Glycosyl-Radical-Based Synthesis of C-Alkyl Glycosides via Photomediated Defluorinative gem-Difluoroallylation

Visible Light-Promoted Ir(III)-Catalyzed Stereoselective Synthesis of Azauracil-C-Nucleosides from 1-Bromosugar

A visible light-promoted, mild, and efficient Ir(III)-catalyzed synthesis of C-nucleosides is reported, utilizing 1-bromosugar as the glycosyl radical precursor and 6-azauracil as the nucleobase. The method exhibits high α-selectivity and excellent functional group tolerance. Spectroscopic evidence shows that the coupling reaction happens via initial reductive quenching of the Ir(III) catalyst under visible light. Density functional theory calculation reveals the reason for complete α-selectivity. Finally, biologically active 6-aza pseudouridine analogues were synthesized, making the process a valuable platform for C-nucleosides.

A brief account of the application of glycosyl halide as glycosyl radical precursor towards glycosylation through visible light catalysis

Glycosylation is potentially one of the most important reactions in glycochemistry, where precise control over the anomeric selectivity is crucial for synthesizing biologically relevant glycoconjugates. Over the last two decades, the advent of visible light chemistry for the generation of glycosyl radicals and its application in the synthesis of glycosides has revolutionized the impact on the stereoselectivity of glycosylation. The visible light-driven radical-based reactions offer exceptional functional group tolerance, operate under mild reaction conditions, and have emerged as a powerful tool for synthesizing potent glycosyl compounds, which include aryl/alkyl glycosides, 2-deoxy sugar, glycosylamino acid, glycosylpeptides, and other glyco-conjugates. In this review article, a brief account of the evolution of glycosyl radical chemistry, with a focus on the visible light-mediated activation of glycosyl halides and its transformative impact on glycoside synthesis, has been discussed.

Photoinduced Excited-State Pd(0)-Catalyzed Stereoselective C-Glycosylation of 1-Bromosugar with Quinoxalin-2(1H)-one

Herein, an α-selective synthesis of C-glycoside with 1-bromosugars and quinoxalin-2(1H)-ones is reported, leveraging visible light-mediated excited-state palladium catalysis. This process involves the generation of glycosyl radicals from easily accessible 1-bromosugar precursors, which can capture quinoxalinone under photoinduced Pd(0) catalysis. The reaction operates under mild conditions, offering excellent axial selectivity with a diverse array of substrates. This methodology will provide a valuable platform for constructing potent biologically active C-glycosides.

Glycosyl Radical-Based Synthesis of C-Alkyl Glycosides Bearing a Cyclopropane via a Deoxygenative Giese Addition-Reduction-Cyclization Cascade

We have developed a glycosyl radical-based synthesis of C-alkyl glycosides through a deoxygenative Giese addition-reduction-cyclization cascade, in which readily available 1-hydroxy carbohydrates serve as precursors for glycosyl radicals and aryl alkenes function as radical acceptors. This reaction not only provides an effective method for accessing a previously underexplored class of functionalized cyclopropanes but also enhances the application of Giese addition in the synthesis of C-alkyl glycosides by derivatizing the radical intermediate generated through polar cyclization to yield a cyclopropane.

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.

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.

Radical Deoxygenative Three-Component Reaction of Alcohols, Aryl Alkenes, and Cyanopyridines

We report herein a deoxygenative radical multicomponent reaction involving alcohols, aryl alkenes, and cyanopyridine under photoredox conditions. This method is photoredox-neutral, suitable for late-stage modification, and compatible with a wide array of alcohols as alkyl radical sources, including primary, secondary, and tertiary alcohols. This reaction comprises a radical relay mechanism encompassing the Giese addition of aryl alkenes by alkyl radicals, followed by the decyanative pyridination of benzyl radicals.

Organophotocatalytic access to C-glycosides: multicomponent coupling reactions using glycosyl bromides

Photochemical multi-component coupling reactions initiated by the activation of glycosyl bromides in the presence of 1,4-bis(diphenylamino)benzene (BDB) as an organic photocatalyst were developed. C-glycosides accompanied by olefin (di)functionalization were obtained. This method allows us to access various C-glycosides with alkene, carbonyl, alcohol, ether, and amide functionalities.

