Showing papers on "Koenigs–Knorr reaction published in 2006"

Synthesis and transformation of glycosyl azides.

Abstract: Publisher Summary This chapter discusses the synthesis and transformation of glycosyl azides. Owing to their functional group, glycosyl azides constitute important and versatile derivatives for carbohydrate chemistry. Because of the dipole character of organic azides, they can function both as nucleophiles and electrophiles and readily undergo dipolar cycloadditions. In addition, as configurationally stable groups, azides are well suited as starting materials for the formation of other nitrogen-containing functionalities such as amines, amides, ureas, carbodiimides, and others. The only method known up to 1974 for preparing glycosyl azides was from acylated glycosyl halides by the treatment with sodium or silver azide. However, because of the reactivity of glycosyl halides with water, the very low solubility of sodium azide in organic solvents, and the thermal ability of silver azide, there were considerable difficulties in preparing the corresponding glycosyl azides. This chapter discusses the synthesis of 1,2- trans glycosyl azides from glycosyl halides. It also explains the synthesis of 1,2- trans -glycopyranosyl azides by intramolecular rearrangement and elaborates the reduction and oxidation of glycosyl azides.

High-Yield Syntheses of Tetra-O-benzyl-α-D-glucopyranosyl bromide and Tetra-O-pivaloyl-α-D-glucopyranosyl bromide and their Advantage in the Koenigs-Knorr Reaction

Abstract: Several improved approaches for the preparation of tetra-O-benzyl-α-D-glucopyranosyl bromide and tetra-O-pivaloyl-α-D-glucopyranosyl bromide are discussed. The importance of these compounds, which are useful glycosyl donors, was demonstrated by successful preparation of cholesteryl glucopyranosides in an almost neutral medium without the formation of orthoesters. In addition, accurate 1H and 13C NMR resonance assignments of the synthesized cholesteryl glycosides were performed by 2D NMR spectroscopy.

O-glycoside synthesis under neutral conditions in concentrated solutions of LiClO4 in organic solvents employing benzyl-protected glycosyl donors

Abstract: The benzyl-protected glucosyl trichloroacetimidates, phosphates, and halides 1 are activated under neutral conditions and without the addition of any further promoter in 1 M solutions of LiClO4 in ether, CH2Cl2, CHCl3, or CH3CN and react under these conditions with the model alcohols 3 to give the glycosides 4 in moderate yields. If the α-imidate 1a or the β-phosphate 1d is used as glycosyl donor, in the majority of the cases 1:1 mixtures of the anomers are obtained. In contrast, the β-imidate 1b gives a distinct excess of the α-glycosides and if the α-phosphate 1c is employed, the β-anomers are formed preferentially. Whereas the glycosyl chloride 1f and the glycosyl bromide 1e are not the donors of choice under these conditions, from the β-fluoride 1g the desired O-glycosides are readily obtained. In 3–5 M solutions of LiClO4 in ether instead of the expected glycosides benzyl-protected 1,6-anhydroglucose 6 is formed and the imidazolylcarbonylactivated carbohydrate 1h reacts with the alcohols 3 to give the glycosyl carbonates 5. Whereas CH2Cl2 and CHCl3 do not influence the stereoselectivity of the glycosylations in ether or CH3CN, the solvent seems to participate in the steric control of the O-glycoside formation.

High-Yield Syntheses of Tetra-O-benzyl- a-D-glucopyranosyl bromide and Tetra-O- pivaloyl-a-D-glucopyranosyl bromide and their Advantage in the Koenigs-Knorr Reaction

Abstract: Summary. Several improved approaches for the preparation of tetra-O-benzyl-� -D-glucopyranosyl bromide and tetra-O-pivaloyl-� -D-glucopyranosyl bromide are discussed. The importance of these compounds, which are useful glycosyl donors, was demonstrated by successful preparation of cholesteryl glucopyranosides in an almost neutral medium without the formation of orthoesters. In addition, accurate 1 Ha nd 13 C NMR resonance assignments of the synthesized cholesteryl glycosides were performed by 2D NMR spectroscopy.