Synthesis of rare sugar isomers through site-selective epimerization

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.

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.

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.

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.

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.

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.

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.

Advances in glycoside and oligosaccharide synthesis

The structural complexity of glycans poses a serious challenge in the chemical synthesis of glycosides, oligosaccharides and glycoconjugates. Glycan complexity, determined by composition, connectivity, and configuration far exceeds what nature achieves with nucleic acids and proteins. Consequently, glycoside synthesis ranks among the most complex tasks in organic synthesis, despite involving only a simple type of bond-forming reaction. Here, we introduce the fundamental principles of glycoside bond formation and summarize recent advances in glycoside bond formation and oligosaccharide synthesis.

Interconverting mirror-image molecules

A light-driven multitasking catalyst enhances chirality in molecular mixtures.

Diversification of Glycosyl Compounds via Glycosyl Radicals

Glycosyl radical functionalization is one of the central topics in synthetic carbohydrate chemistry. Recent advances in metal-catalyzed cross-coupling chemistry and metallaphotoredox catalysis provided powerful platforms for glycosyl radical diversification. In particular, the discovery of new glycosyl radical precursors in conjunction with these advanced reaction technologies have significantly expanded the space for glycosyl compound synthesis. In this Review, we highlight the most recent progress in this area starting from 2021, and the reports included will be categorized based on different reaction types for better clarity.

Late-Stage Molecular Editing Enabled by Ketone Chain-Walking Isomerization

Herein, a method for the isomerization of ketones in a manner akin to the chain-walking reaction of alkenes is described. Widely available and inexpensive pyrrolidine and elemental sulfur are deployed as catalysts to achieve this reversible transformation. Key to the utility of this approach was the elucidation of a stereochemical model to determine the thermodynamically favored product of the reaction and the kinetic selectivity observed. With the distinct selectivity profile of our ketone chain-walking process, the isomerization of various steroids was demonstrated to rapidly access novel steroids with "unnatural" oxidation patterns.

Non-Native Site-Selective Enzyme Catalysis

The ability to site-selectively modify equivalent functional groups in a molecule has the potential to streamline syntheses and increase product yields by lowering step counts. Enzymes catalyze site-selective transformations throughout primary and secondary metabolism, but leveraging this capability for non-native substrates and reactions requires a detailed understanding of the potential and limitations of enzyme catalysis and how these bounds can be extended by protein engineering. In this review, we discuss representative examples of site-selective enzyme catalysis involving functional group manipulation and C-H bond functionalization. We include illustrative examples of native catalysis, but our focus is on cases involving non-native substrates and reactions often using engineered enzymes. We then discuss the use of these enzymes for chemoenzymatic transformations and target-oriented synthesis and conclude with a survey of tools and techniques that could expand the scope of non-native site-selective enzyme catalysis.

Functional-group translocation of cyano groups by reversible C-H sampling

Chemical transformations that introduce, remove or manipulate functional groups are ubiquitous in synthetic chemistry1. Unlike conventional functional-group interconversion reactions that swap one functionality for another, transformations that alter solely the location of functional groups are far less explored. Here, by photocatalytic, reversible C-H sampling, we report a functional-group translocation reaction of cyano (CN) groups in common nitriles, allowing for the direct positional exchange between a CN group and an unactivated C-H bond. The reaction shows high fidelity for 1,4-CN translocation, frequently contrary to inherent site selectivity in conventional C-H functionalizations. We also report the direct transannular CN translocation of cyclic systems, providing access to valuable structures that are non-trivial to obtain by other methods. Making use of the synthetic versatility of CN and a key CN translocation step, we showcase concise syntheses of building blocks of bioactive molecules. Furthermore, the combination of C-H cyanation and CN translocation allows access to unconventional C-H derivatives. Overall, the reported reaction represents a way to achieve site-selective C-H transformation reactions without requiring a site-selective C-H cleavage step.

