Alkyl/Glycosyl Sulfoxides as Radical Precursors and Their Use in the Synthesis of Pyridine Derivatives

Exploration of Glycosyl Dithioimidocarbonates in Photoinduced Desulfurative Cross-Coupling Reactions

Readily synthesized bench-stable glycosyl dithioimidocarbonates are useful C-glycoside precursors. Under mild photochemical conditions, these glycosides undergo desulfurative glycosyl radical generation in the presence of weak acid, 4CzIPN, and Hantzsch ester. These radicals perform well in Geise-like reactions to yield C-glycosides with high stereoselectivity.

Stereoselective and site-divergent synthesis of C-glycosides

Carbohydrates play important roles in medicinal chemistry and biochemistry. However, their synthesis relies on specially designed glycosyl donors, which are often unstable and require multi-step synthesis. Furthermore, the catalytic and stereoselective installation of arylated quaternary stereocentres on sugar rings remains a formidable challenge. Here we report a facile and versatile method for the synthesis of diverse C-R (where R is an aryl, heteroaryl, alkenyl, alkynyl or alkyl) glycosides from readily available and bench-stable 1-deoxyglycosides. The reaction proceeds under mild conditions and exhibits high stereoselectivity across a broad range of glycosyl units. This protocol can be used to synthesize challenging 2-deoxyglycosides, unprotected glycosides, non-classical glycosides and deuterated glycosides. We further developed the catalyst-controlled site-divergent functionalization of carbohydrates for the synthesis of various unexplored carbohydrates containing arylated quaternary stereocentres that are inaccessible by existing methods. The synthetic utility of this strategy is further demonstrated in the synthesis of pharmaceutically relevant molecules and carbohydrates.

A Radical Activation Strategy for Versatile and Stereoselective N-Glycosylation

Previous N-glycosylation approaches have predominately involved acidic conditions, facing challenges of low stereoselectivity and limited scope. Herein, we introduce a radical activation strategy that enables versatile and stereoselective N-glycosylation using readily accessible glycosyl sulfinate donors under basic conditions and exhibits exceptional tolerance towards various N-aglycones containing alkyl, aryl, heteroaryl and nucleobase functionalities. Preliminary mechanistic studies indicate a pivotal role of iodide, which orchestrates the formation of a glycosyl radical from the glycosyl sulfinate and subsequent generation of the key intermediate, a configurationally well-defined glycosyl iodide, which is subsequently attacked by an N-aglycone in a stereospecific SN2 manner to give the desired N-glycosides. An alternative route involving the coupling of a glycosyl radical and a nitrogen-centered radical is also proposed, affording the exclusive 1,2-trans product. This novel approach promises to broaden the synthetic landscape of N-glycosides, offering a powerful tool for the construction of complex glycosidic structures under mild conditions.

Glycosylation with sulfoxide-based glycosyl donors

Sulfoxides have emerged as pivotal constituents in modern carbohydrate chemistry. As anomeric leaving groups, sulfinyl moieties may occupy positions directly at the anomeric position or at a more remote site. This feature article is focused on the evolution and notable advancements of glycosyl sulfoxide donors in glycosylation reactions. Its objective is to elucidate the obstacles and prospects within this evolving research domain, with the aim of enhancing comprehension and progress in the field of carbohydrate chemistry.

C-H Functionalization of Heteroarenes via Electron Donor-Acceptor Complex Photoactivation

C-H Functionalization of heteroarenes stands as a potent instrument in organic synthesis, and with the incorporation of visible light, it emerged as a transformative game-changer. In this domain, electron donor-acceptor (EDA) complex, formed through the pairing of an electron-rich substrate with an electron-accepting molecule, has garnered substantial consideration in recent years due to the related avoidance of the requirement of photocatalyst as well as oxidant. EDA complexes can undergo photoactivation under mild conditions and exhibit high functional group tolerance, making them potentially suitable for the functionalization of biologically relevant heteroarenes. This review article provides an overview of recent advancements in the field of C-H functionalization of heteroarenes via EDA complex photoactivation with literature coverage up to April, 2024.

