Transition-Metal-Free Synthesis of C-Glycosylated Phenanthridines via K2S2O8-Mediated Oxidative Radical Decarboxylation of Uronic Acids

Synthesis of C4-Acyl Glycosides by Cross-Ketonization: The Importance of Precise Temperature Control in Photochemistry

We describe herein a method for the synthesis of reversed C-acyl glycosides by direct cross-ketonization of linear carboxylic acids with carboxy sugars under metallaphotoredox conditions. This reaction proves to be highly sensitive to reaction temperature, with the reproducibility and yield of ketonization highly dependent upon precise temperature control. Analysis of the reaction mechanism suggests this sensitivity reflects the need to match the rate of Ni catalytic turnover with the rate of photocatalytic decarboxylation of the carboxy sugar. The insights gained from this study have important implications for the study of all metallaphotoredox reactions as well as other photoreactions that involve dual catalytic cycles.

Pd-Catalyzed Stereospecific Glycosyl Cross-Coupling of Reversed Anomeric Stannanes for Modular Synthesis of Nonclassical C-Glycosides

Nonclassical C-glycosides, distinguished by their unique glycosidic bond connection mode, represent a promising avenue for the development of carbohydrate-based drugs. However, the accessibility of nonclassical C-glycosides hinders broader investigations into their structural features and modes of action. Herein, we present the first example of Pd-catalyzed stereospecific glycosylation of nonclassical anomeric stannanes with aryl or vinyl halides. This method furnishes desired nonclassical aryl and vinyl C-glycosides in good to excellent yields, while allowing for exclusive control of nonclassical anomeric configuration. Of significant note is the demonstration of the generality and practicality of this nonclassical C-glycosylation approach across more than 50 examples, encompassing various protected and unprotected saccharides, deoxy sugars, oligopeptides, and complex molecules. Furthermore, biological evaluation indicates that nonclassical C-glycosylation modifications of drug molecules can positively impact their biological activity. Additionally, extensive computational studies are conducted to elucidate the rationale behind differences in reaction reactivity, unveiling a transmetalation transition state containing silver (Ag) within a six-membered ring. Given its remarkable controllability, predictability, and consistently high chemical selectivity and stereospecificity regarding nonclassical anomeric carbon and Z/E configuration, the method outlined in this study offers a unique solution to the longstanding challenge of accessing nonclassical C-glycosides with exclusive stereocontrol.

Access to Reverse Glycosyl Azides and Rare Sugar-Based Glycosyl Azides via Radical Decarboxylative Azidation: Divergent Synthesis of 4'-C-Azidonucleosides as Potential Antiviral Agents

The radical decarboxylative azidation of structurally diverse uronic acids has been established as an efficient approach to reverse glycosyl azides and rare sugar-derived glycosyl azides under the action of Ag2CO3, 3-pyridinesulfonyl azide, and K2S2O8. The power of this method has been highlighted by the divergent synthesis of 4'-C-azidonucleosides using Vorbrüggen glycosylation of nucleobases with 4-C-azidofuranosyl acetates. The antiviral assessment of the resulting nucleosides revealed one compound as a potential inhibitor of covalently closed circular DNA.

Generation and Coupling of Radical Species from α-Alkoxy Bridgehead Carboxylic Acid, Selenide, Telluride, Acyl Selenide, and Acyl Telluride

α-Alkoxy bridgehead radicals enable intermolecular construction of sterically congested C-C bonds due to their sterically accessible nature. We implemented these radical species into total syntheses of various densely oxygenated natural products and demonstrated their exceptional versatility. Herein, we employed different precursors to generate the same α-alkoxy bridgehead radical and compared the efficacy of the precursors for coupling reactions. Specifically, the bridgehead radical of the trioxaadamantane structure was formed from α-alkoxy carboxylic acid, selenide/telluride, and acyl selenide/acyl telluride, and reacted with 4-((tert-butyldimethylsilyl)oxy)cyclopent-2-en-1-one and 5-oxo-1-cyclopentene-1-carbonitrile. The efficiency of the bridgehead radical formation and subsequent coupling reaction significantly depended on the structures of the precursors and acceptors as well as the reaction conditions. Our findings provide new insights for selecting the appropriate substrates of key coupling reactions in the total synthesis of complex natural products.

Photoredox-catalyzed C-heteroaryl glycosylation of biphenyl isocyanides with glycosyl bromides

8,9-Dimethoxyphenanthridine derivatives, as potential antitumor drugs, need modification to improve their biocompatibility and water solubility. Reported here is a strategy to access C-heteroaryl glycosides by photoredox catalysis. C6-glycosylated phenanthridine derivatives are synthesized from biphenyl isocyanides and glycosyl bromides. The reaction conditions are mild and widely applicable, with anomeric α selectivity and good functional group tolerance.

