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Free keywords:
PERCHLORATE-DIETHYL-ETHER; LEWIS ACID CATALYSIS; DIELS-ALDER REACTION; SILVER PERCHLORATE; OLIGOSACCHARIDES; PHOSPHATES; CALICHEAMICIN-GAMMA-1(I); THIOGLYCOSIDES; CHLORIDES
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 alpha-imidate la or the beta-phosphate Id is used as glycosyl donor, in the majority of the cases 1:1 mixtures of the anomers are obtained. In contrast, the beta-imidate 1b gives a distinct excess of the alpha-glycosides and if the alpha-phosphate 1c is employed, the beta-anomers are formed preferentially. Whereas the glycosyl chloride If and the glycosyl bromide 1e are not the donors of choice under these conditions, from the beta-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 imidazolylcarbonyl-activated 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.
