogen atom transfer, which circumvent the limitations imposed by the triplet energy and redox potential of racemic substrates, remain underexplored. Conceptually, the most attractive method for tertiary stereocenter deracemization might be hydrogen atom abstraction followed by hydrogen atom donation. However, implementing such a strategy poses significant challenges, primarily because the enantioenriched products are also reactive if the chiral catalyst is unable to differentiate between the two enantiomers. Herein we report a distinct dual hydrogen atom transfer strategy for photochemical deracemization of δ- and γ-lactams, achieving high enantioenrichment and deuterium incorporation despite the inherent reactivity of the products. Mechanistic studies reveal that benzophenone enables nonselective hydrogen atom abstraction while a tetrapeptide-derived thiol dictates the enantioselectivity of the hydrogen atom donation step. More importantly, a pyridine-based alcohol was found to play crucial roles in facilitating the hydrogen atom abstraction as well as enhancing the enantioselectivity of the hydrogen atom donation step.

Organometallic Photocatalyst-Promoted Synthesis and Modification of Carbohydrates under Photoirradiation

Carbohydrates are natural, renewable, chemical compounds that play crucial roles in biological systems. Thus, efficient and stereoselective glycosylation is an urgent task for the preparation of pure and structurally well-defined carbohydrates. Photoredox catalysis has emerged as a powerful tool in carbohydrate chemistry, providing an alternative for addressing some of the challenges of glycochemistry. Over the last few decades, Ir- and Ru-based organometallic photocatalysts have attracted significant interest because of their high stability, high-energy triplet state, strong visible-light absorption, long luminescence lifetime, and amenability to ligand modification. This review highlights the recent progress in the organometallic photocatalyst-promoted synthesis and modification of carbohydrates under photoirradiation, as well as the related benefits and drawbacks.

Evaluation of cross-linkers in the design of immobilized multi isomerase cascade for the preparation of rare sugars

The cascade of sugar isomerases is one of the most practical methods for producing rare sugars, and enzyme immobilization endows it with high economic efficiency, operational convenience and reusability. However, the most employed cross-linker glutaraldehyde (GA) has the disadvantages of enzyme deactivation and limitation of substrate binding. Herein, three compounds, glyoxal, GA, and 2,5-furandicarboxaldehyde (DFF) were evaluated within a previously developed cascade comprising ribose-5-phosphate isomerase and D-tagatose-3-epimerase to prepare D-ribulose form D-xylose. Analyses of surface morphology, element and chemical bond revealed that all compounds effectively cross-linked the isomerases. High concentration of the cross-linkers was generally beneficial for binding protein and preventing enzyme leak during reusing cycles. Glyoxal performed the highest immobilization rate, though it hadn't been employed as a cross-linker for enzyme immobilization. DFF mediated cross-linking revealed the highest activity recovery, substrate conversion and residual activity after reusing cycles, suggesting better biocompatibility than glyoxal and GA. After 8 rounds of recycling, the residual activity of enzyme immobilized by DFF was 61.4 %, ∼30 % higher than that of GA. This study proved a potential alternative cross-linker DFF for the immobilization of enzyme cascade with high activity recovery and reusability, which could promote the efficient production of high value-added products from biomass monosaccharides.

The Golden Age of Thermally Activated Delayed Fluorescence Materials: Design and Exploitation

Since the seminal report by Adachi and co-workers in 2012, there has been a veritable explosion of interest in the design of thermally activated delayed fluorescence (TADF) compounds, particularly as emitters for organic light-emitting diodes (OLEDs). With rapid advancements and innovation in materials design, the efficiencies of TADF OLEDs for each of the primary color points as well as for white devices now rival those of state-of-the-art phosphorescent emitters. Beyond electroluminescent devices, TADF compounds have also found increasing utility and applications in numerous related fields, from photocatalysis, to sensing, to imaging and beyond. Following from our previous review in 2017 ( Adv. Mater. 2017, 1605444), we here comprehensively document subsequent advances made in TADF materials design and their uses from 2017-2022. Correlations highlighted between structure and properties as well as detailed comparisons and analyses should assist future TADF materials development. The necessarily broadened breadth and scope of this review attests to the bustling activity in this field. We note that the rapidly expanding and accelerating research activity in TADF material development is indicative of a field that has reached adolescence, with an exciting maturity still yet to come.