Catalytic Asymmetric Hydrogen Atom Transfer: Enantioselective Hydroamination of Alkenes

We report a highly enantioselective radical-based hydroamination of enol esters with sulfonamides jointly catalyzed by an Ir photocatalyst, Brønsted base, and tetrapeptide thiol. This method is demonstrated for the formation of 23 protected β-amino-alcohol products, achieving selectivities up to 97:3 er. The stereochemistry of the product is set through selective hydrogen atom transfer from the chiral thiol catalyst to a prochiral C-centered radical. Structure-selectivity relationships derived from structural variation of both the peptide catalyst and olefin substrate provide key insights into the development of an optimal catalyst. Experimental and computational mechanistic studies indicate that hydrogen-bonding, π-π stacking, and London dispersion interactions are contributing factors for substrate recognition and enantioinduction. These findings further the development of radical-based asymmetric catalysis and contribute to the understanding of the noncovalent interactions relevant to such transformations.

Isotopic Fractionation as a Mechanistic Probe in Light-Driven C-H Bond Exchange Reactions

Here, we report a diagnostic framework for elucidating the mechanisms of photoredox-based hydrogen isotope exchange (HIE) reactions based on hydrogen/deuterium (H/D) fractionation. Traditional thermal HIE methods generally proceed by reversible bond cleavage and bond reformation steps that share a common transition state. However, bond cleavage and bond reformation in light-driven HIE reactions can proceed via multiple, non-degenerate sets of elementary steps, complicating both mechanistic analysis and attendant optimization efforts. Building on classical treatments of equilibrium isotope effects, the fractionation method presented here extracts information regarding the nature of the key bond-forming and bond-breaking steps by comparing the extent of deuterium incorporation into an exchangeable C-H bond in the substrate relative to the H/D isotopic ratio of a solvent reservoir. We show that the extent of fractionation is sensitive to the mechanism of the exchange process and provides a means to distinguish between degenerate and non-degenerate mechanisms for isotopic exchange. In model systems, the mechanisms implied by the fractionation method align with those predicted by thermochemical considerations. We then employed the method to study HIE reactions whose mechanisms are ambiguous on thermodynamic grounds.

Enantio- and Diastereoselective Mannich Reactions of ß-Dicarbonyls by Second Stage Diastereoconvergent Crystallization

The synthesis of chiral α-monosubstituted-ß-dicarbonyls is a challenging task in asymmetric catalysis due to the rapid, typically uncontrolled, product racemization or epimerization under most reaction conditions. For this reason, diastereoselective additions of unsubstituted ß-dicarbonyls to π-electrophiles are unusual. Herein, we disclose a simple catalytic crystallization-driven enantio- and diastereoselective Mannich reaction for the synthesis of stereodefined α-monosubstituted-ß-keto esters, dissymmetric ß-diesters, dissymmetric ß-diketones, and ß-keto amides that productively leverages product epimerization in solution. Mechanistic studies suggest a scenario where the initial enantioselective, diastereodivergent skeletal assembly is catalyzed by a chiral tertiary amine organocatalyst, which then facilitates second stage crystallization-induced diastereoconvergence to provide the challenging α-stereocenter in excellent stereoselectivity.

Catalytic Approaches to Chemo- and Site-Selective Transformation of Carbohydrates

Carbohydrates are a fundamental unit playing pivotal roles in all the biological processes. It is thus essential to develop methods for synthesizing, functionalizing, and manipulating carbohydrates for further understanding of their functions and the creation of sugar-based functional materials. It is, however, not trivial to develop such methods, since carbohydrates are densely decorated with polar and similarly reactive hydroxy groups in a stereodefined manner. New approaches to chemo- and site-selective transformations of carbohydrates are, therefore, of great significance for revolutionizing sugar chemistry to enable easier access to sugars of interest. This review begins with a brief overview of the innate reactivity of hydroxy groups of carbohydrates. It is followed by discussions about catalytic approaches to enhance, override, or be orthogonal to the innate reactivity for the transformation of carbohydrates. This review avoids making a list of chemo- and site-selective reactions, but rather focuses on summarizing the concept behind each reported transformation. The literature references were sorted into sections based on the underlying ideas of the catalytic approaches, which we hope will help readers have a better sense of the current state of chemistry and develop innovative ideas for the field.