Polarity-Matched Initiation of Radical-Polar Crossover Reactions for the Synthesis of C-Allyl Glycosides with gem-Difluoroalkene Groups

Herein, we disclose a general method for the assembly of C-allyl glycosides containing gem-difluoroalkene groups via a radical-polar crossover strategy. Central to the success of this process is the polarity matching between the benzenesulfinate radical and the glycosyl donor, which facilitates the initiation of the glycosyl radical and the subsequent formation of gem-difluoroalkene sugar derivatives. This method demonstrated good compatibility with various glycosyl donors and functional groups. Furthermore, we showcase the utility of this method in modifying amino acids, potentially paving the way for analogous modifications to peptides.

Synthesis and Biological Activities of Scleropentaside D

We report the first total synthesis of scleropentaside D, a unique C-glycosidic ellagitannin, from the ketal derivative of scleropentaside A employing site-selective O4-protection of C-acyl glycoside and copper-catalyzed oxidative coupling reaction of galloyl groups as the key steps. Our study confirms the proposed structure of this natural product, scleropentaside D, and demonstrates its effectiveness as an inhibitor of α-glycosidase.

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.

Pd-catalyzed stereoselective synthesis of chromone C-glycosides

Herein, we present an efficient Pd-catalysed method for stereoselective synthesis of chromone C-glycosides from various glycals. We successfully applied this method to various glycals with different protecting groups, yielding the corresponding glycosides in 41-78% yields. Additionally, we investigated the potential of this approach for the late-stage modification of natural products and pharmaceutical compounds linked to glycals, leading to the synthesis of their respective glycosides. Furthermore, we extended our research to gram-scale synthesis and demonstrated its applicability in producing various valuable products, including 2-deoxy-chromone C-glycosides. In summary, our work introduces a novel library of chromone glycosides, which holds promise for advancing drug discovery efforts.

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.

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.

Visible-Light-Mediated Synthesis of C-Alkyl Glycosides via Glycosyl Radical Addition and Aryl Migration

A visible-light-induced glycoarylation of activated olefins has been accomplished. Glycosyl radicals are generated via radical transfer strategies between (TMS)3SiOH and glycosyl bromides. Subsequent radical translocation and rapid 1,4-aryl migration form β-sugar amide derivatives, and eight types of sugars are compatible with this reaction. Further, the cascade reaction produced a quaternary carbon center with good functional group adaptability and high regioselectivity in mild conditions.

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.

Copper-Catalyzed Stereoselective Borylation and Palladium-Catalyzed Stereospecific Cross-Coupling to Give Aryl C-Glycosides

Metabolically stable C-glycosides are an essential family of compounds in bioactive natural products, therapeutic agents, and biological probes. For their application, development of synthetic methods by connecting glycosides and aglycons with strict stereocontrol at the anomeric carbon, as well as with high functional-group compatibility and environmental compatibility is a pivotal issue. Although Suzuki-Miyaura-type C(sp³ )-C(sp² ) cross-coupling using glycosyl boronates is a potential candidate for the construction of C-glycosides, neither the cross-coupling itself nor the facile synthesis of the coupling precursor, glycosyl boronates, have been achieved to date. Herein, it was succeeded to develop a copper-catalyzed stereoselective one-step borylation of glycosyl bromides to glycosyl boronates and palladium-catalyzed stereospecific cross-coupling of β-glycosyl borates with aryl bromides to give aryl β-C-glycosides, in which the β-configuration of the anomeric carbon of the glycosyl trifluoroborates is stereoretentively transferred to that of the resulting aryl C-glycosides.

Visible-Light-Induced Copper-Catalyzed Asymmetric C(sp3)-C(sp3)-H Glycosylation: Access to C-Glycopeptides

Herein, a practical and highly efficient method for visible-light-induced copper-catalyzed N-aminoquinoline-directed asymmetric C(sp³)-C(sp³)-H glycosylation was reported. At the same time, C(sp³)-C(sp³)-H glycosylation of nondeoxysugars with amino acids to construct C-glycopeptides was achieved. This approach promoted the synthesis of various C-glycopeptides and provided a new model for the synthesis of C-glycoamino acids.