Regiodivergent Functionalization of Protected and Unprotected Carbohydrates using Photoactive 4-Tetrafluoropyridinylthio Fragment as an Adaptive Activating Group

The selective functionalization of carbohydrates holds a central position in synthetic carbohydrate chemistry, driving the ongoing quest for ideal approaches to manipulate these compounds. In this study, we introduce a general strategy that enables the regiodivergent functionalization of saccharides. The use of electron-deficient photoactive 4-tetrafluoropyridinylthio (SPyf) fragment as an adaptable activating group, facilitated efficient functionalization across all saccharide sites. More importantly, this activating group can be directly installed at the C1, C5 and C6 positions of biomass-derived carbohydrates in a single step and in a site-selective manner, allowing for the efficient and precision-oriented modification of unprotected saccharides and glycans.

Catalytic Enantioselective Hydrogen Atom Abstraction Enables the Asymmetric Oxidation of Meso Diols

Desymmetrization of meso diols is an important strategy for the synthesis of chiral oxygen-containing building blocks. Oxidative desymmetrization is an important subclass, but existing methods are often constrained by the need for activated alcohol substrates. We disclose a conceptually distinct strategy toward oxidative diol desymmetrization that is enabled by catalytic enantioselective hydrogen atom abstraction. Following single electron oxidation of a cinchona alkaloid-derived catalyst, enantiodetermining hydrogen atom abstraction generates a desymmetrized ketyl radical intermediate which reacts with either DIAD or O2 before in situ elimination to form valuable hydroxyketone products. A range of cyclic and acyclic meso diols are competent, defining the absolute configuration of up to four stereocenters in a single operation. As well as providing rapid access to complex hydroxyketones, this work emphasizes the broad synthetic potential of harnessing hydrogen atom abstraction in an enantioselective manner.

Stretchable Unsymmetrical Piezoelectric BiO2-x Deposited-Hydrogel as Multimodal Triboelectric Nanogenerators for Biomechanical Motion Harvesting

Enhancing the output performance of triboelectric nanogenerators (TENGs) is essential for increasing their application in smart devices. Oxygen-vacancy-rich BiO2-x nanosheets (BiO2-x NSs) are advanced-engineered nanomaterials with excellent piezoelectric properties. Herein, a stretchable unsymmetrical BiO2-x NSs deposited-hydrogel made of polyacrylamide (PAM) as a multimodal TENG is rationally fabricated, and the performance of TENG can be tailored by controlling the BiO2-x NSs deposition amount and spatial distribution. The alteration of resistance caused by the Poisson effect of PAM/BiO2-x composite hydrogel (H-BiO2-x) can be used as a piezoresistive sensor, and the piezoelectricity of BiO2-x NSs can effectively enhance the density of transfer charge, thus improving the output performance of the H-BiO2-x-based TENG. In addition, the chemical cross-linking between the BiO2-x NSs and the PAM polymer chain allows the hydrogel electrode to have a higher tensile capacity (867%). Used for biomechanical motion signal detection, the sensors made of H-BiO2-x have high sensitivity (gauge factor = 6.93) and can discriminate a range of forces (0.1-5.0 N) at low frequencies (0.5-2.0 Hz). Finally, the prepared TENG can collect biological energy and convert it into electricity. Consequently, the improved TENG shows a good application prospect as multimodal biomechanical sensors by combining piezoresistive, piezoelectric, and triboelectric effects.

Enantioconvergent and Site-Selective Etherification of Carbohydrate Polyols through Chiral Copper Radical Catalysis

Going beyond currently reported two electron transformations that formed the core backdrop of asymmetric catalytic site-selective carbohydrate polyol functionalizations, we herein report a seminal demonstration of an enantioconvergent copper catalyzed site-selective etherification of minimally protected saccharides through a single-electron radical pathway. Further, this strategy paves a rare strategy, through which a carboxamide scaffold that is present in some glycomimetics of pharmacological relevance, can be selectively introduced. In light of the burgeoning interest in chiral radical catalysis, and the virtual absence of such stereocontrol broadly in carbohydrate synthesis, our strategy showcased the unknown capability of chiral radical copper catalysis as a contemporary tool to address the formidable site-selectivity challenge on a remarkable palette of naturally occurring saccharides. When reducing sugars were employed, a further dynamic kinetic resolution type glycosylation can be activated by the catalytic system to selectively generate the challenging β-O-glycosides.

Dual-Catalytic Structural Isomerisation as a Route to α-Arylated Ketones

Isomerisation reactions provide streamlined routes to organic compounds which are otherwise hard to directly synthesise. The most common forms are positional, geometrical or stereochemical isomerisations which involve the relocation of a double bond or a change in relative location of groups in space. In contrast, far fewer examples of structural (or constitutional) isomerisation exist where the connectivity between atoms is altered. The development of platforms capable of such rearrangement poses a unique set of challenges because chemical bonds must be selectively cleaved, and new ones formed without overall addition or removal of atoms. Here, we show that a dual catalytic system can enable the structural isomerisation of readily available allylic alcohols into more challenging-to-synthesise α-arylated ketones via a H-atom transfer initiated semi-pinacol rearrangement. Key to our strategy is the combination of a cobalt catalyst and photocatalyst under reductive, protic conditions which allows intermediates to propagate catalytic turnover. By providing an unusual disconnection to structural motifs which are difficult to access through direct arylation, we anticipate inspiring other advanced catalytic isomerisation strategies that will further retrosynthetic logic for complex molecule synthesis.

