Diastereoselective Decarboxylative Alkynylation of Anomeric Carboxylic Acids Using Cu/Photoredox Dual Catalysis

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.

Photocatalytic decarboxylation of free carboxylic acids and their functionalization

Visible light mediated decarboxylative functionalization of carboxylic acids and their derivatives has recently emerged as a novel and powerful toolkit for small molecule activation in diverse carbon-carbon and carbon-hetero bond forming reactions. Naturally abundant highly functionalized bench-stable carboxylic acid analogs have been employed as promising alternatives to non-trivial organometallic reagents for mild and eco-benign synthetic transformation with traceless CO2 by-products. In this highlight article, we focus on the development of various photodecarboxylative functionalization strategies along with intra/inter-molecular cyclization via concerted single electron transfer (SET) or energy transfer (ET) pathways. Moreover, widely explored carboxylic acids are systematically classified here into four categories; i.e., α-keto, aliphatic, α,β-unsaturated, and aromatic analogs for a concise overview to the readership. The association of decarboxylative radical species with coupling partners to construct C-C and C-N/O/S/P/X bonds for each analogous acid has been presented in brief.

Synthesis of C-Alkyl Glycosides from Alkyl Bromides and Glycosyl Carboxylic Acids via Ni/Photoredox Dual Catalysis

C-Alkyl glycosides, an important class of C-glycosides, are widely found in various drugs and natural products. The synthesis of C-alkyl glycosides has attracted considerable attention. Herein, we developed a Ni/photoredox catalyzed decarboxylative C(sp3)-C(sp3) coupling reaction of stable glycosylcarboxylic acids with simple aliphatic bromides to generate C-alkyl glycosides. The method successfully linked several functional molecular fragments (natural products or drugs) to a sugar moiety, showing the extensive application prospects of this transformation. Controlled experiments and DFT calculations demonstrated that the reaction pathway contains a free radical process, and a possible mechanism is proposed.

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.

Dehydroxylative radical N-glycosylation of heterocycles with 1-hydroxycarbohydrates enabled by copper metallaphotoredox catalysis

N-Glycosylated heterocycles play important roles in biological systems and drug development. The synthesis of these compounds heavily relies on ionic N-glycosylation, which is usually constrained by factors such as labile glycosyl donors, precious metal catalysts, and stringent conditions. Herein, we report a dehydroxylative radical method for synthesizing N-glycosides by leveraging copper metallaphotoredox catalysis, in which stable and readily available 1-hydroxy carbohydrates are activated for direct N-glycosylation. Our method employs inexpensive photo- and copper- catalysts and can tolerate some extent of water. The reaction exhibits a broad substrate scope, encompassing 76 examples, and demonstrates high stereoselectivity, favoring 1,2-trans selectivity for furanoses and α-selectivity for pyranoses. It also exhibits high site-selectivity for substrates containing multiple N-atoms. The synthetic utility is showcased through the late-stage functionalization of bioactive compounds and pharmaceuticals like Olaparib, Axitinib, and Metaxalone. Mechanistic studies prove the presence of glycosyl radicals and the importance of copper metallaphotoredox catalysis.

Developments in the chemistry and biology of 1,2,3-triazolyl-C-nucleosides

In the last 50 years, nucleoside analogs have been introduced to drug therapy as antivirals for different types of cancer due to their interference in cellular proliferation. Among the first line of nucleoside treatment drugs, ribavirin (RBV) is a synthetic N-nucleoside with a 1,2,4-triazole moiety that acts as a broad-spectrum antiviral. It is on the World Health Organization (WHO) list of essential medicines. However, this important drug therapy causes several side effects due to its nonspecific mechanism of action. There is thus a need for a continuous study of its scaffold. A particular approach consists of connecting  d-ribose to the nitrogen-containing base with a C-C bond. It provides more stability against enzymatic action and a better pharmacologic profile. The coronavirus disease (COVID) pandemic has increased the need for more solutions for the treatment of viral infections. Among these solutions, remdesivir, the first C-nucleoside, has been approved by the Food and Drug Administration (FDA) for clinical use against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). It drew attention to the study of the C-nucleoside scaffold. Different C-nucleoside patterns have been synthesized over the years. They show many important activities against viruses and cancer cell lines. 1,2,3-Triazolyl-C-nucleoside derivatives are a prolific and efficient subclass of RBV analogs close to the already-known RBV with a C-C bond modification. These compounds are often prepared by alkynylation of the  d-ribose ring followed by azide-alkyne cycloaddition. They are reported to be active against the Crimean-Congo hemorrhagic fever virus and several tumoral cell lines, showing promising biological potential. In this review, we explore such approaches to 1,2,3-triazolyl-C-nucleosides and their evolution over the years.

