Methyl glycosides glycosyl halides

The first synthesis of methyl 4-thiomaltoside (36b) (34%), 4-thiocellobioside (39d) (52%) and 4-thiodigalactobioside (86) (56%) was reported 18 years ago by using the Sn2 displacement of 1,2-cis- or 1,2-trans- glycosyl halides (9 a), (7 a) and (7c) with methyl 4-thio-a-D-glycopyranoside sodium salts generated from the acetylated methyl glycosides (84) and (85a) (Scheme 25) [14a]. 

Activation of thioglucosides. Thioglucosides are useful for glycosidation since they are more reactive than glycosyl halides. They are known to be activated by methyl triflate, but this triflate is a potent carcinogen. Methylsulfenyl trifluoromethanesulfonate (1) is also an activator and has the advantage that it can be prepared in situ. Thus the 1-thio-p-D-glucopyranoside (p—2) couples with a-D-... 

Some of the most important syntheses of glycosides start from glycosyl halides, which are converted into acetylated glycosides, with Walden inversion, by reaction with alcohols or phenols in the presence of acid-binding compounds. For example, methyl tetra-0-acetyl-/ -D-glucoside is obtained from a-aceto-bromoglucose by this method (the Koenigs-Knorr reaction) 942... 

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