A chiral hydrogen atom abstraction catalyst for the enantioselective epimerization of meso-diols

Hydrogen atom abstraction is an important elementary chemical process but is very difficult to carry out enantioselectively. We have developed catalysts, readily derived from the Cinchona alkaloid family of natural products, which can achieve this by virtue of their chiral amine structure. The catalyst, following single-electron oxidation, desymmetrizes meso-diols by selectively abstracting a hydrogen atom from one carbon center, which then regains a hydrogen atom by abstraction from a thiol. This results in an enantioselective epimerization process, forming the chiral diastereomer with high enantiomeric excess. Cyclic and acyclic 1,2-diols are compatible, as are acyclic 1,3-diols. Additionally, we demonstrate the viability of combining our approach with carbon-carbon bond formation in Giese addition. Given the increasing number of synthetic methods involving hydrogen atom transfer steps, we anticipate that this work will have a broad impact in the field of enantioselective radical chemistry.

Radical Polarity

The polarity of a radical intermediate profoundly impacts its reactivity and selectivity. To quantify this influence and predict its effects, the electrophilicity/nucleophilicity of >500 radicals has been calculated. This database of open-shell species entails frequently encountered synthetic intermediates, including radicals centered at sp3, sp2, and sp hybridized carbon atoms or various heteroatoms (O, N, S, P, B, Si, X). Importantly, these computationally determined polarities have been experimentally validated for electronically diverse sets of >50 C-centered radicals, as well as N- and O- centered radicals. High correlations are measured between calculated polarity and quantified reactivity, as well as within parallel sets of competition experiments (across different radical types and reaction classes). These multipronged analyses show a strong relationship between the computed electrophilicity, ω, of a radical and its relative reactivity (krel vs Δω slopes up to 40; showing mere Δω of 0.1 eV affords up to 4-fold rate enhancement). We expect this experimentally validated database will enable reactivity and selectivity prediction (by harnessing polarity-matched rate enhancement) and assist with troubleshooting in synthetic reaction development.

Optimizing Droplet-Based Electricity Generator via a Low Sticky Hydrophobic Droplet-Impacted Surface

Droplet-based electricity generators (DEGs) are increasingly recognized for their potential in converting renewable energy sources. This study explores the interplay of surface hydrophobicity and stickiness in improving DEG efficiency. It find that the high-performance C-WaxDEGs leverage both these properties. Specifically, DEGs incorporating polydimethylsiloxane (PDMS) with carnauba wax (C-wax) exhibit increased output as surface stickiness decreases. Through experimental comparisons, PDMS with 1wt.% C-wax demonstrated a significant power output increase from 0.07 to 1.2 W m- 2, which attribute to the minimized adhesion between water molecules and the polymer surface, achieved by embedding C-wax into PDMS surface to form microstructures. This improvement in DEG performance is notable even among samples with similar surface potentials and contact angles, suggesting that C-wax's primary contribution is in reducing surface stickiness rather than altering other surface properties. The further investigations into the C-WaxDEG variant with 1wt.% C-wax PDMS uncover its potential as a sensor for water quality parameters such as temperature, pH, and heavy metal ion concentration. These findings open avenues for the integration of C-WaxDEGs into flexible electronic devices aimed at environmental monitoring.

Ready chemistry with a rare sugar: Altrobioside synthesis and analysis of conformational characteristics

We describe the synthesis of the full set of the so far unknown methyl altrobiosides and the initial analysis of the conformational dynamic which occurs in some of the synthesized compounds. d-Altrose chemistry has largely been neglected as it is a rare sugar and has first to be synthesized from glucose or mannose, respectively. Nevertheless, d-altrose is particularly interesting as the energy barrier between the complementary chair conformations is rather low and therefore dynamic mixtures of conformers might occur. We describe the ready synthesis of the selectively protected altrosyl acceptors for the glycosidation from d-mannose and the altrosyl-trichloroacetimidate as useful glycosyl donor to achieve the (1 → 2), (1 → 3), (1 → 4), and (1 → 6)-α-linked altrobiosides. The diastereomeric α- and β-O-(d-altropyranosyl)-trichloroacetimidates adopt different ring conformations as analyzed by NMR and VCD spectroscopy. Also, the pyranose ring conformations of the obtained altrobiosides apparently differ from a regular 4C1 chair according to NMR analysis and are influenced by the regiochemistry of the interglycosidic linkage.

