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

Stereoselective P(III)-Glycosylation for the Preparation of Phosphinated Sugars

Most of the reported work focus on the development of O-, N-, C- and S-glycosylation methods. However, no study explores P(III)-glycosylation reaction. Herein we describe a convenient protocol to realize P(III)-glycosylation process. A simple β-phosphino ester is adopted as P(III)-transfer reagent for this new type of glycosylation via a nucleophilic substitution and release strategy. Diverse phosphine units are introduced to the anomeric center of various sugars efficiently and with excellent stereoselectivity. The value of this method is showcased by the prepared P(III)-sugars as novel linkers in bioactive molecule conjugation, new chiral ligands in metal-catalyzed asymmetric allylic substitutions and organocatalysts. Preliminary mechanistic studies corroborated the designed P(III)-transfer process.

Visible-light-promoted direct desulfurization of glycosyl thiols to access C-glycosides

C-Glycosides are essential for the study of biological processes and the development of carbohydrate-based drugs. Despite the tremendous hurdles, glycochemists have often fantasized about the efficient, highly stereoselective synthesis of C-glycosides with the shortest steps under mild conditions. Herein, we report a desulfurative radical protocol to synthesize C-alkyl glycosides and coumarin C-glycosides under visible-light induced conditions without the need of an extra photocatalyst, in which stable and readily available glycosyl thiols that could be readily obtained from native sugars are activated in situ by pentafluoropyridine. The benefits of this procedure include high stereoselectivity, broad substrate scope, and easy handling. Mechanistic studies indicate that the in situ produced tetrafluoropyridyl S-glycosides form key electron donor-acceptor (EDA) complexes with Hantzsch ester (for C-alkyl glycosides) or Et3N (for coumarin C-glycosides), which, upon irradiation with visible light, trigger a cascade of glycosyl radical processes to access C-glycosides smoothly.

Palladium-Catalyzed Carbonylative Cyclization of 1-Alkynyl-2-iodo-d-glucal

Cascade reactions are important synthetic tools for the synthesis of heterocyclic molecules, particularly those catalyzed by palladium. Herein, we report a palladium-catalyzed aminocarbonylative cyclization of new 1-alkynyl-2-iodo-d-glucals, which undergo a tandem carbonylative cyclization in the presence of various amine nucleophiles. A broad range of aromatic and aliphatic amines were applied as coupling partners, resulting in the selective and high-yield synthesis of glycosides fused to pyridinones. A plausible mechanism is proposed, proceeding via a tandem palladium aminocarbonylation followed by a palladium-catalyzed endo-dig cyclization.

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.

Late-Stage Glycosylation of Peptides by Methionine-Directed β-C(sp3)-H Functionalization with 1-Iodoglycals

Using l-methionine (Met) as the endogenous directing group, we developed Pd-catalyzed β-C(sp3)-H glycosylation of peptides with 1-iodoglycals. A wide range of tri- to hexapeptides containing the Ala-Met motifs underwent Ala C-H glycosylation under the standard conditions to give the glycopeptides smoothly. 15 proteinogenic amino acids (with easily removable protecting groups) were well tolerated. Control experiments indicated that Met acted as a N,S-bidentate directing group and exhibited an effect superior to other amino acid residues such as l-aspartic acid (Asp), l-asparagine (Asn), and S-protected l-cysteine (Cys). In addition, further transformation by HFIP-promoted 1,4-elimination furnished another type of glycopeptide with the 1,3-diene motif, which provides a handle for further derivatization.

Electrochemical Glycosylation via Halogen-Atom-Transfer for C-Glycoside Assembly

Glycosyl donor activation emerged as an enabling technology for anomeric functionalization, but aimed primarily at O-glycosylation. In contrast, we herein disclose mechanistically distinct electrochemical glycosyl bromide donor activations via halogen-atom transfer and anomeric C-glycosylation. The anomeric radical addition to alkenes led to C-alkyl glycoside synthesis under precious metal-free reaction conditions from readily available glycosyl bromides. The robustness of our e-XAT strategy was further mirrored by C-aryl and C-acyl glycosides assembly through nickela-electrocatalysis. Our approach provides an orthogonal strategy for glycosyl donor activation with expedient scope, hence representing a general method for direct C-glycosides assembly.