Preparation of glycosyl carboxylic acids via stereoselective synthesis and oxidative cleavage of C-vinyl glycosides

We have developed an improved cyanide-free strategy for the synthesis of glycosyl carboxylic acids, employing stereoselective C-vinyl glycosylation and oxidative cleavage of C-vinyl glycosides as key steps. Compared to our previous work, the amount of NaIO₄ required for the oxidative cleavage step is reduced significantly from 18 equivalents to 4.5 equivalents. This modification not only is advantageous in terms of operation and costs but also avoids the over-oxidation problem, thus greatly expanding the substrate scope, which is evidenced by the fact that 10 out of 21 glycosyl carboxylic acids synthesized are undocumented. With differently O5-protected furanosyl acids in hand, we demonstrate that an electron-rich protecting group is beneficial for the decarboxylative arylation of furanosyl carboxylic acids. This represents a rare example of protecting groups affecting the reaction efficiency in radical C-glycosylation. As C-vinyl glycosides can be prepared stereoselectively and the oxidative step is stereoretentive, the approach provides an effective means to access 1,2-trans or 1,2-cis glycosyl acids, which would be a valuable alternative to the cyanide-based synthesis of glycosyl carboxylic acids.

Visible-Light-Promoted Stereoselective C(sp3 )-H Glycosylation for the Synthesis of C-Glycoamino Acids and C-Glycopeptides

The glycosylative modification of peptides could improve the pharmacological properties of peptide drugs and deliver them efficiently to the target sites. Compared with O-/N-glycosides, C-glycosides exhibit more metabolic stability. We here disclose the first example of visible-light-promoted and Cu-catalyzed stereoselective C-glycosylation. The mild reaction conditions are compatible with various carbohydrate substrates, as demonstrated with a series of monosaccharides and a disaccharide, and are amenable to the synthesis of a wide variety of C-glycoamino acids and C-glycopeptidomimetics with good yields and excellent stereoselectivities. The dual-functional photocatalyst formed in situ via coordination of the glycine derivative and the chiral phosphine Cu complex could not only catalyze the photoredox process but also control the stereoselectivity of the glycosylation reaction.

C(sp3)–F Bond Transformation of Perfluoroalkyl Compounds Mediated by Visible-Light Photocatalysis: Spin-Center Shifts and Radical/Polar Crossover Processes via Anionic Intermediates

Due to its large bond energy, precisely controllable C–F bond activation is a significant challenge in organic synthesis. A single C(sp3)–F bond transformation of perfluoroalkyl groups is particularly desirable to supply functionalized perfluoroalkyl compounds offering properties that are potentially useful in pharmaceutical and materials chemistry. Recently, the single defluorinative transformation of perfluoroalkyl compounds has been developed via visible-light photocatalysis. Herein, we summarize this field via two main topics. Topic 1 covers the transformations of C(sp3)–F bonds in either perfluoroalkylarenes or perfluoroalkane carbonyl compounds via a defluorinative spin-center shift in the radical anion intermediates. Topic 2 addresses the defluorinative transformations of α-trifluoromethyl alkenes to give gem-difluoroalkenes via a radical/polar crossover process.

1 Introduction

2 C(sp3)–F Transformations via Defluorinative Spin-Center Shifts

3 C(sp3)–F Transformations via a Radical/Polar Crossover Process

4 Conclusions

De Novo Synthesis of Orthogonally-Protected C2-Fluoro Digitoxoses and Cymaroses: Development and Application for the Synthesis of Fluorinated Digoxin

Inspired by Roush's pioneering work on rare sugars, we have developed a scalable, stereoselective, de novo synthesis of orthogonally protected C2-fluoro digitoxose and cymarose, utilizing Sharpless kinetic resolution and organocatalytic fluorination as key steps. The utility of this strategy is demonstrated by the synthesis of a fluorinated analogue of digoxin, which indicates the fluorine on the sugar ring may have a significant impact on biological activity.