Efficient Ionovoltaic Energy Harvesting via Water-Induced p-n Junction in Reduced Graphene Oxide

Water motion-induced energy harvesting has emerged as a prominent means of facilitating renewable electricity from the interaction between nanostructured materials and water over the past decade. Despite the growing interest, comprehension of the intricate solid-liquid interfacial phenomena related to solid state physics remains elusive and serves as a hindrance to enhancing energy harvesting efficiency up to the practical level. Herein, the study introduces the energy harvester by utilizing inversion on the majority charge carrier in graphene materials upon interaction with water molecules. Specifically, various metal electrode configurations are employed on reduced graphene oxide (rGO) to unravel its distinctive charge carriers that experience the inversion in semiconductor type upon water contact, and exploit this characteristic to leverage the efficacy of generated electricity. Through the strategic arrangement of the metal electrodes on rGO membrane, the open-circuit voltage (Voc) and short-circuit current (Isc) have exhibited a remarkable augmentation, reaching 1.05 V and 31.6 µA, respectively. The demonstration of effectively tailoring carrier dynamics via electrode configuration expands the practicality by achieving high power density and elucidating how the water-induced carrier density modulation occurs in 2D nanomaterials.

Synchronous Switching Strategy to Enhance the Real-Time Powering and Charging Performance of Triboelectric Nanogenerator

Triboelectric nanogenerators (TENGs) are of great significance as sustainable power sources that harvest energy from the human body and environment. Nevertheless, due to TENG's impedance-dependent output voltage characteristics, in traditional strategy (TS), real-timely powering a sensor with TENG has a poor sensing on/off ratio (or response), and directly charging a capacitor with TENG shows a low charging efficiency. This degraded real-time powering and charging performance of TENG compared to a commercial constant voltage source is a huge challenge of the TENG field for a long time. This work proposes a synchronous switching strategy (SSS) for TENG to real-timely power sensors or charge capacitors without degrading its performance. Compared with TS, this new strategy enables sensors to have 5-7 times sensing on/off ratio enhancement when using TENG as a power source, reaching the powering ability of a commercial constant voltage source, it makes the powering performance of TENG stable under different driving frequency, improving the powering robustness of TENG. In addition, compared with TS, SSS can also enhance the charging efficiency of TENG in every charging cycle by up to 2.4 times when charging capacitors. This work contributes to real-timely powering or charging the distributed, mobile and wireless electronics using TENG.

Switchable Enantio- and Diastereoselective Michael Additions of β-Keto Amides to Nitroolefins: Crystallization-Based Inversion of Kinetic Stereocontrol

Asymmetric catalytic reactions rely on chiral catalysts that induce highly ordered transition states capable of imparting stereoselectivity in the bond-forming step(s). Productive deviations from this paradigm are rare yet hold the potential for accessing different stereoisomers using the same catalyst. Here, we present an enantio- and diastereoselective Michael addition of β-keto amides to nitroolefin electrophiles proceeding via an unusual scenario where the kinetic diastereocontrol imparted by the catalyst may be overridden by crystallization to provide the complementary stereoisomer of the product.

Catalytic glycosylation for minimally protected donors and acceptors

Oligosaccharides have myriad functions throughout biological processes1,2. Chemical synthesis of these structurally complex molecules facilitates investigation of their functions. With a dense concentration of stereocentres and hydroxyl groups, oligosaccharide assembly through O-glycosylation requires simultaneous control of site, stereo- and chemoselectivities3,4. Chemists have traditionally relied on protecting group manipulations for this purpose5-8, adding considerable synthetic work. Here we report a glycosylation platform that enables selective coupling between unprotected or minimally protected donor and acceptor sugars, producing 1,2-cis-O-glycosides in a catalyst-controlled, site-selective manner. Radical-based activation9 of allyl glycosyl sulfones forms glycosyl bromides. A designed aminoboronic acid catalyst brings this reactive intermediate close to an acceptor through a network of non-covalent hydrogen bonding and reversible covalent B-O bonding interactions, allowing precise glycosyl transfer. The site of glycosylation can be switched with different aminoboronic acid catalysts by affecting their interaction modes with substrates. The method accommodates a wide range of sugar types, amenable to the preparation of naturally occurring sugar chains and pentasaccharides containing 11 free hydroxyls. Experimental and computational studies provide insights into the origin of selectivity outcomes.

