Methyl acid-catalyzed anomerization

This process is the main course for the acid-catalyzed anomerization of sugar acetates, methyl glycosides, and related compounds in non-aqueous solvents.104,244,245 260,263 In many instances, formation of the cyclic carbonium ion is assisted by participation of a neighboring acetyl group,104 but this process is not a necessary feature. Anomerization of tetra-O-acetyl-2-deoxy-a-D-arafomo-hexopyranose takes place through a cyclic carbonium ion as depicted.245[Pg.43]

This reaction has been used m an imaginative way to determine the ring size of glycosides Once all the free hydroxyl groups of a glycoside have been methylated the glycoside is subjected to acid catalyzed hydrolysis Only the anomeric methoxy group IS hydrolyzed under these conditions—another example of the ease of carbocation for matron at the anomeric position... 

The Lewis acid catalyzed C-glycosylation method is probably one of the most efficient to prepare C-glycosides of ketoses, the anomeric carbocation intermediate being in this case more stabilized by a further substituent. We applied this procedure to synthesize C-fructosides [15] (Scheme 8).The reaction of methyl... 

P. J. