Reductive fragmentation of carbohydrate anomeric alkoxy radicals. Synthesis of alditols with potential utility as chiral synthons

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.

Total Synthesis of (+)-Polyoxamic Acid via Visible-Light-Mediated Photocatalytic β-Scission and 1,5-Hydrogen Atom Transfer of Glucose Derivative

A total synthesis of polyoxamic acid has been achieved. The key feature of the synthetic route is a visible-light-mediated β-scission and carbon-to-carbon 1,5-hydrogen atom transfer (1,5-HAT) to provide the functionalized alditol under mild conditions. This type of carbon-to-carbon 1,5-HAT initiated by C(sp³)-centered radicals has been scarcely reported. Furthermore, the reaction was adapted for flow chemistry, facilitating the total synthesis of polyoxamic acid.

Synthetic Approaches to Phosphasugars (2-oxo-1,2-oxaphosphacyclanes) Using the Anomeric Alkoxyl Radical β-Fragmentation Reaction as the Key Step

The anomeric alkoxyl radical β-fragmentation (ARF) of carbohydrates possessing an electron-withdrawing group (EWG) at C2, promoted by PhI(OAc)₂/I₂, gives rise to an acyclic iodide through which a pentavalent atom of phosphorus can be introduced via the Arbuzov reaction. After selective hydrolysis and subsequent cyclization, the phosphonate or phosphinate intermediates can be converted into 2-deoxy-1-phosphahexopyranose and 2-deoxy-1-phosphapentopyranose sugars. The ARF of carbohydrates with an electron-donor group (EDG) at C2 proceeds by a radical-polar crossover mechanism, and the cyclization occurs by nucleophilic attack of a conveniently positioned phosphonate or phosphinate group to the transient oxocarbenium ion. This alternative methodology leads to 5-phosphasugars with a 4-deoxy-5-phosphapentopyranose framework. The structure and conformation of the 2-oxo-1,2-oxaphosphinane and 2-oxo-1,2-oxaphospholane ring systems in different carbohydrate models have been studied by NMR and X-ray crystallography.

Iron-Catalyzed Radical Cleavage/C-C Bond Formation of Acetal-Derived Alkylsilyl Peroxides

A novel radical-based approach for the iron-catalyzed selective cleavage of acetal-derived alkylsilyl peroxides, followed by the formation of a carbon-carbon bond is reported. The reaction proceeds under mild reaction conditions and exhibits a broad substrate scope with respect to the acetal moiety and the carbon electrophile. Mechanistic studies suggest that the present reaction proceeds through a free-radical process involving carbon radicals generated by the homolytic cleavage of a carbon-carbon bond within the acetal moiety. A synthetic application of this method to sugar-derived alkylsilyl peroxides is also described.

Glycosyl nitrates in synthesis: streamlined access to glucopyranose building blocks differentiated at C-2

In an attempt to refine a CAN-mediated synthesis of 1,3,4,6-tetra-O-acetyl-α-d-glucopyranose (2-OH glucose) we unexpectedly discovered that this reaction proceeds via the intermediacy of glycosyl nitrates. Improved mechanistic understanding of this reaction led to the development of a more versatile synthesis of 2-OH glucose from a variety of precursors. Also demonstrated is the conversion of 2-OH glucose into a variety of building blocks differentially protected at C-2, a position that is generally hard to protect regioselectively in the glucopyranose series.

Immobilization of glycans on solid surfaces for application in glycomics

Carbohydrates are an important class of biomolecules which are involved in a multitude of cellular functions. In the field of glycomics, the structure and function of various carbohydrates, oligosaccharides, glycans and their conjugates are constantly under investigation. In the continuing quest to understand the roles of carbohydrates in their interactions with proteins, immunogens, and other cell-surface carbohydrates, scientists have developed methods for observing the effects of specific saccharide sequences on various cellular components. Carbohydrate immobilization has allowed researchers to study the impact of specific sequences, leading to a deeper understanding of many cellular processes. The goal of this review is to highlight the chemical reactions and interactions that have been used for glycan immobilization.

Anti-inflammatory components of Euphorbia humifusa Willd

Two new compounds, euphorbinoside (1) and dehydropicrorhiza acid methyl diester (2), along with 24 known compounds (3-26) were isolated from Euphorbia humifusa Willd. The effects of these compounds on soluble epoxide hydrolase (sEH) inhibitory activity were evaluated. Flavonoid compounds (10-21) exhibited high sEH inhibitory activity. Among them, compounds 12, 13, and 19 greatly inhibited sEH enzymatic activity, with IC50 values as low as 18.05±1.17, 18.64±1.83, and 17.23±0.84 μM, respectively. In addition, the effects of these compounds on lipopolysaccharide (LPS)-induced nitric oxide (NO) and tumor necrosis factor alpha (TNF-α) production by RAW 264.7 cells were investigated. Compounds 3-6, 8, 18, 20-23, and 25-26 inhibited the production of both NO and TNF-α, with IC50 values ranging from 11.1±0.9 to 45.3±1.6 μM and 14.4±0.5 to 44.5±1.2 μM, respectively.

Beneficial Effect of Internal Hydrogen Bonding Interactions on the β-Fragmentation of Primary Alkoxyl Radicals. Two-Step Conversion of d-Xylo- and d-Ribofuranoses into l-Threose and d-Erythrose, Respectively

Primary alkoxyl free radicals were generated from their readily synthesized N-phthalimido derivatives under reductive conditions. Primary alkoxyl radicals derived from their corresponding xylo- and ribofuranose derivatives underwent, exclusively, an unusual beta-fragmentation affording L-threose and D-erythrose derivatives, respectively. This occurs because the alkoxyl radical is capable of achieving an internal hydrogen-bonding interaction leading to a stable six-membered ring intramolecular hydrogen-bonded structure. When the hydroxyl group is protected, the beta-fragmentation pathway is prevented and the hydrogen atom transfer (HAT) pathway occurs. Computational studies provided strong support for the experimental observations.