Facile synthesis of C-aryl glycals from sugar-derived lactones

A general entry to C(1) aryl-substituted glycals from the corresponding sugar lactones is described. This approach features one-pot access to aryl glycals. A variety of aryllithium reagents can be used and the method is compatible with various 2-deoxysugar lactones.     

A New Procedure for Highly Regio‐ and Stereoselective Iodoacetoxylation of Protected Glycals and α‐1,2‐Cyclopropanated Sugars

Protected glycals and α‐1,2‐cyclopropanated sugars were converted in high yields and selectivities in less than 2 h at low temperatures to 2‐deoxy‐2‐iodoglycosyl acetates or novel 2‐deoxy‐2‐iodomethylglycosyl acetates using the simple, inexpensive reagent mixture of ammonium iodide, hydrogen peroxide, and acetic anhydride/acetic acid in acetonitrile. The protected glycals gave rise to 2‐deoxy‐2‐bromoglycosyl acetates when ammonium bromide was used instead of the iodide, although longer reaction times were required and selectivities were inferior. Other simple olefins such as styrene and indene were also converted to their corresponding 1,2‐trans‐iodoacetates.     

Direct Ferrier rearrangement on unactivated glycals catalyzed by indium(III) chloride

Anhydrous InCl₃ has been shown to efficiently catalyze the Ferrier rearrangement by a direct allylic substitution of the hydroxyl group at C-3 position of glycals to afford the corresponding 2,3-unsaturated glycosides in high yields at ambient temperature. This methodology obviates the need for protecting and/or activating the C-3 hydroxyl group of glycals. The reaction works in equal ease with both 4,6-di-O-benzyl-d-glucal and 4,6-di-O-benzyl-d-galactal. The mildness of InCl₃ makes this approach compatible for glycosyl acceptors with acid labile groups. The generality of the reaction has been demonstrated with a diversity of alcohols, phenols, and sugar nucleophiles.     

The first examples of cyclizations of a glycal with enamines leading to oxa-aza bicyclononene scaffolds

Glycals undergo smooth coupling with β-enaminoketones and β-enaminoesters generated in situ from 1,3-dicarbonyl compounds and arylamines in the presence of 10 mol % of InCl₃ in refluxing dichloroethane to produce oxa-aza bicyclononene scaffolds in excellent yields with high selectivity. The use of InCl₃ makes this protocol simple, convenient and easy to scale-up.     

Aminolysis of glycal-derived allyl epoxides and activated aziridines. Effects of the absence of coordination processes on the regio- and stereoselectivity

The addition of primary and secondary aliphatic amines to glycal-derived allyl epoxides is completely 1,2-regio- and anti-stereoselective, whereas mixtures of the corresponding anti-1,2- [3-N-(substituted-amino) glycals] and anti-1,4-addition products (N-glycosyl amines) are obtained with N-(mesyl)-aziridines. In this way, structural moieties, otherwise difficult to synthesize, are obtained by means of a very simple protocol. The regio- and stereoselectivity observed with epoxides is the consequence of an isomerization process, whereas the result obtained with aziridines is explained by the absence of an effective substrate-nucleophile (amine) coordination.     

A simple, efficient alternative for highly stereoselective iodoacetoxylation of protected glycals

Protected glycals are converted in high yields and selectivities in less than 2 h at low temperatures to 2-deoxy-2-iodoglycosyl acetates using the simple, inexpensive reagent mixture of ammonium iodide, hydrogen peroxide and acetic anhydride/acetic acid in acetonitrile. The corresponding 2-deoxy-2-bromoglycosyl acetates are obtained using ammonium bromide instead of the iodide, although longer reaction times are required and selectivities are inferior.     

Oxime-based methods for synthesis of stereodefined acyclic polyfunctionalized δ-azido-nitriles and 5-substituted isoxazoles from carbohydrate derivatives

Hydroxylamine-mediated syntheses of stereodefined acyclic polyfunctionalized δ-azido-nitriles and 5-substituted isoxazoles, bearing a differentially protected glycerol moiety, from 2-deoxysugars and glycals are described.     

Formation of C-Glycosides by Ferrier Reaction

Peracetylated glycals were treated with terminal alkyne derivatives under Lewis acid catalysis to result in the formation of the pure α- or β- anomerically substituted hex-2-enopyranosides carrying 2(E)-halogenated olefins.     

Amidoglycosylation of Polymer-Bound Glycals: A Complete Solid-Phase Synthesis of the Oligosaccharide Domain of the Lewisb Blood Group Determinant

A new general method for the preparation of N-acetylglucosamine glycosidic linkages has been developed for solid-phase synthesis. The complete pentasaccharide domain of the Lewis^b blood group antigen, which is a mediator in the formation of ulcers and gastric cancer, has now been synthesized on solid support (see 1 ; the shaded circle is the polystyrene support; TIPS=triisopropylsilyl).     

Efficient synthesis of alkyl 2,3-unsaturated glucopyranosides from glycals mediated by ytterbium(III) triflate-trialkyl aluminum

Treatment of glycals with trialkylaluminum in the presence of a catalytic amount of Yb(OTf)₃ leads to the corresponding alkyl 2,3-unsaturated glycosides in good to excellent yields. Reactions of protected glycals are achieved under very mild conditions.