Mass Spectrometry Approach for Differentiation of Positional Isomers of Saccharides: Toward Direct Analysis of Rare Sugars

Rare sugars have gained popularity in recent years due to their use in antiaging treatments, their ability to sweeten with few calories, and their ability to heal infections. Rare sugars are found in small quantities in nature, and they exist typically as isomeric forms of traditional sugars, rendering some challenges in their isolation, synthesis, and characterization. In this work, we present the first direct mass spectrometric approach for differentiating structural isomers of sucrose that differ only by their glycosidic linkages. The method employed a noncontact nanoelectrospray (nESI) platform capable of analyzing minuscule volumes (5 μL) of saccharides via the formation of halide adducts ([M+X]-; X = Cl and Br). Tandem mass spectrometry analysis of the five structural isomers of sucrose afforded diagnostic fragment ions that can be used to distinguish each isomer. Detailed mechanisms showcasing the distinct fragmentation pattern for each isomer are discussed. The method was applied to characterize and confirm the presence of all five selected rare sugars in raw honey complex samples. Aside from the five natural α isomers of sucrose, the method was also suitable for differentiating some β isomers of the same glycosidic linkages, provided the monomeric sugar units are different. The halide adduct formation via the noncontact nESI source was also proven to be effective for oligosaccharides such as raffinose, β-cyclodextrin, and maltoheptaose. The results from this study encourage the future development of methods that function with simple operation to enable straightforward characterization of small quantities of rare sugars.

Key characters of glycodrugs

Glycodrug is an important chemical medicine category derived from biological carbohydrates and their mimics. However, the fundamental logic and features of glycodrugs are obscure issues. To make it easier to understand, four key characters of glycodrugs are extracted for reference. First, Glc relating drugs are key guards in glycometabolism. Second, Rib, GlcN/GalN, and Sia relating drugs are efficient modulators in life signaling regulations. Third, rare sugar relating drugs are effective weapons against various pathogens. Finally, glycosylation modifications are helpful strategies for druggability enhancement. In light of such key characters, more innovative glycodrugs will emerge in the future.

Stereochemical Editing

Stereochemical editing has recently risen to prominence, allowing the direct editing of organic molecules with stereocenter(s) to adjust their relative stereochemistry at a late-stage. Several seminal light-driven stereochemical editing reactions such as deracemization and epimerization have been successively developed. Recently, Wendlandt and co-workers reported a versatile photochemical epimerization of unactivated tertiary stereogenic centers to rapidly prepare the stereoisomers that were previously challenging to access.

A Retro-Aldol Reaction Prompted the Evolvability of a Phosphotransferase System for the Utilization of a Rare Sugar

The evolution of the bacterial phosphotransferase system (PTS) linked to glycolysis is dependent on the availability of naturally occurring sugars. Although bacteria exhibit sugar specificities based on carbon catabolite repression, the acquisition and evolvability of the cellular sugar preference under conditions that are suboptimal for growth (e.g., environments rich in a rare sugar) are poorly understood. Here, we generated Escherichia coli mutants via a retro-aldol reaction to obtain progeny that can utilize the rare sugar d-tagatose. We detected a minimal set of adaptive mutations in the d-fructose-specific PTS to render E. coli capable of d-tagatose utilization. These E. coli mutant strains lost the tight regulation of both the d-fructose and N-acetyl-galactosamine PTS following deletions in the binding site of the catabolite repressor/activator protein (Cra) upstream from the fruBKA operon and in the agaR gene, encoding the N-acetylgalactosamine (GalNAc) repressor, respectively. Acquired d-tagatose catabolic pathways then underwent fine-tuned adaptation via an additional mutation in 1-phosphofructose kinase to adjust metabolic fluxes. We determined the evolutionary trajectory at the molecular level, providing insights into the mechanism by which enteric bacteria evolved a substrate preference for the rare sugar d-tagatose. Furthermore, the engineered E. coli mutant strain could serve as an in vivo high-throughput screening platform for engineering non-phosphosugar isomerases to produce rare sugars. IMPORTANCE Microorganisms generate energy through glycolysis, which might have preceded a rapid burst of evolution, including the evolution of cellular respiration in the primordial biosphere. However, little is known about the evolvability of cellular sugar preferences. Here, we generated Escherichia coli mutants via a retro-aldol reaction to obtain progeny that can utilize the rare sugar d-tagatose. Consequently, we identified mutational hot spots and determined the evolutionary trajectory at the molecular level. This provided insights into the mechanism by which enteric bacteria evolved substrate preferences for various sugars, accounting for the widespread occurrence of these taxa. Furthermore, the adaptive laboratory evolution-induced cellular chassis could serve as an in vivo high-throughput screening platform for engineering tailor-made non-phosphorylated sugar isomerases to produce low-calorigenic rare sugars showing antidiabetic, antihyperglycemic, and antitumor activities.