Palladium-catalyzed Suzuki-Miyaura cross-couplings of stable glycal boronates for robust synthesis of C-1 glycals

C-1 Glycals serve as pivotal intermediates in synthesizing diverse C-glycosyl compounds and natural products, necessitating the development of concise, efficient and user-friendly methods to obtain C-1 glycosides is essential. The Suzuki-Miyaura cross-coupling of glycal boronates is notable for its reliability and non-toxic nature, but glycal donor stability remains a challenge. Herein, we achieve a significant breakthrough by developing stable glycal boronates, effectively overcoming the stability issue in glycal-based Suzuki-Miyaura coupling. Leveraging the balanced reactivity and stability of our glycal boronates, we establish a robust palladium-catalyzed glycal-based Suzuki-Miyaura reaction, facilitating the formation of various C(sp2)-C(sp), C(sp2)-C(sp2), and C(sp2)-C(sp3) bonds under mild conditions. Notably, we expand upon this achievement by developing the DNA-compatible glycal-based cross-coupling reaction to synthesize various glycal-DNA conjugates. With its excellent reaction reactivity, stability, generality, and ease of handling, the method holds promise for widespread appication in the preparation of C-glycosyl compounds and natural products.

Stereoselective assembly of C-oligosaccharides via modular difunctionalization of glycals

C-oligosaccharides are found in natural products and drug molecules. Despite the considerable progress made during the last decades, modular and stereoselective synthesis of C-oligosaccharides continues to be challenging and underdeveloped compared to the synthesis technology of O-oligosaccharides. Herein, we design a distinct strategy for the stereoselective and efficient synthesis of C-oligosaccharides via palladium-catalyzed nondirected C1-H glycosylation/C2-alkenylation, cyanation, and alkynylation of 2-iodoglycals with glycosyl chloride donors while realizing the difunctionalization of 2-iodoglycals. The catalysis approach tolerates various functional groups, including derivatives of marketed drugs and natural products. Notably, the obtained C-oligosaccharides can be further transformed into various C-glycosides while fully conserving the stereochemistry. The results of density functional theory (DFT) calculations support oxidative addition mechanism of alkenyl-norbornyl-palladacycle (ANP) intermediate with α-mannofuranose chloride and the high stereoselectivity of glycosylation is due to steric hindrance.

Synthesis of C-Oligosaccharides via Ni-Catalyzed Reductive Hydroglycosylation

C-Oligosaccharides are metabolically stable surrogates of native glycans containing O/N/S-glycosidic linkages and thus have therapeutic potential. Here we report a straightforward approach to the synthesis of vinyl C-linked oligosaccharides via the Ni-catalyzed reductive hydroglycosylation of alkynyl glycosides with glycosyl bromides.

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.

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.

Remote C-H Glycosylation by Ruthenium(II) Catalysis: Modular Assembly of meta-C-Aryl Glycosides

The prevalence of C-aryl glycosides in biologically active natural products and approved drugs has long motivated the development of efficient strategies for their selective synthesis. Cross-couplings have been frequently used, but largely relied on palladium catalyst with prefunctionalized substrates, while ruthenium-catalyzed C-aryl glycoside preparation has thus far proven elusive. Herein, we disclose a versatile ruthenium(II)-catalyzed meta-C-H glycosylation to access meta-C-aryl glycosides from readily available glycosyl halide donors. The robustness of the ruthenium catalysis was reflected by mild reaction conditions, outstanding levels of anomeric selectivity and exclusive meta-site-selectivity.

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.