The sugar cube: Network control and emergence in stereoediting reactions

Stereochemical editing strategies have recently enabled the transformation of readily accessible substrates into rare and valuable products. Typically, site selectivity is achieved by minimizing kinetic complexity by using protecting groups to suppress reactivity at undesired sites (substrate control) or by using catalysts with tailored shapes to drive reactivity at the desired site (catalyst control). We propose "network control," a contrasting paradigm that exploits hidden interactions between rate constants to greatly amplify modest intrinsic biases and enable precise multisite editing. When network control is applied to the photochemical isomerization of hexoses, six of the eight possible diastereomers can be selectively obtained. The amplification effect can be viewed as a mesoscale phenomenon between the limiting regimes of kinetic control in simple chemical systems and metabolic regulation in complex biological systems.

Synthesis of alcohols: streamlined C1 to Cn hydroxyalkylation through photoredox catalysis

Naturally occurring and readily available α-hydroxy carboxylic acids (AHAs) are utilized as platforms for visible light-mediated oxidative CO2-extrusion furnishing α-hydroxy radicals proved to be versatile C1 to Cn hydroxyalkylating agents. The direct decarboxylative Giese reaction (DDGR) is operationally simple, not requiring activator or sacrificial oxidants, and enables the synthesis of a diverse range of hydroxylated products, introducing connectivity typically precluded from conventional polar domains. Notably, the methodology has been extended to widely used glycolic acid resulting in a highly efficient and unprecedented C1 hydroxyhomologation tactic. The use of flow technology further facilitates scalability and adds green credentials to this synthetic methodology.

Organic Synthesis Away from Equilibrium: Contrathermodynamic Transformations Enabled by Excited-State Electron Transfer

ConspectusChemists have long been inspired by biological photosynthesis, wherein a series of excited-state electron transfer (ET) events facilitate the conversion of low energy starting materials such as H2O and CO2 into higher energy products in the form of carbohydrates and O2. While this model for utilizing light-driven charge transfer to drive catalytic reactions thermodynamically "uphill" has been extensively adapted for small molecule activation, molecular machines, photoswitches, and solar fuel chemistry, its application in organic synthesis has been less systematically developed. However, the potential benefits of these approaches are significant, both in enabling transformations that cannot be readily achieved using conventional thermal chemistry and in accessing distinct selectivity regimes that are uniquely enabled by excited-state mechanisms. In this Account, we present work from our group that highlights the ability of visible light photoredox catalysis to drive useful organic transformations away from their equilibrium positions, addressing a number of long-standing synthetic challenges.We first discuss how excited-state ET enabled the first general methods for the catalytic anti-Markovnikov hydroamination of unactivated alkenes with alkyl amines. In these reactions, an excited-state iridium(III) photocatalyst reversibly oxidizes secondary amine substrates to their corresponding aminium radical cations (ARCs). These electrophilic N-centered radicals can then react with olefins to furnish valuable tertiary amine products with complete anti-Markovnikov regioselectivity. Notably, some of these products are less thermodynamically stable than their corresponding amine and alkene starting materials. We next present a strategy for light-driven C-C bond cleavage within various aliphatic alcohols mediated by homolytic activation of alcohol O-H bonds by excited-state proton-coupled electron transfer (PCET). The resulting alkoxy radical intermediates then undergo C-C β-scission to ultimately provide isomeric linear carbonyl products that are often higher in energy than their cyclic alcohol precursors. Applications of this chemistry for the light-driven depolymerization of lignin biomass, commercial phenoxy resin, hydroxylated polyolefin derivatives, and thermoset polymers are presented as well. We then describe a method for the contrathermodynamic positional isomerization of highly substituted olefins by means of cooperative photoredox and chromium(II) catalysis. In this work, generation of an allylchromium(III) species that can undergo highly regioselective in situ protodemetalation enables access to a less substituted and thermodynamically less stable positional isomer. Product selectivity in this reaction is determined by the large differential in oxidation potentials between differently substituted olefin isomers. Lastly, we discuss a light-driven deracemization reaction developed in collaboration with the Miller group, wherein a racemic urea substrate undergoes spontaneous optical enrichment upon visible light irradiation in the presence of an iridium(III) chromophore, a chiral Brønsted base, and a chiral peptide thiol. Excellent levels of enantioselectivity are achieved via sequential and synergistic proton transfer (PT) and H atom transfer (HAT) steps. Taken together, these examples highlight the ability of excited-state ET events to enable access to nonequilibrium product distributions across a wide range of catalytic, redox-neutral transformations in which photons are the only stoichiometric reagents.