Total Synthesis of Nikkomycin Sz and Nikkomycin Soz

The nikkomycins Sz/Soz are a class of locked nucleoside antibiotics that share a common [5,6] trans-bicyclic core. Herein we present an efficient synthesis of these nikkomycins from diene, using neighboring group participation N-glycosylation and stereoselective oxidation state installation. This synthetic strategy overcomes several challenges due to the poor redox tolerance of the uracil base, the high strain of the trans-fused furanopyran C8 monosaccharides, and the acid-sensitive glycosidic bond when dealing with the deoxynucleotide natural product nikkomycin Sz.

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 Reverse Glycosyl Fluorides via Organophotocatalytic Decarboxylative Fluorination of Uronic Acids

An efficient protocol for organophotocatalytic synthesis of reverse glycosyl fluorides (RGFs) is established relying on 9-mesityl-10-methyl-acridinium (Mes-Acr+)-mediated oxidative decarboxylative fluorination of uronic acids. Both pentofuranoid and hexopyranoid uronic acids are...

Photocatalytic xanthate-based radical addition/cyclization reaction sequence toward 2-biphenyl isocyanides: synthesis of 6-alkylated phenanthridines

A photocatalytic xanthate-based radical addition/cyclization reaction cascade toward 2-biphenylisocyanides is described as a practical and modular approach to 6-alkylated phenanthridines. The use of xanthates as radical precursors allowed the synthesis of diversely 6-substituted phenanthridines. Electrophilic radicals derived from nitriles, aromatic and aliphatic ketones, malonates, and amide derivatives, as well as radicals derived from phthalimidomethyl and benzylic derivatives were successfully introduced. The reaction proceeds under mild conditions without a stoichiometric amount of oxidant. Thirty novel phenanthridine scaffolds were synthesized with yields ranging from 24 to 76%.

Synthesis of Reverse Glycosyl Fluorides and Rare Glycosyl Fluorides Enabled by Radical Decarboxylative Fluorination of Uronic Acids

An efficient protocol for synthesizing reverse glycosyl fluorides is described, relying on silver-promoted decarboxylative fluorination of structurally diverse pentofuran- and hexopyranuronic acids under the mild reaction conditions. The potential applications of the reaction are further demonstrated by converting readily available d-uronic acid derivatives into uncommon d-/l-glycosyl fluorides through a C1-to-C5 switch strategy. The reaction mechanism is corroborated by 5-exo-trig radical cyclization of allyl α-d-C-glucopyranuronic acid triggered by decarboxylative fluorination.

Cyanide-Free Synthesis of Glycosyl Carboxylic Acids and Application for the Synthesis of Scleropentaside A

We have developed a cyanide-free strategy for the synthesis of glycosyl carboxylic acids, which can provide 1,2-trans or 1,2-cis glycosyl carboxylic acids and is compatible with common protecting groups. The synthetic utility was demonstrated by the synthesis of 12 unreported glycosyl acids and the total synthesis of scleropentaside A.

Radical Dehydroxymethylative Fluorination of Carbohydrates and Divergent Transformations of the Resulting Reverse Glycosyl Fluorides

A mild and convenient method for the synthesis of reverse glycosyl fluorides (RGFs) has been developed that is based on the silver-promoted radical dehydroxymethylative fluorination of carbohydrates. A salient feature of the reaction is that furanoid and pyranoid carbohydrates furnish structurally diverse RGFs bearing a wide variety of functional groups in good to excellent yields. Intramolecular hydrogen atom transfer experiments revealed that the reaction involves an underexploited radical fluorination that proceeds via β-fragmentation of sugar-derived primary alkoxyl radicals. Structurally divergent RGFs were obtained by catalytic C-F bond activation, and our method thus offers a concise and efficient strategy for the synthesis of reverse glycosides by late-stage diversification of RGFs. The potential of this method is showcased by the preparation and diversification of sotagliflozin, leading to the discovery of a promising SGLT2 inhibitor candidate.

Recent development in the synthesis of C-glycosides involving glycosyl radicals

C-Glycosylation involving glycosyl radical intermediates is a particularly effective approach to access C-glycosides, which are core units of a great number of natural products, bioactive compounds and marketed drugs.

Cleavage of carbon–carbon bonds by radical reactions

This review provides insights into the in situ generated radicals triggered carbon–carbon bond cleavage reactions.

Transformations of Isonitriles with Bromoalkanes Using Photoredox Gold Catalysis

Isonitriles have excellent electronic compatibility to react with free radicals. Recently, photoredox catalysis has emerged as a powerful tool for the construction of C-C bonds with few protocols for alkylative heterocycle synthesis through isonitrile addition. Herein, we describe the photocatalytic generation of alkyl radicals from unactivated bromoalkanes as part of an efficient cross-coupling strategy for the diversification of isonitriles using a dimeric gold(I) photoredox catalyst, [Au₂(dppm)₂]Cl₂.