Direct radical functionalization of native sugars

Naturally occurring (native) sugars and carbohydrates contain numerous hydroxyl groups of similar reactivity1,2. Chemists, therefore, rely typically on laborious, multi-step protecting-group strategies3 to convert these renewable feedstocks into reagents (glycosyl donors) to make glycans. The direct transformation of native sugars to complex saccharides remains a notable challenge. Here we describe a photoinduced approach to achieve site- and stereoselective chemical glycosylation from widely available native sugar building blocks, which through homolytic (one-electron) chemistry bypasses unnecessary hydroxyl group masking and manipulation. This process is reminiscent of nature in its regiocontrolled generation of a transient glycosyl donor, followed by radical-based cross-coupling with electrophiles on activation with light. Through selective anomeric functionalization of mono- and oligosaccharides, this protecting-group-free 'cap and glycosylate' approach offers straightforward access to a wide array of metabolically robust glycosyl compounds. Owing to its biocompatibility, the method was extended to the direct post-translational glycosylation of proteins.

Stereoselective synthesis of α-glycosyl azides: allyl glycosyl sulfones as radical precursors

Despite their critical importance in drug development and biochemistry, efficiently synthesizing α-glycosyl azides has continued to pose significant challenges. In this report, we introduce a universal and practical radical reaction for the stereoselective synthesis of α-glycosyl azides using bench-stable allyl glycosyl sulfones as the donor. This method is characterized by its mild reaction conditions, high stereoselectivity, and extensive scope of glycosyl units. Moreover, the accessibility of several structurally complex drug-sugar conjugates underscores the practicality of our approach.

Preparing glycosyl benzothiazoles from 2-isocyanoaryl thioethers and glycosyl radicals under thermal conditions

Herein, we report a method for preparing glycosyl benzothiazoles via radical cascade cyclization, in which glycosyl radicals are generated from readily available and bench-stable allyl glycosyl sulfones. This cascade reaction proceeds under simple conditions and tolerates a broad substrate scope in high yield with excellent stereoselectivity. Mechanistic studies support that the reactions proceed via the intermediacy of imidoyl radicals, which attack the appended sulfide unit by a SH2 process to forge the thiazole ring.

Direct Construction of C-Alkyl Glycosides from Non-Activated Olefins via Nickel-Catalyzed C(sp3)─C(sp3) Coupling Reaction

Among C-glycosides, C-alkyl glycosides are significant building blocks for natural products and glycopeptides. However, research on efficient construction methods for C-alkyl glycosides remains relatively limited. Compared with Michael acceptors, non-activated olefins are more challenging substrates and have rarely been employed in the construction of C-glycosides. Here, a highly efficient and convenient approach for the synthesis of C-alkyl glycosides through a nickel-catalyzed C(sp3)-C(sp3) coupling reaction is presented. A distinctive feature of this method is its utilization of non-activated olefins as the anomeric radical acceptors for hydroalkylation, allowing for the direct formation of C-glycoside bonds in a single step. Furthermore, this method demonstrates excellent compatibility with a broad scope of highly reactive functional groups. Mechanistic investigations suggest that the reaction proceeds via a free radical pathway, leading predominantly to the formation of products with α-configuration. Overall, this innovative methodology offers a versatile and practical approach for the synthesis of C-alkyl glycosides, offering new avenues for the production of intricate glycosides with potential applications in drug discovery and chemical biology.

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.

Photosensitizer-free visible-light-promoted glycosylation enabled by 2-glycosyloxy tropone donors

Photochemical glycosylation has attracted considerable attention in carbohydrate chemistry. However, to the best of our knowledge, visible-light-promoted glycosylation via photoactive glycosyl donor has not been reported. In the study, we report a photosensitizer-free visible-light-mediated glycosylation approach using a photoactive 2-glycosyloxy tropone as the donor. This glycosylation reaction proceeds at ambient temperature to give a wide range of O-glycosides or oligosaccharides with yields up to 99%. This method is further applied in the stereoselective preparation of various functional glycosyl phosphates/phosphosaccharides, the construction of N-glycosides/nucleosides, and the late-stage glycosylation of natural products or pharmaceuticals on gram scales, and the iterative synthesis of hexasaccharide. The protocol features uncomplicated conditions, operational simplicity, wide substrate scope (58 examples), excellent compatibility with functional groups, scalability of products (7 examples), and high yields. It provides an efficient glycosylation method for accessing O/N-glycosides and glycans.

Electroreductive Desulfurative Transformations with Thioethers as Alkyl Radical Precursors

Thioethers are highly prevalent functional groups in organic compounds of natural and synthetic origin but remain remarkably underexplored as starting materials in desulfurative transformations. As such, new synthetic methods are highly desirable to unlock the potential of the compound class. In this vein, electrochemistry is an ideal tool to enable new reactivity and selectivity under mild conditions. Herein, we demonstrate the efficient use of aryl alkyl thioethers as alkyl radical precursors in electroreductive transformations, along with mechanistic details. The transformations proceed with complete selectivity for C(sp3 )-S bond cleavage, orthogonal to that of established transition metal-catalyzed two-electron routes. We showcase a hydrodesulfurization protocol with broad functional group tolerance, the first example of desulfurative C(sp3 )-C(sp3 ) bond formation in Giese-type cross-coupling and the first protocol for electrocarboxylation of synthetic relevance with thioethers as starting materials. Finally, the compound class is shown to outcompete their well-established sulfone analogues as alkyl radical precursors, demonstrating their synthetic potential for future desulfurative transformations in a one-electron manifold.