Multifaceted View on the Mechanism of a Photochemical Deracemization Reaction

Upon irradiation in the presence of a chiral benzophenone catalyst (5 mol %), a racemic mixture of a given chiral imidazolidine-2,4-dione (hydantoin) can be converted almost quantitatively into the same compound with high enantiomeric excess (80-99% ee). The mechanism of this photochemical deracemization reaction was elucidated by a suite of mechanistic experiments. It was corroborated by nuclear magnetic resonance titration that the catalyst binds the two enantiomers by two-point hydrogen bonding. In one of the diastereomeric complexes, the hydrogen atom at the stereogenic carbon atom is ideally positioned for hydrogen atom transfer (HAT) to the photoexcited benzophenone. Detection of the protonated ketyl radical by transient absorption revealed hydrogen abstraction to occur from only one but not from the other hydantoin enantiomer. Quantum chemical calculations allowed us to visualize the HAT within this complex and, more importantly, showed that the back HAT does not occur to the carbon atom of the hydantoin radical but to its oxygen atom. The achiral enol formed in this process could be directly monitored by its characteristic transient absorption signal at λ ≅ 330 nm. Subsequent tautomerization leads to both hydantoin enantiomers, but only one of them returns to the catalytic cycle, thus leading to an enrichment of the other enantiomer. The data are fully consistent with deuterium labeling experiments and deliver a detailed picture of a synthetically useful photochemical deracemization reaction.

Site-Selective Electrochemical Oxidation of Glycosides

Quinuclidine-mediated electrochemical oxidation of glycopyranosides provides C3-ketosaccharides with high selectivity and good yields. The method is a versatile alternative to Pd-catalyzed or photochemical oxidation and is complementary to the 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO)-mediated C6-selective oxidation. Contrary to the electrochemical oxidation of methylene and methine groups, the reaction proceeds without oxygen.

Synthesis of the Three Most Expensive l-Hexose Thioglycosides from d-Glucose

The biologically important l-hexoses, which are less widespread than d-hexoses, cannot be obtained from natural sources or can only be extracted very costly. Due to the complexity of their synthesis, their commercially available derivatives (which are sold mostly in free form) are also very expensive, which is further exacerbated by the current rapid rise in prices. In the present work, starting from the cheapest d-hexose, d-glucose, using inexpensive and readily available chemicals, a reaction pathway was developed in which the three most expensive l-hexoses (l-idose, l-altrose, and l-talose) were successfully prepared in orthogonally protected thioglycoside form, ready for glycosylation. The l-ido and l-talo derivatives were synthesized by C-5 epimerization of the corresponding 5,6-unsaturated thioglycosides. From the l-ido derivatives, the orthogonally protected thioglycosides of l-altrose were then prepared by C-4 epimerization. Different approaches to the preparation of the key intermediates, 5,6-unsaturated thioglycoside derivatives, were systematically investigated in the presence of various protecting groups (ether and ester) and using commercially available reagents.

