Photoinitiated Glycosylation at 350 nm

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.

Photosensitizer-free visible-light-promoted glycosylation enabled by 2-glycosyloxy tropone donors

Photochemical glycosylation has attracted considerable attention in carbohydrate chemistry. However, to the best of our knowledge, visible-light-promoted glycosylation via photoactive glycosyl donor has not been reported. In the study, we report a photosensitizer-free visible-light-mediated glycosylation approach using a photoactive 2-glycosyloxy tropone as the donor. This glycosylation reaction proceeds at ambient temperature to give a wide range of O-glycosides or oligosaccharides with yields up to 99%. This method is further applied in the stereoselective preparation of various functional glycosyl phosphates/phosphosaccharides, the construction of N-glycosides/nucleosides, and the late-stage glycosylation of natural products or pharmaceuticals on gram scales, and the iterative synthesis of hexasaccharide. The protocol features uncomplicated conditions, operational simplicity, wide substrate scope (58 examples), excellent compatibility with functional groups, scalability of products (7 examples), and high yields. It provides an efficient glycosylation method for accessing O/N-glycosides and glycans.

Benzyne-Promoted, 1,2-cis-Selective O-Glycosylation with Benzylchalcogenoglycoside Donors

Here, we show that the reaction of benzylchalcogenoglycosides with benzyne in the presence of alcohols results in highly 1,2-cis-selective O-glycosylation in a solvent-dependent manner. Thioglycosides, selenoglycosides, and alcohols with a range of nucleophilicities lead to a productive reaction, and unusual protecting groups, auxiliary groups, and additives are avoided.

Anomeric Thioglycosides Give Different Anomeric Product Distributions under NIS/TfOH Activation

The reaction of a series of anomeric thioglycosides with various glycosyl acceptors and N-iodosuccinimide/catalytic triflic acid was investigated with respect to reactivity and anomeric selectivity. In general, β-configured donors were found to give a more β-selective reaction outcome compared to their α-configured counterparts. The relative reactivity of various thioglycosides was measured through competition experiments, and the following order was established: phenyl, tolyl, methyl, ethyl, isopropyl, and 1-adamantyl.

Visible-light-promoted 3,5-dimethoxyphenyl glycoside activation and glycosylation

A new glycosylation method promoted by visible light with 3,5-dimethoxyphenyl glycoside as the donor was developed. This protocol delivers both O-glycosides and N-glycosides in moderate to excellent yields using a wide range of O-nucleophiles and nucleobases as the glycosyl acceptors.

On-water synthesis of glycosyl selenocyanate derivatives and their application in the metal free organocatalytic preparation of nonglycosidic selenium linked pseudodisaccharide derivatives

Glycosyl selenocyanate derivatives were prepared in very good yield by the treatment of glycosyl halide or triflate derivatives with potassium selenocyanate in water. A variety of selenium linked pseudodisaccharide derivatives were prepared in excellent yield using glycosyl selenocyanates as stable building blocks in the presence of hydrazine hydrate under metal-free organocatalytic reaction conditions.

Room-Temperature Pd-Catalyzed Synthesis of 1-(Hetero)aryl Selenoglycosides

A general protocol for functionalization of an anomeric selonate anion at room temperature has been reported. By using the PdG3 XantPhos catalyst, the cross-coupling between the in situ-generated glycosyl selenolate and a broad range of (hetero)aryl and alkenyl iodides furnished a series of functionalized selenoglycosides in excellent yields with perfect control of the anomeric configuration.

Blue Light Photocatalytic Glycosylation without Electrophilic Additives

Photocatalytic formation of glycosidic bonds employing stable and readily accessible O-glycosyl derivatives of 2,2,6,6-tetramethylpiperidin-1-ol is presented that employs an iridium-based photocatalyst and blue LEDs. The reaction proceeds at room temperature and in the absence of additives other than 4 Å molecular sieves. Stereoselectivities are modest but nevertheless dependent on the anomeric configuration of the donor, suggesting a substantial degree of concerted character.

Recent advances in photoinduced glycosylation: oligosaccharides, glycoconjugates and their synthetic applications

Carbohydrates have been demonstrated to perform imperative act in biological processes. This review highlights recent uses of photoinduced glycosylation in carbohydrate chemistry for the synthesis of oligosaccharides, thiosugars, glycoconjugates and glycoprotein.

Dual Catalysis Strategies in Photochemical Synthesis

The interaction between an electronically excited photocatalyst and an organic molecule can result in the genertion of a diverse array of reactive intermediates that can be manipulated in a variety of ways to result in synthetically useful bond constructions. This Review summarizes dual-catalyst strategies that have been applied to synthetic photochemistry. Mechanistically distinct modes of photocatalysis are discussed, including photoinduced electron transfer, hydrogen atom transfer, and energy transfer. We focus upon the cooperative interactions of photocatalysts with redox mediators, Lewis and Brønsted acids, organocatalysts, enzymes, and transition metal complexes.

Light-driven highly efficient glycosylation reactions

A novel and efficient light-driven glycosylation strategy was developed with wide substrate scope/generality.

A propos of glycosyl cations and the mechanism of chemical glycosylation; the current state of the art

An overview of recent advances in glycosylation with particular emphasis on mechanism is presented. The mounting evidence for both the existence of glycosyl oxocarbenium ions as fleeting intermediates in some reactions, and the crucial role of the associated counterion in others is discussed. The extremes of the SN1 and SN2 manifolds for the glycosylation reaction are bridged by a continuum of mechanisms in which it appears likely that most examples are located.

Photo-induced glycosylation using reusable organophotoacids

The glycosylation reactions of glycosyl trichloroacetimidates and several alcohols using an organophotoacid as an activator under photoirradiation proceeded smoothly to give the corresponding glycosides in high yields. The organophotoacid was recovered and reused without any loss of efficiency.

Light-induced O-glycosylation of unprotected deoxythioglycosyl donors

The O-glycosylation of several unprotected deoxythioglycosides and alcohols using 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) under long-wavelength UV irradiation was effectively realized using a single electron transfer (SET) mechanism. In some cases, the use of p-methoxyboronic acid was very effective in temporary protection of the 1,3-diol (C4 and C6 positions) of the unprotected glycosyl donors to increase the yields of the obtained O-glycosides.

An α-selective, visible light photocatalytic glycosylation of alcohols with selenoglycosides

Exceptionally mild procedures for the visible light photocatalytic activation of selenoglycoside donors in the presence of alcohol acceptors have been developed. This process is demonstrated with both 1-phenylselenyl-2,3,4,6-tetra-O-benzyl glucoside (1) and 1-phenylselenyl-2,3,4,6-tetra-O-benzyl galactoside (2). Catalysis is effected with both metal (Ru(bpy)3) and organocatalysts (diphenyldiselenide). Reactions afford, in all cases, primarily the α-anomers with selectivities that vary with solvent. This represents the first example of a visible light-promoted O-glycosylation.