1,2-cis Alkyl glycosides: straightforward glycosylation from unprotected 1-thioglycosyl donors

Stereoselective protecting-group-free synthesis of alkyl glycosides using dibenzyloxy triazine type glycosyl donors

Chemical O-glycosylation is a key step for the synthesis of sugar-containing molecules such as glycolipids. However, traditional carbohydrate chemistry is characterized by extensive use of protective groups, resulting in laborious manipulations and poor atom economy. Here, we present a protecting-group-free glycosylation strategy employing dibenzyloxy-1,3,5-triazin-2-yl glycosides (DBT-glycosides) as glycosyl donors. The DBT-glycosyl donors could be prepared directly through an alkaline nucleophilic substitution from unprotected sugars in aqueous media. The O-glycosylation of alcohols by using DBT-glycosyl donors has been carried out under mild hydrogenolytic conditions, affording the corresponding alkyl glycosides stereo-selectively in good yields.

Stereoselective oxidative O-glycosylation of disarmed glycosyl iodides with alcohols using PIDA as the promoter

The direct and practical oxidative anomeric O-glycosylation of glycosyl iodides with an array of alcohols as glycosyl acceptors is presented. Using phenyliodine(III) diacetate (PIDA) as the promoter of the reaction, at ambient temperature, an enviromentally benign and operationally simple protocol has been developed providing access stereoselectively to 1,2-trans-O-glycosides.

Selectivity of 1-O-Propargyl-d-Mannose Preparations

Thanks to their ability to bind to specific biological receptors, mannosylated structures are examined in biomedical applications. One of the most common ways of linking a functional moiety to a structure is to use an azide-alkyne click reaction. Therefore, it is necessary to prepare and isolate a propargylated mannose derivative of high purity to maintain its bioactivity. Three known preparations of propargyl-α-mannopyranoside were revisited, and products were analysed by NMR spectroscopy. The preparations were shown to yield by-products that have not been described in the literature yet. Our experiments showed that one-step procedures could not provide pure propargyl-α-mannopyranoside, while a three-step procedure yielded the desired compound of high purity.

Anomeric alkylations and acylations of unprotected mono- and disaccharides mediated by pyridoneimine in aqueous solutions

A site-specific deprotonation followed by alkylations and acylations of sugar hemiacetals to the corresponding alkyl glycosides and acylated sugars in aqueous solutions is disclosed herein. Pyridoneimine as a new base is developed to mediate the deprotonation of readily available sugar hemiacetals and further reactions with alkylation and acylation agents.

Protecting-group-free S-glycosylation towards thioglycosides and thioglycopeptides in water

A facile and green S-glycosylation method has been developed featuring protecting-group-free and proceeding-in-water like enzymatic synthesis. Glycosylation of fluoride donors with thiol sugar acceptors using Ca(OH)2 as a promoter afforded various thioglycosides in good yields with exclusive stereoselectivity. This method also enabled the successful production of S-linked oligosaccharides and S-linked glycopeptides.

Preparation and Uses of Chlorinated Glycerol Derivatives

Crude glycerol (C3H8O3) is a major by-product of biodiesel production from vegetable oils and animal fats. The increased biodiesel production in the last two decades has forced glycerol production up and prices down. However, crude glycerol from biodiesel production is not of adequate purity for industrial uses, including food, cosmetics and pharmaceuticals. The purification process of crude glycerol to reach the quality standards required by industry is expensive and dificult. Novel uses for crude glycerol can reduce the price of biodiesel and make it an economical alternative to diesel. Moreover, novel uses may improve environmental impact, since crude glycerol disposal is expensive and dificult. Glycerol is a versatile molecule with many potential applications in fermentation processes and synthetic chemistry. It serves as a glucose substitute in microbial growth media and as a precursor in the synthesis of a number of commercial intermediates or fine chemicals. Chlorinated derivatives of glycerol are an important class of such chemicals. The main focus of this review is the conversion of glycerol to chlorinated derivatives, such as epichlorohydrin and chlorohydrins, and their further use in the synthesis of additional downstream products. Downstream products include non-cyclic compounds with allyl, nitrile, azide and other functional groups, as well as oxazolidinones and triazoles, which are cyclic compounds derived from ephichlorohydrin and chlorohydrins. The polymers and ionic liquids, which use glycerol as an initial building block, are highlighted, as well.