Phosphorylation-Driven Production of d-Allulose from d-Glucose by Coupling with an ATP Regeneration System

d-Allulose is a desirable sucrose substitute with potential applications in food and health care. d-Allulose can be synthesized using d-glucose as a substrate through coupling glucose isomerase with d-allulose 3-epimerase (DAEase); however, the product yield is typically less than 20% at reaction equilibrium and thus limits its use in industrial applications. Here, a 3R-ketose phosphorylation pathway coupled with an adenosine triphosphate (ATP) regeneration system was developed for the efficient synthesis of d-allulose in Escherichia coli using d-glucose as a substrate. The l-rhamnulose kinase (RhaB) was used to break the inherent reaction equilibrium due to its substrate specificity, resulting in increases in d-allulose titer by 69.9% to 4.96 ± 0.49 g/L. By optimizing the whole cell transformation conditions and designing an ATP regeneration module, d-allulose production reached 17.62 ± 0.77 g/L from 30 g/L d-glucose with a final yield of 0.73 g/g without the addition of exogenous ATP. To evaluate the potential industrial application of this multienzyme cascade system, d-allulose was produced from cane molasses (124.16 ± 2.69 g/L glucose equivalent) with a final d-allulose titer of 62.60 ± 3.76 g/L. The present study provides a practical enzymatic approach for the economical synthesis of d-allulose.

Light-empowered contra-thermodynamic stereochemical editing

Creating, conserving and modifying the stereochemistry of organic compounds has been the subject of significant research efforts in synthetic chemistry. Most synthetic routes are designed according to the stereoselectivity-determining step. Stereochemical editing is an alternative strategy, wherein the chiral-defining or geometry-defining steps are independent of the construction of the major scaffold or complexity. It enables late-stage alterations of stereochemistry and can generate isomers from a single compound. However, in many instances, stereochemical editing processes are contra-thermodynamic, meaning the transformation is unfavourable. To overcome this barrier, photocatalysis uses photogenerated radical species and introduces thermochemical biases. A range of synthetically valuable contra-thermodynamic stereochemical editing processes have been invented, including deracemization of chiral molecules, positional alkene isomerization and dynamic epimerization of sugars and diols. In this Review, we highlight the fundamental mechanisms of visible-light photocatalysis and the general reactivity modes of the photogenerated radical intermediates towards contra-thermodynamic stereochemical editing processes.

Methanol-Dependent Carbon Fixation for Irreversible Synthesis of d-Allulose from d-Xylose by Engineered Escherichia coli

d-Allulose is a rare hexose with great application potential, owing to its moderate sweetness, low energy, and unique physiological functions. The current strategies for d-allulose production, whether industrialized or under development, utilize six-carbon sugars such as d-glucose or d-fructose as a substrate and are usually based on the principle of reversible Izumoring epimerization. In this work, we designed a novel route that coupled the pathways of methanol reduction, pentose phosphate (PP), ribulose monophosphate (RuMP), and allulose monophosphate (AuMP) for Escherichia coli to irreversibly synthesize d-allulose from d-xylose and methanol. After improving the expression of AlsE by SUMO fusion and regulating the carbon fluxes by knockout of FrmRAB, RpiA, PfkA, and PfkB, the titer of d-allulose in fed-batch fermentation reached ≈70.7 mM, with a yield of ≈0.471 mM/mM on d-xylose or ≈0.512 mM/mM on methanol.

Photocatalytic deracemisation of cobalt(III) complexes with fourfold stereogenicity

The deracemisation of fourfold stereogenic cobalt(III) diketonates with a chiral photocatalyst is described. With only 0.5 mol% menthyl Ru(bpy)32+ photocatalyst, an enantiomeric enrichment of up to 88 : 12 e.r. was obtained for the major meridional diastereomers. Moreover, a distribution of configurationally stable diastereomers distinct from the thermodynamic ratio was observed upon reaching the photostationary state.

