C-branched carbohydrates are of current interest for glycochemistry, are widely found in nature and serve as important subunits in many antibiotics, bacterial polysaccharides and macrolides. Among C-functionalized saccharides, 2-C-branched carbohydrates represent challenging structures for synthetic chemists, since in contrast to C-glycosides they are not easily accessible from glycosyl bromides or other simple precursors. In this perspective we want to summarize recent approaches to 2-C-branched carbohydrates over the past fifteen years. The two main strategies are based on ring-opening of 1,2-cyclopropanated carbohydrates by various reagents, as well as radical additions to glycals and further transformations, developed in our group. Both methods are characterized by high stereoselectivities and good yields and give access to a broad variety of functionalized carbohydrate 2-C-analogs.

Recent advances in the stereoselective synthesis of carbohydrate 2-C-analogs.

Applications of 2-C-Formyl-Glycals in Organic Synthesis

Preparation and reactions of iodo sugars

Treatment of methyl 2-C-formylmethyl-2-deoxy-β-D-glucopyranoside (5) or methyl 2-C-acetylmethyl-2-deoxy-β-D-glucopyranoside (1) with H2SO4–HOAc–Ac2O gave 2-acetoxyl-4,5-bis(benzyloxy)-6-[(benzyloxy)methyl]hexahydrofuro[2,3-b]pyran (6) and acetyl 2-C-acetylmethyl-2-deoxy-α-D-glucopyranoside (7) respectively, which were further reacted with nucleophiles in the presence of TMSOTf and offered a series of 2-substituted and 2,2-disubstituted perhydrofuro[2,3-b]pyran derivatives in high yield with excellent diastereoselectivity.

A neighboring group participation strategy: direct and highly diastereoselective synthesis of 2-substituted and 2,2-bisubstituted perhydrofuro[2,3-b]pyran derivatives

This article reviews the progress in the area of radicals in carbohydrate chemistry with special attention to new developments published during the past decade. No attempt has been made to present a comprehensive review of the literature but, rather, some specific examples are given to highlight the most synthetically valuable processes. The major focus is on inter- and intramolecular carbon–carbon bond-forming radical reactions, with emphasis on the preparation of C-glycosides, C-ketosides, C-disaccharides, and branched-chain sugars, as well as on the synthesis of polyfunctionalized carbocycles by radical cyclization of acyclic carbohydrate derivatives. We also describe methods for the formation of carbon–hydrogen bonds, and synthesis in which a heteroatom-centered radical plays a clear role in the carbon–heteroatom (C–N, C–P, C–O, C–S/Se, and C–Br) bond formation. Finally, a few selected examples of alkoxyl radical β-fragmentation reactions, carried out under oxidative or reductive conditions, are included to illustrate their potential in the preparation of chiral synthons.


Keywords:

carbohydrates;
radicals;
C-glycosides;
C-disaccharides;
carbasugars;
glycos-1-yl radicals;
carbon-centered radicals;
heteroatom-centered radicals

/pdf/radicals-and-carbohydrates-5cypsoma9m.pdf

Radicals and Carbohydrates

https://www.chem.ucla.edu/~jung/pdfs/194.pdf

Preparation of various C-2 branched carbohydrates using intramolecular radical reactions.

A new and efficient method for the facile synthesis of C-2 branched carbohydrates has been developed using an intramolecular radical cyclization–fragmentation reaction. The desired C-2 branched glucopyranosides were isolated in 40–84% yield. Additionally, an unexpected furanoside was obtained from a tributyltin iodide-promoted rearrangement of the radical intermediate. The C-2 formyl glycal was also isolated in good yield using tris(trimethylsilyl)silane (TTMSS) as the reducing agent. This method was extended to synthesize a β C-2 branched glucopyranoside, a C-2 branched galactoside and a C-2 cyano glucopyranoside.

Sung Won Tina Choe

Papers

Preparation of various C-2 branched carbohydrates using intramolecular radical reactions.

Preparation of Various C‐2 Branched Carbohydrates Using Intramolecular Radical Reactions.