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.

"Boomerang" Strategy in Carbohydrate Chemistry: Diastereoselective Synthesis of C-Glycosylated Benzothiazoles from ortho-Isocyanophenyl Thioglycosides

This paper reveals a novel "boomerang" strategy in the expedient and diastereoselective synthesis of C-nucleoside analogues. Bench-stable ortho-isocyanophenyl thioglycosides can be converted to glycosyl radicals through rapid and efficient C-S bond homolysis when they are irradiated by visible light. The glycosyl radicals are subsequently trapped by the corresponding leaving group or other radical acceptors to provide diverse C-nucleoside analogues under mild conditions.

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.

Stereoselective alkyl C-glycosylation of glycosyl esters via anomeric C-O bond homolysis: efficient access to C-glycosyl amino acids and C-glycosyl peptides

C-Glycosyl peptides possess excellent metabolic stability and therapeutic properties and thus play critical roles in biological studies as well as drug discoveries. However, the limited accessibility of C-glycosyl amino acids has significantly hindered the broader research of their structural features and mode of action. Herein, for the first time we disclose a novel visible-light-driven radical conjugate addition of 1,4-dihydropyridine (DHP)-derived glycosyl esters with dehydroalanine derivatives, generating C-glycosyl amino acids and C-glycosyl peptides in good yields with excellent stereoselectivities. Redox-active glycosyl esters, as readily accessible and bench-stable radical precursors, could be easily converted to glycosyl radicals via anomeric C(sp³)-O bond homolysis under mild conditions. Importantly, the generality and practicality of this transformation were fully demonstrated in >40 examples including 2-dexosugars, oligosaccharides, oligopeptides, and complex drug molecules. Given its mild reaction conditions, robust sugar scope, and high anomeric control and diastereoselectivity, the method presented herein could find widespread utility in the preparation of C(sp³)-linked sugar-based peptidomimetics.

Photocatalytic Transformation of Biomass and Biomass Derived Compounds-Application to Organic Synthesis

Biomass and biomass-derived compounds have become an important alternative feedstock for chemical industry. They may replace fossil feedstocks such as mineral oil and related platform chemicals. These compounds may also be transformed conveniently into new innovative products for the medicinal or the agrochemical domain. The production of cosmetics or surfactants as well as materials for different applications are examples for other domains where new platform chemicals obtained from biomass can be used. Photochemical and especially photocatalytic reactions have recently been recognized as being important tools of organic chemistry as they make compounds or compound families available that cannot be or are difficultly synthesized with conventional methods of organic synthesis. The present review gives a short overview with selected examples on photocatalytic reactions of biopolymers, carbohydrates, fatty acids and some biomass-derived platform chemicals such as furans or levoglucosenone. In this article, the focus is on application to organic synthesis.

Recent advances on synthesis of C-glycosides

In recent years, C-glycosides have emerged as significant building blocks for many naturally occurring alkaloids and pharmaceutically active drug molecules. Therefore, significant efforts have been devoted to the construction of structurally important C-glycosidic linkages in carbohydrate compounds. Herein, we have summarized the recent developments of diverse synthesis of C-glycoside core between the time period from 2019 to 2022 focusing on different catalytic strategies, such as (i) transition-metal, and (ii) metal-free catalytic approaches. Further, the transition metal catalyzed C-glycosylations have been categorized into four sub classes: (a) metal based C-H activation, (b) cross-coupling reaction, (c) glycosyl radical intermediate-based process, and (d) Others.

One-pot synthesis of heteroaryl diketoalkynyl C-glycoside and dialkynyl di-C-glycoside analogues by three-component successive coupling reaction

The synthesis of heteroaryl diketoalkynyl C-glycoside and dialkynyl di-C-glycoside analogues has been accomplished by successive coupling of heteroaromatics, oxalyl chloride and terminal sugar alkynes in one pot. The three-component coupling reaction catalyzed by CuI gives heteroaryl diketoalkynyl C-glycosides. The same three-component coupling in the presence of n-BuLi produces dialkynyl di-C-glycosides, and the 1:1 of molar ratio of heteroaromatics to terminal sugar alkynes affords the corresponding esters of dialkynyl di-C-glycosides. The desired products have been obtained in good to excellent yields. This sequential one-pot method is mild and efficient, suitable for different heteroaromatics and terminal sugar alkynes. The sugar alkynes include furanosides, pyranosides, and acyclic sugars. Twenty-seven examples have been given. The mechanism for the formation of the desired products has been elucidated.