Stereochemical editing logic powered by the epimerization of unactivated tertiary stereocenters

The stereoselective synthesis of complex targets requires the precise orchestration of chemical transformations that simultaneously establish the connectivity and spatial orientation of desired bonds. In this work, we describe a complementary paradigm for the synthesis of chiral molecules and their isomers, which tunes the three-dimensional structure of a molecule at a late stage. Key to the success of this strategy is the development of a mild and highly general photocatalytic method composed of decatungstate polyanion and disulfide cocatalysts, which enable the interconversion of unactivated tertiary stereogenic centers that were previously configurationally fixed. We showcase the versatility of this method-and the implementation of stereoediting logic-by the rapid construction of chiral scaffolds that would be challenging to access using existing tools and by the late-stage stereoediting of complex targets.

Visible Light-Mediated, Diastereoselective Epimerization of Morpholines and Piperazines to More Stable Isomers

We report a photocatalyzed epimerization of morpholines and piperazines that proceeds by reversible hydrogen atom transfer (HAT) and provides an efficient strategy for editing the stereochemical configurations of these saturated nitrogen heterocycles, which are prevalent in drugs. The more stable morpholine and piperazine isomers are obtained from the more synthetically accessible but less stable stereoisomers, and a broad scope is demonstrated in terms of substitution patterns and functional group compatibility. The observed distributions of diastereomers correlate well with the relative energies of the diastereomer pairs as determined by density functional theory (DFT) calculations. Mechanistic studies, including luminescence quenching, deuterium labeling reactions, and determination of reversibility support a thiyl radical mediated HAT pathway for the epimerization of morpholines. Investigation of piperazine epimerization established that the mechanism is more complex and led to the development of thiol free conditions for the highly stereoselective epimerization of N,N'-dialkyl piperazines for which a previously unrecognized radical chain HAT mechanism is proposed.

Engineering of Escherichia coli for D-allose fermentative synthesis from D-glucose through izumoring cascade epimerization

D-Allose is a potential alternative to sucrose in the food industries and a useful additive for the healthcare products in the future. At present, the methods for large-scale production of D-allose are still under investigation, most of which are based on in vitro enzyme-catalyzed Izumoring epimerization. In contrast, fermentative synthesis of D-allose has never been reported, probably due to the absence of available natural microorganisms. In this work, we co-expressed D-galactose: H+ symporter (GalP), D-glucose isomerase (DGI), D-allulose 3-epimerase (DAE), and ribose-5-phosphate isomerase (RPI) in Escherichia coli, thereby constructing an in vivo Izumoring pathway for yielding D-allose from D-glucose. The carbon fluxes and carbon catabolite repression (CCR) were rationally regulated by knockout of FruA, PtsG, Glk, Mak, PfkA, and PfkB involved in the pathways capable of phosphorylating D-fructose, D-glucose, and fructose-6-phosphate. Moreover, the native D-allose transporter was damaged by inactivation of AlsB, thus driving the reversible Izumoring reactions towards the target product. Fermentation was performed in the M9 medium supplemented with glycerol as a carbon source and D-glucose as a substrate. The results show that the engineered E. coli cell factory was able to produce approximately 127.35 mg/L of D-allose after 84 h. Our achievements in the fermentative production of D-allose in this work may further promote the green manufacturing of rare sugars.

Rapid and Accurate Estimation of Activation Free Energy in Hydrogen Atom Transfer-Based C-H Activation Reactions: From Empirical Model to Artificial Neural Networks

A well-performing machine learning (ML) model is obtained by using proper descriptors and artificial neural network (ANN) algorithms, which can quickly and accurately predict activation free energy in hydrogen atom transfer (HAT)-based sp³ C-H activation. Density functional theory calculations (UωB97X-D) are used to establish the reaction system data sets of methoxyl (CH₃O·), trifluoroethoxyl (CF₃CH₂O·), tert-butoxyl (tBuO·), and cumyloxyl (CumO·) radicals. The simplified Roberts' equation proposed in our recent study works here [R ² = 0.84, mean absolute error (MAE) = 0.85 kcal/mol]. Its performance is comparable with univariate Mulliken-type electronegativity (χ) with the ANN model. The ANN model with bond dissociation free energy, χ, α-unsaturation, and Nolan buried volume (%V _buried) successively improves R ² and MAE to 0.93 and 0.54 kcal/mol, respectively. It reproduces the test sets of trichloroethoxyl (CCl₃CH₂O·) with R ² = 0.87 and MAE = 0.89 kcal/mol and accurately predicts the relative experimental barrier of the HAT reactions with CumO· and the site selectivity of CH₃O·.