Scalable and Ultra-Sensitive Nanofibers Coaxial Yarn-Woven Triboelectric Nanogenerator Textile Sensors for Real-Time Gait Analysis

Yarn-woven triboelectric nanogenerators (TENGs) have greatly advanced wearable sensor technology, but their limited sensitivity and stability hinder broad adoption. To address these limitations, Poly(VDF-TrFE) and P(olyadiohexylenediamine (PA66)-based nanofibers coaxial yarns (NCYs) combining coaxial conjugated electrospinning and online conductive adhesive coating are developed. The integration of these NCYs led to enhanced TENGs (NCY-TENGs), notable for their flexibility, stretchability, and improved sensitivity, which is ideal for capturing body motion signals. One significant application of this technology is the fabrication of smart insoles from NCY-TENG plain-woven fabrics. These insoles are highly sensitive and possess antibacterial, breathable, and washable properties, making them ideal for real-time gait monitoring in patients with diabetic foot conditions. The NCY-TENGs and their derivatives show immense potential for a variety of wearable electronic devices, representing a considerable advancement in the field of wearable sensors.

Synthetic techniques for thermodynamically disfavoured substituted six-membered rings

Six-membered rings are ubiquitous structural motifs in bioactive compounds and multifunctional materials. Notably, their thermodynamically disfavoured isomers, like disubstituted cyclohexanes featuring one substituent in an equatorial position and the other in an axial position, often exhibit enhanced physical and biological activities in comparison with their opposite isomers. However, the synthesis of thermodynamically disfavoured isomers is, by its nature, challenging, with only a limited number of possible approaches. In this Review, we summarize and compare synthetic methodologies that produce substituted six-membered rings with thermodynamically disfavoured substitution patterns. We place particular emphasis on elucidating the crucial stereoinduction factors within each transformation. Our aim is to stimulate interest in the synthesis of these unique structures, while simultaneously providing synthetic chemists with a guide to approaching this synthetic challenge.

Iron-Catalyzed Photoredox Alcohol α-C-H Alkylation and Tandem Intramolecular Cyclization: Facile Access to Multisubstituted 2,3-Dihydrofurans and γ-Butyrolactones

Multisubstituted furans occupy a pivotal position within the realms of synthetic chemistry and pharmacological science due to their distinctive chemical configurations and inherent properties. We herein introduce a tandem difunctionalization protocol of alcohols for the efficient synthesis of multisubstituted 2,3-dihydrofurans and γ-butyrolactones through the combination of photocatalysis and iron catalysis under mild conditions. Photoredox alcohol α-C(sp3)-H activation and Pinner-type intramolecular cyclization are two key processes. This method features significant convenience, economic benefits, and environmental friendliness.

Development of a Divergent Synthesis of Pleurotinoid Natural Products

Herein, we describe the evolution of our syntheses of the pleurotinoid natural products pleurotin (1), pleurogrisein (3), and 4-hydroxypleurogrisein (4). An approach based on a proximity-induced intramolecular Diels-Alder cycloaddition of a transient ortho-quinone dimethide (e.g., 6, Scheme 1) was inferior to an alternative construction featuring Gao's titanium(IV)-mediated photoenolization Diels-Alder coupling of ortho-tolualdehyde 20 with functionalized hydrindenone 22. While this pairing exhibited the desired stereoface selectivity and produced cis-fused hydrindanone 23, the successful realization of our syntheses of 1, 3, and 4 required a post-Diels-Alder epimerization of the unactivated stereocenter at C-5 in compound 23. Ultimately, it was possible to generate a reactive oxygen-centered radical via a reductive homolytic cleavage of the N-O bond in 23 and capitalize on its ability to break the C5-H bond in an intramolecular 1,5-hydrogen atom transfer (HAT). The carbon radical arising from this pivotal 1,5-HAT was subsequently trapped in situ by an exogenous thiol in a kinetically controlled HAT reaction to establish the natural configuration at C-5. The successful flipping of the cis-hydrindane in 23 to the challenging trans configuration in 24 provided a firm foundation for a formal synthesis of pleurotin (1), as well as syntheses of pleurogrisein (3) and 4-hydroxypleurogrisein (4).

Thermodynamic H-Abstraction Abilities of Nitrogen Centered Radical Cations as Potential Hydrogen Atom Transfer Catalysts in Y-H Bond Functionalization

Y-H bond functionalization has always been the focus of research interest in the area of organic synthesis. Direct hydrogen atom transfer (HAT) from the Y-H bond is one of the most efficient and practical methods to activate the Y-H bond. Recently, nitrogen centered radical cations were broadly utilized as H-abstraction catalysts to activate Y-H bonds via the HAT process. As a type of HAT catalyst, the H-affinity of nitrogen centered radical cations is a significant thermodynamic parameter to quantitatively evaluate the thermodynamic H-abstraction potentials of nitrogen centered radical cations. In this work, the pK a values of 120 protonated N-containing compounds in acetonitrile (AN) are predicted, and the H-affinities of 120 nitrogen centered radical cations in AN are derived from the reduction potentials of nitrogen centered radical cations and pK a of protonated N-containing compounds using Hess' law. This work focuses on the H-abstraction abilities of 120 nitrogen centered radical cations in AN to enrich the molecule library of novel HAT catalysts or H-abstractors and provides valuable thermodynamic guidelines for the application of nitrogen centered radical cations in Y-H bond functionalization.