Stereoselective oxidative glycosylation of anomeric nucleophiles with alcohols and carboxylic acids

Oligosaccharides, one of the most abundant biopolymers, are involved in numerous biological processes. Although many efforts have been put in preparative carbohydrate chemistry, achieving optimal anomeric and regioselectivities remains challenging. Herein we describe an oxidative glycosylation method between anomeric stannanes and oxygen nucleophiles resulting in the formation of a C−O bond with consistently high anomeric control for glycosyl donors bearing a free C2-hydroxyl group. These reactions are promoted by hypervalent iodine reagents with catalytic or stoichiometric amounts of Cu or Zn salts. The generality of this transformation is demonstrated in 42 examples. Mechanistic studies indicate that the oxidative glycosylation is initiated by the hydroxyl-guided delivery of the hypervalent iodine and tosylate into the anomeric position, and results in excellent 1,2-trans selectivity. The unique mechanistic paradigm, high selectivities, and mild reaction conditions make this method suitable for the synthesis of oligosaccharides and for integration with other methodologies such as automated synthesis.

Glycosylation of partially protected sugars is usually limited by suboptimal regio- and stereo-selectivities. Here, the authors show a general oxidative glycosylation between anomeric stannanes with a nonprotected hydroxyl group and oxygen nucleophiles, additionally providing mechanistic insights into the origin of selectivity.

Strategies toward protecting group-free glycosylation through selective activation of the anomeric center

Glycosylation is an immensely important biological process and one that is highly controlled and very efficient in nature. However, in a chemical laboratory the process is much more challenging and usually requires the extensive use of protecting groups to squelch reactivity at undesired reactive moieties. Nonetheless, by taking advantage of the differential reactivity of the anomeric center, a selective activation at this position is possible. As a result, protecting group-free strategies to effect glycosylations are available thanks to the tremendous efforts of many research groups. In this review, we showcase the methods available for the selective activation of the anomeric center on the glycosyl donor and the mechanisms by which the glycosylation reactions take place to illustrate the power these techniques.

Solution and Solid-Phase Stereocontrolled Synthesis of 1,2-cis-Glycopyranosides with Minimally Protected Glycopyranosyl Donors Catalyzed by BF3-N,N-Dimethylformamide Complex

Methods are described for the stereoselective synthesis of 1,2-cis glycopyranosides in the d-galacto, d-gluco, and 2-azido-2-deoxy-d-glucopyranoside series utilizing minimally protected (3-bromo-2-pyridyloxy) β-d-glycopyranosyl donors in the presence of BF3-N,N-dimethylformamide (DMF) as a catalyst and a variety of alcohol acceptors relying on the "remote activation concept". Precursors to antifreeze glycopeptide components are synthesized in excellent yields and high α/β ratios. The method is adaptable to one-pot sequential glycosidation as well as to solid-supported synthesis giving access to diverse sets of minimally protected α-d-glycopyranosides as major products.

Aqueous Glycosylation of Unprotected Sucrose Employing Glycosyl Fluorides in the Presence of Calcium Ion and Trimethylamine

We report a synthetic glycosylation reaction between sucrosyl acceptors and glycosyl fluoride donors to yield the derived trisaccharides. This reaction proceeds at room temperature in an aqueous solvent mixture. Calcium salts and a tertiary amine base promote the reaction with high site-selectivity for either the 3'-position or 1'-position of the fructofuranoside unit. Because nonenzymatic aqueous oligosaccharide syntheses are underdeveloped, mechanistic studies were carried out in order to identify the origin of the selectivity, which we hypothesized was related to the structure of the hydroxyl group array in sucrose. The solution conformation of various monodeoxysucrose analogs revealed the co-operative nature of the hydroxyl groups in mediating both this aqueous glycosyl bond-forming reaction and the site-selectivity at the same time.

Stereocontrolled 1,2-cis glycosylation as the driving force of progress in synthetic carbohydrate chemistry

Recent developments in stereoselective 1,2-cis glycosylation that have emerged during the past decade are surveyed herein.

Recent developments in stereoselective 1,2-cis glycosylation that have emerged during the past decade are surveyed herein. For detailed coverage of the previous achievements in the field the reader is referred to our earlier reviews: A. V. Demchenko, Curr. Org. Chem. , 2003, 7 , 35–79 and Synlett , 2003, 1225–1240.