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.