CuX Dual Catalysis: Construction of Oxazolo[2,3-b][1,3]oxazines via a Tandem CuAAC/Ring Cleavage/[4+2+3] Annulation Reaction

CuX as a simple dual catalyst strategy that promotes the tandem transformations of fused oxazolo[2,3-b][1,3]oxazines has been developed. Copper catalyzed terminal ynones, sulfonyl azides, and nitriles for the CuAAC/ring cleavage/[4+2] annulation reaction, while the halogen catalyzed ring cleavage and [2+3] annulation of oxiranes to form the final fused products. This study provides a four-component, one-pot strategy for synthesizing complex fused heterocycles from simple ingredients and expands the application of CuAAC in organic synthesis.

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.

Dealkenylative Alkynylation Using Catalytic FeII and Vitamin C

In this paper, we report the synthesis of alkyl-tethered alkynes through ozone-mediated and FeII-catalyzed dealkenylative alkynylation of unactivated alkenes in the presence of alkynyl sulfones. This one-pot reaction, which employs a combination of a catalytic FeII salt and l-ascorbic acid, proceeds under mild conditions with good efficiency, high stereoselectivity, and broad functional group compatibility. In contrast to our previous FeII-mediated reductive fragmentation of α-methoxyhydroperoxides, the FeII-catalyzed process was devised through a thorough kinetic analysis of the multiple competing radical (redox) pathways. We highlight the potential of this dealkenylative alkynylation through multiple post-synthetic transformations and late-stage diversifications of complex molecules, including natural products and pharmaceuticals.

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.

Synthesis of C-Oligosaccharides through Versatile C(sp3 )-H Glycosylation of Glycosides

C-oligosaccharides are pharmacologically relevant because they are more hydrolysis-resistant than O-oligosaccharides. Despite indisputable advances, C-oligosaccharides continue to be underdeveloped, likely due to a lack of efficient and selective strategies for the assembly of the interglycosidic C-C linkages. In contrast, we, herein, report a versatile and robust strategy for the synthesis of structurally complex C-oligosaccharides via catalyzed C(sp3 )-H activations. Thus, a wealth of complex interglycosidic (2→1)- and (1→1)-C-oligosaccharides becomes readily available by palladium-catalyzed C(sp3 )-H glycoside glycosylation. The isolation of key palladacycle intermediates and experiments with isotopically-labeled compounds identified a trans-stereoselectivity for the C(sp3 )-H glycosylation. The glycoside C(sp3 )-H activation manifold was likewise exploited for the diversification of furanoses, pyranoses and disaccharides.

Synthesis of C-Alkynyl Glycosides by Photoredox-Catalyzed Reductive Coupling of Alkynyl Bromides with Glycosyl Bromides

A general, convenient, and highly α stereoselective approach to access C-alkynyl glycosides via the photoredox-catalyzed reductive coupling of alkynyl bromides and glycoside bromides has been developed. Cheap and small-load eosin Y is used as the photocatalyst, and organic base N,N-diisopropylethylamine serves as the reducing reagent. This strategy features readily available starting materials, diverse substrates, mild conditions, and high α stereoselectivity. Moreover, a glycoconjugated peptide could also be achieved using this strategy.

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

We have developed a stereoselective, glycosyl radical-based method for the synthesis of C-alkyl glycosides via a photomediated defluorinative gem-difluoroallylation reaction. We demonstrate for the first time that glycosyl radicals, generated from glycosyl bromides, can readily participate in a photomediated radical polar crossover process, affording a diverse array of gem-difluoroalkene containing C-glycosides. Notable features of this method include scalability, mild conditions, broad substrate scope, and suitability for the late-stage modification of complex molecules.

Synthesis of C-acyl furanosides via the cross-coupling of glycosyl esters with carboxylic acids

C-Acyl furanosides are versatile synthetic precursors to a variety of natural products, nucleoside analogues, and pharmaceutical molecules. This report addresses the unmet challenge in preparing C-acyl furanosides by developing a cross-coupling reaction between glycosyl esters and carboxylic acids. A key step is the photoredox activation of the glycosyl ester, which promotes the homolysis of the strong anomeric C–O bond through CO₂ evolution to afford glycosyl radicals. This method embraces a large scope of furanoses, pyranoses, and carboxylic acids, and is readily applicable to the synthesis of a thymidine analogue and diplobifuranylone B, as well as the late-stage modification of (+)-sclareolide. The convenient preparation of the redox active glycosyl ester from native sugars and the compatibility with common furanoses exemplifies the potential of this method in medicinal chemistry.

A cross-coupling of glycosyl esters with carboxylic acids to prepare C-acyl furanosides and pyranosides. The reaction proceeds through photoredox activation of the glycosyl ester to afford glycosyl radicals.