Glucopyranosyl bromide reaction with alcohols

The glycals are easily converted into the 1,2-dihalo-derivatives, which in principle can act as glycosyl donors. However, these derivatives have not found wide application in glycoside synthesis, mainly because of the low facial selectivity in the initial addition of the electrophilic species [143-145]. In an example of a successful application, 2-deoxy-2-bromo-a-D-glucopyranosyl bromide [146] has been shown to give predominantly the 2-deoxy-P-D-glucopyranosides in silver-triflate-promoted reactions with alcohols. 

A route to a-glycosides involves treatment of a 2,3,4,6-tetra-O-benzyl-a-D-glucopyranosyl bromide with an alcohol, tetraethylammonium bromide, and diisopropylethylamine in CH2C12. Explain the stereoselectivity of this reaction. 

Tin(II) triflate has been used to catalyse the condensation between tetra-O-acetyl-a-D-glucopyranosyl bromide and primary and secondary alcohol groups on other monosaccharide derivatives. The products had the 3-configuration exclusively but the yields were modest (30-65%). In a related study the influences of catalyst, solvent, and temperature on the reaction of 0-(tetra-O-benzyl-a-D-glucopyranosyDtrichloroacetimidate with primary and secondary sugar alcohols were examined. With boron trifluoride in dichloro-methane at -I8—good yields (e.g. 80 for a secondary alcohol) and anomeric ratios (a,3, ca. 1 4) were obtained. 

Thiem [3] introduced the use of 2-bromo-2-deoxy-a-D-glucopyranosyl bromides 96 for the P-selective glycosylation of complex aglycones involved in aureolic acid antibiotics. These bromides were prepared by the reaction of dibromomethyl methyl ether [67] with a methyl 2,3-0-isopropylidene-a-D-mannopyranoside 95 and gave predominantly (P a 10 l) 2-deoxy-2-bromo-P-glucopyranosides in their silver triflate-promoted reactions with sugar alcohols. Radical debromination afforded the 2-deoxy-P-glycosides 97 in excellent yields (Scheme 23). 

In this procedure, the chiral alcohol is converted to the corresponding tetra-0-benzoyl-)8-glucoside by reaction with commercial 2,3,4,6-tetra-O-benzoyl-a-D-glucopyranosyl bromide in the presence of silver trifluoromethanesulfonate and 1,1,3,3-tetramethyl-urea. The NMR spectrum of the product is then compared with that of the free alcohol. The observed differences between the two spectra are caused by the anisotropic effect of the benzoyl groups and the... 

When methanesulfonyl chloride was allowed to react with 2,3,4,6-tetra-O-ben-zylglucopyranose and collidine in dichloromethane, the a-glucopyranosyl chloride was isolated regardless of whether the quaternary ammonium bromide was included. Addition of methanol to the reaction mixture resulted in the formation of an anomeric mixtiue of methyl glycosides. Similar results were obtained with toluenesulfonyl chloride, although it was noted that the initial sulfonylation was somewhat slower [8,9]. The use of tosyl chloride in the dehydrative coupling of alcohols with pyranoses was later revisited by Szeja and his coworkers, with the difference that aqueous... 

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