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 C-H Functionalization of Azines

Azines, such as pyridines, quinolines, pyrimidines, and pyridazines, are widespread components of pharmaceuticals. Their occurrence derives from a suite of physiochemical properties that match key criteria in drug design and is tunable by varying their substituents. Developments in synthetic chemistry, therefore, directly impact these efforts, and methods that can install various groups from azine C-H bonds are particularly valuable. Furthermore, there is a growing interest in late-stage functionalization (LSF) reactions that focus on advanced candidate compounds that are often complex structures with multiple heterocycles, functional groups, and reactive sites. Because of factors such as their electron-deficient nature and the effects of the Lewis basic N atom, azine C-H functionalization reactions are often distinct from their arene counterparts, and the application of these reactions in LSF contexts is difficult. However, there have been many significant advances in azine LSF reactions, and this review will describe this progress, much of which has occurred over the past decade. It is possible to categorize these reactions as radical addition processes, metal-catalyzed C-H activation reactions, and transformations occurring via dearomatized intermediates. Substantial variation in reaction design within each category indicates both the rich reactivity of these heterocycles and the creativity of the approaches involved.

Pyridine C(sp2)-H bond functionalization under transition-metal and rare earth metal catalysis

Pyridine is a crucial heterocyclic scaffold that is widely found in organic chemistry, medicines, natural products, and functional materials. In spite of the discovery of several methods for the synthesis of functionalized pyridines or their integration into an organic molecule, new methodologies for the direct functionalization of pyridine scaffolds have been developed during the past two decades. In addition, transition-metal-catalyzed C-H functionalization and rare earth metal-catalyzed reactions have flourished over the past two decades in the development of functionalized organic molecules of concern. In this review, we discuss recent achievements in the transition-metal and rare earth metal-catalyzed C-H bond functionalization of pyridine and look into the mechanisms involved.

Chemical synthesis of oligosaccharides and their application in new drug research

Oligosaccharides are the ubiquitous molecules of life. In order to translate human bioglycosylation into clinical applications, homogeneous samples of oligosaccharides and glycoconjugates can be obtained by chemical, enzymatic or other biological methods for systematic studies. However, the structural complexity and diversity of glycans and their conjugates present a major challenge for the synthesis of such molecules. This review summarizes the chemical synthesis methods of oligosaccharides, the application of oligosaccharides in the field of medicinal chemistry according to their related biological activities, and shows the great prospect of oligosaccharides in the field of pharmaceutical chemistry.

Photoinduced Dehydrogenative Borylation via Dihydrogen Bond Bridged Electron Donor and Acceptor Complexes

Air-stable amine- and phosphine-boranes are discovered as donors to integrate with pyridinium acceptor for generating photoactive electron-donor-acceptor (EDA) complexes. Experimental results and DFT calculations suggest a dihydrogen bond bridging the donor and acceptor. Irradiating the EDA complex enables an intra-complex single electron transfer to give a boron-centered radical for dehydrogenative borylation with no need of external photosensitizer and radical initiator. The deprotonation of Wheland-like radical intermediate rather than its generation is believed to determine the good ortho-selectivity based on DFT calculations. A variety of α-borylated pyridine derivatives have been readily synthesized with good functional group tolerance.

Generation and Use of Glycosyl Radicals under Acidic Conditions: Glycosyl Sulfinates as Precursors

We herein report a method that enables the generation of glycosyl radicals under highly acidic conditions. Key to the success is the design and use of glycosyl sulfinates as radical precursors, which are bench-stable solids and can be readily prepared from commercial starting materials. This development allows the installation of glycosyl units onto pyridine rings directly by the Minisci reaction. We further demonstrate the utility of this method in the late-stage modification of complex drug molecules, including the anticancer agent camptothecin. Experimental studies provide insight into the reaction mechanism.