Characterization of a novel ribose-5-phosphate isomerase B from Curtobacterium flaccumfaciens ZXL1 for D-allose production

Enzymatic preparation of rare sugars as an alternative to traditional sweeteners is an effective strategy to achieve a low-calorie healthy diet. Ribose-5-phosphate isomerase B (RpiB) is a key enzyme in the non-oxidative branch of the catalytic pentose phosphate pathway. Here, we investigated the potential of Curtobacterium flaccumfaciens ZXL1 (C. flaccumfaciens ZXL1) derived RpiB (CfRpiB) in D-allose preparation. The optimal reaction conditions for recombinant CfRpiB were found experimentally to be pH 7.0, 55 °C, and no metal ions. The kinetic parameters Km, kcat, and catalytic efficiency kcat/Km were 320 mM, 4769 s-1, and 14.9 mM-1 s-1 respectively. The conversion of D-allulose by purified enzyme (1 g L-1 ) to D-allose was 13% within 1 h. In addition, homology modeling and molecular docking were used to predict the active site residues: Asp13, Asp14, Cys72, Gly73, Thr74, Gly77, Asn106, and Lys144.

Asymmetric Pd/Organoboron-Catalyzed Site-Selective Carbohydrate Functionalization with Alkoxyallenes Involving Noncovalent Stereocontrol

Herein, we demonstrate the robustness of a synergistic chiral Pd/organoboron system in tackling a challenging suite of site-, regio-, enantio- and diastereoselectivity issues across a considerable palette of biologically relevant carbohydrate polyols, when prochiral alkoxyallenes were employed as electrophiles. In view of the burgeoning role of noncovalent interactions (NCIs) in stereoselective carbohydrate synthesis, our mechanistic experiments and DFT modeling of the reaction path unexpectedly revealed that NCIs such as hydrogen bonding and CH-π interactions between the resting states of the Pd-π-allyl complex and the borinate saccharide are critically involved in the stereoselectivity control. Our strategy thus illuminates the untapped potential of harnessing NCIs in the context of transition metal catalysis to tackle stereoselectivity challenges in carbohydrate functionalization.

D-allose, a typical rare sugar: properties, applications, and biosynthetic advances and challenges

D-allose, a C-3 epimer of D-glucose and an aldose-ketose isomer of D-allulose, exhibits 80% of sucrose's sweetness while being remarkably low in calories and nontoxic, making it an appealing sucrose substitute. Its diverse physiological functions, particularly potent anticancer and antitumor effects, render it a promising candidate for clinical treatment, garnering sustained attention. However, its limited availability in natural sources and the challenges associated with chemical synthesis necessitate exploring biosynthetic strategies to enhance production. This overview encapsulates recent advancements in D-allose's physicochemical properties, physiological functions, applications, and biosynthesis. It also briefly discusses the crucial role of understanding aldoketose isomerase structure and optimizing its performance in D-allose synthesis. Furthermore, it delves into the challenges and future perspectives in D-allose bioproduction. Early efforts focused on identifying and characterizing enzymes responsible for D-allose production, followed by detailed crystal structure analysis to improve performance through molecular modification. Strategies such as enzyme immobilization and implementing multi-enzyme cascade reactions, utilizing more cost-effective feedstocks, were explored. Despite progress, challenges remain, including the lack of efficient high-throughput screening methods for enzyme modification, the need for food-grade expression systems, the establishment of ordered substrate channels in multi-enzyme cascade reactions, and the development of downstream separation and purification processes.

Synthesis of Sucrose-Mimicking Disaccharide by Intramolecular Aglycone Delivery

Rare sugars are known for their ability to suppress postprandial blood glucose levels. Therefore, oligosaccharides and disaccharides derived from rare sugars could potentially serve as functional sweeteners. A disaccharide [α-d-allopyranosyl-(1→2)-β-d-psicofuranoside] mimicking sucrose was synthesized from rare monosaccharides D-allose and D-psicose. Glycosylation using the intermolecular aglycon delivery (IAD) method was employed to selectively form 1,2-cis α-glycosidic linkages of the allopyranose residues. Moreover, β-selective psicofuranosylation was performed using a psicofuranosyl acceptor with 1,3,4,6-tetra-O-benzoyl groups. This is the first report on the synthesis of non-reducing disaccharides comprising only rare d-sugars by IAD using protected ketose as a unique acceptor; additionally, this approach is expected to be applicable to the synthesis of functional sweeteners.