Energy- and atom-efficient chemical synthesis with endergonic photocatalysis

Endergonic photocatalysis is the use of light to perform catalytic reactions that are thermodynamically unfavourable. While photocatalysis has become a powerful tool in facilitating chemical transformations, the light-energy efficiency of these processes has not gathered much attention. Exergonic photocatalysis does not take full advantage of the light energy input, producing low-energy products and heat, whereas endergonic photocatalysis incorporates a portion of the photon energy into the reaction, yielding products that are higher in free energy than the reactants. Such processes can enable catalytic, atom-economic syntheses of reactive compounds from bench-stable materials. With respect to environmental friendliness and carbon neutrality, endergonic photocatalysis is also of interest to large-scale industrial manufacturing, where better energy efficiency, less waste and value addition are highly sought. We therefore assess here the thermochemistry of several classes of reported photocatalytic transformations to showcase current advances in endergonic photocatalysis and point to their industrial potential.

Exploiting photoredox catalysis for carbohydrate modification through C–H and C–C bond activation

Photoredox catalysis has recently emerged as a powerful synthetic platform for accessing complex chemical structures through non-traditional bond disconnection strategies that proceed through free-radical intermediates. Such synthetic strategies have been used for a range of organic transformations; however, in carbohydrate chemistry they have primarily been applied to the generation of oxocarbenium ion intermediates in the ubiquitous glycosylation reaction. In this Review, we present more intricate light-induced synthetic strategies to modify native carbohydrates through homolytic C-H and C-C bond cleavage. These strategies allow access to glycans and glycoconjugates with profoundly altered carbohydrate skeletons, which are challenging to obtain through conventional synthetic means. Carbohydrate derivatives with such structural motifs represent a broad class of natural products integral to numerous biochemical processes and can be found in active pharmaceutical substances. Here we present progress made in C-H and C-C bond activation of carbohydrates through photoredox catalysis, focusing on the operational mechanisms and the scope of the described methodologies.

Collective total synthesis of C4-oxygenated securinine-type alkaloids via stereocontrolled diversifications on the piperidine core

Securinega alkaloids have fascinated the synthetic chemical community for over six decades. Historically, major research foci in securinega alkaloid synthesis have been on the efficient construction of the fused tetracyclic framework that bears a butenolide moiety and tertiary amine-based heterocycles. These "basic" securinega alkaloids have evolved to undergo biosynthetic oxidative diversifications, especially on the piperidine core. However, a general synthetic solution to access these high-oxidation state securinega alkaloids is lacking. In this study, we have completed the total synthesis of various C4-oxygenated securinine-type alkaloids including securingines A, C, D, securitinine, secu'amamine D, phyllanthine, and 4-epi-phyllanthine. Our synthetic strategy features stereocontrolled oxidation, rearrangement, and epimerization at N1 and C2-C4 positions of the piperidine core within (neo)securinane scaffolds. Our discoveries provide a fundamental synthetic solution to all known securinine-type natural products with various oxidative and stereochemical variations around the central piperidine ring.

Site-selective C-H alkylation of myo-inositol via organic photoredox catalysis

Site-selective photoredox reactions with aromatic olefins enable direct alkylation of unprotected myo-inositol at C4. The efficacy of these reactions can be finely tuned by modifying the structures of HAT reagents. These reactions open the possibility of selective C-H alkylations of myo-inositol without the need for multi-step protection-deprotection strategies.