Diboron(4)-Catalyzed Remote [3+2] Cycloaddition of Cyclopropanes via Dearomative/Rearomative Radical Transmission through Pyridine

Ring structures such as pyridine, cyclopentane or their combinations are important motifs in bioactive molecules. In contrast to previous cycloaddition reactions that necessitated a directly bonded initiating functional group, this work demonstrated a novel through-(hetero)arene radical transmission concept for selective activation of a remote bond. An efficient, metal-free and atom-economical [3+2] cycloaddition between 4-pyridinyl cyclopropanes and alkenes or alkynes has been developed for modular synthesis of pyridine-substituted cyclopentanes, cyclopentenes and bicyclo[2.1.1]hexanes that are difficult to access using known methods. This complexity-building reaction was catalyzed by a very simple and inexpensive diboron(4) compound and took place via dearomative/rearomative processes. The substrate scope was broad and more than 100 new compounds were prepared in generally high yields. Mechanistic experiments and density function theory (DFT) investigation supported a radical relay catalytic cycle involving alkylidene dihydropyridine radical intermediates and boronyl radical transfer.

N-Functionalized Pyridinium Salts: A New Chapter for Site-Selective Pyridine C-H Functionalization via Radical-Based Processes under Visible Light Irradiation

The radical-mediated C-H functionalization of pyridines has attracted considerable attention as a powerful tool in synthetic chemistry for the direct functionalization of the C-H bonds of the pyridine scaffold. Classically, the synthetic methods for functionalized pyridines often involve radical-mediated Minisci-type reactions under strongly acidic conditions. However, the site-selective functionalization of pyridines in unbiased systems has been a long-standing challenge because the pyridine scaffold contains multiple competing reaction sites (C2 vs C4) to intercept free radicals. Therefore, prefunctionalization of the pyridine is required to avoid issues observed with the formation of a mixture of regioisomers and overalkylated side products.Recently, N-functionalized pyridinium salts have been attracting considerable attention in organic chemistry as promising radical precursors and pyridine surrogates. The notable advantage of N-functionalized pyridinium salts lies in their ability to enhance the reactivity and selectivity for synthetically useful reactions under acid-free conditions. This approach enables exquisite regiocontrol for nonclassical Minisci-type reactions at the C2 and C4 positions under mild reaction conditions, which are suitable for the late-stage functionalization of bioactive molecules with greater complexity and diversity. Over the past five years, a variety of fascinating synthetic applications have been developed using various types of pyridinium salts under visible light conditions. In addition, a new platform for alkene difunctionalization using appropriately designed N-substituted pyridinium salts as bifunctional reagents has been reported, offering an innovative assembly process for complex organic architectures. Intriguingly, strategies involving light-absorbing electron donor-acceptor (EDA) complexes between pyridinium salts and suitable electron-rich donors further open up new reactivity under photocatalyst-free conditions. Furthermore, we developed enantioselective reactions using pyridinium salts to afford enantioenriched molecules bearing pyridines through single-electron N-heterocyclic carbene (NHC) catalysis.Herein, we provide a broad overview of our recent contributions to the development of N-functionalized pyridinium salts and summarize the cornerstones of organic reactions that successfully employ these pyridinium salts under visible light conditions. The major advances in the field are systematically categorized on the basis of the pyridines' N-substituent, N-X (X = O, N, C, and SO₂CF₃), and its reactivity patterns. Furthermore, the identification of new activation modes and their mechanistic aspects are discussed by providing representative contributions to each paradigm. We hope that this Account will inspire broad interest in the continued innovation of N-functionalized pyridinium salts in the exploration of new transformations.

Desulfinative Alkylation of Heteroarenes via an Electrostatic Electron Donor-Acceptor Complex

Functionalized pyridine and quinoline rings are important components of numerous bioactive molecules and natural products; however, diversification of these rings often requires de novo heterocycle ring synthesis or demanding reaction conditions. We report a method for desulfinative alkylation of pyridine and quinoline N-methoxide salts that operates under both photocatalytic and electrostatic electron donor-acceptor-mediated pathways. Unlike most EDA-mediated processes, this reaction operates in the absence of light and with the desulfination of the donor compound.

β-C-Glycosylation with 2-Oxindole Acceptors via Palladium-Catalyzed Decarboxylative Reactions

This report describes a highly efficient β-selective C-glycosylation of bicyclic galactals with 2-oxindoles through a palladium-catalyzed decarboxylative pathway. A variety of substrates representing both glycosyl donors and acceptors could be transformed in greater than 90% yields under mild reaction conditions. The decarboxylation intermediate of galactal could serve as an efficient base to deprotonate the enol tautomer of 2-oxindole and enhance its nucleophilicity. The β-selective nucleophilic addition at the anomeric center originates from the steric hindrance imposed by the palladium and bulky ligand.