Natural Extracts for Antibacterial Applications

Bacteria-induced epidemics and infectious diseases are seriously threatening the health of people around the world. In addition, antibiotic therapy has been inducing increasingly more serious bacterial resistance, which makes it urgent to develop new treatment strategies to combat bacteria, including multidrug-resistant bacteria. Natural extracts displaying antibacterial activity and good biocompatibility have attracted much attention due to greater concerns about the safety of synthetic chemicals and emerging drug resistance. These antibacterial components can be isolated and utilized as antimicrobials, as well as transformed, combined, or wrapped with other substances by using modern assistive technologies to fight bacteria synergistically. This review summarizes recent advances in natural extracts from three kinds of sources-plants, animals, and microorganisms-for antibacterial applications. This work discusses the corresponding antibacterial mechanisms and the future development of natural extracts in antibacterial fields.

Transformations of carbohydrate derivatives enabled by photocatalysis and visible light photochemistry

This review article highlights the diverse ways in which recent developments in the areas of photocatalysis and visible light photochemistry are impacting synthetic carbohydrate chemistry. The major topics covered are photocatalytic glycosylations, generation of radicals at the anomeric position, transformations involving radical formation at non-anomeric positions, additions to glycals, processes initiated by photocatalytic hydrogen atom transfer from sugars, and functional group interconversions at OH and SH groups. Factors influencing stereo- and site-selectivity in these processes, along with mechanistic aspects, are discussed.

Site-Selective C-O Bond Editing of Unprotected Saccharides

Glucose and its polyhydroxy saccharide analogs are complex molecules that serve as essential structural components in biomacromolecules, natural products, medicines, and agrochemicals. Within the expansive realm of saccharides, a significant area of research revolves around chemically transforming naturally abundant saccharide units to intricate or uncommon molecules such as oligosaccharides or rare sugars. However, partly due to the presence of multiple hydroxyl groups with similar reactivities and the structural complexities arising from stereochemistry, the transformation of unprotected sugars to the desired target molecules remains challenging. One such formidable challenge lies in the efficient and selective activation and modification of the C-O bonds in saccharides. In this study, we disclose a modular 2-fold "tagging-editing" strategy that allows for direct and selective editing of C-O bonds of saccharides, enabling rapid preparation of valuable molecules such as rare sugars and drug derivatives. The first step, referred to as "tagging", involves catalytic site-selective installation of a photoredox active carboxylic ester group to a specific hydroxyl unit of an unprotected sugar. The second step, namely, "editing", features a C-O bond cleavage to form a carbon radical intermediate that undergoes further transformations such as C-H and C-C bond formations. Our strategy constitutes the most effective and shortest route in direct transformation and modification of medicines and other molecules bearing unprotected sugars.

Visible-Light-Mediated, Diastereoselective Epimerization of Exocyclic Amines

Stereoselective α-amino C-H epimerization of exocyclic amines is achieved via photoredox catalyzed, thiyl-radical mediated, reversible hydrogen atom transfer to provide thermodynamically controlled anti/syn isomer ratios. The method is applicable to different substituents and substitution patterns about aminocyclopentanes, aminocyclohexanes, and a N-Boc-3-aminopiperidine. The method also provided efficient epimerization for primary, alkyl and (hetero)aryl secondary, and tertiary exocyclic amines. Demonstration of reversible epimerization, deuterium labeling, and luminescence quenching provides insight into the reaction mechanism.

Creating a Defined Chirality in Amino Acids and Cyclic Dipeptides by Photochemical Deracemization

2,5-Diketopiperazines are cyclic dipeptides displaying a wide range of applications. Their enantioselective preparation has now been found possible from the respective racemates by a photochemical deracemization (53 examples, 74 % to quantitative yield, 71-99 % ee). A chiral benzophenone catalyst in concert with irradiation at λ=366 nm enables to establish the configuration at the stereogenic carbon atom C6 at will. If other stereogenic centers are present in the diketopiperazines they remain unaffected and a stereochemical editing is possible at a single position. Consecutive reactions, including the conversion into N-aryl or N-alkyl amino acids or the reduction to piperazines, occur without compromising the newly created stereogenic center. Transient absorption spectroscopy revealed that the benzophenone catalyst processes one enantiomer of the 2,5-diketopiperazines preferentially and enables a reversible hydrogen atom transfer that is responsible for the deracemization process. The remarkably long lifetime of the protonated ketyl radical implies a yet unprecedented mode of action.