Glycosylation reaction conditions

Glycosylation of 3-amino-5(7)//-[l,2,4]triazolo[4,3-3] l,2,4 triazole 17 with 1-O-acetyl- 2,3,5-tri-O-benzoyl-D-ribo-furanose 18 or 2,3,5-tri-O-benzoyl-D-ribofuranosyl bromide 19 can be selective or nonselective, depending on the reaction conditions (Scheme 1). In the presence of trimethylsilyl triflate or boron trifluoride etherate, the... 

The glycosylations were carried out in dry dichloromethane in the presence of 0.1 equiv of triphenylmethylium perchlorate. It was assumed that the triphenylmethy-lium cation attacks the nitrogen of the thiocyanate group whereas the oxygen of the trityl ether attacks the anomeric carbon in a push-pull fashion. The trityl isothiocyanate formed was found to be unreactive under the reaction conditions. The disaccharide derivatives with (1-6), (1-4), (1-3) and (1-2) linkages have been obtained in good yields. 

In 1964, Kochetkov, Khorlin and Bochkov reported that the reaction of 1,2-alky-lorthoacetates with alcohols in the presence of catalytic amounts of HgBr2 and pTsOH furnished acetylated 1,2-trans glycosides or isomeric orthoesters depending on the reaction conditions [4]. Polar solvents (nitromethane, acetonitrile) and large amounts of catalyst promoted glycosylation (a, Scheme 5.5), whereas solvents of low polarity (dichloroethane) and the use of small amounts of catalyst favored transorthoesterification (b, Scheme 5.5) [16]. 

However, in spite of the fact that good results were obtained by an appropriate choice of reactants and of the reaction conditions, a great deal of attention has been devoted during recent years to the search for successful comprehensive methods for 1,2-m-glycosylations. They are discussed in the next sections. 

The influence of solvents in glycosylation reactions has been observed and discussed extensively already (1,4,74). For instance, the participation of ethers, when anomeric leaving-groups are removed under SNl-type conditions, results [because of the reverse anomeric effect (75,76)] in the genera-... 

Glycosyl acceptor Reaction conditions Anomeric configuration (or.fi) Yield (%) Reference... 

Trichloroacetiiiiidate Glycosyl acceptor Reaction conditions Yield (%) Anomeric configuration (a /0 Reference... 

Trichloroacetimidate Glycosyl acceptor Reaction conditions Reference... 

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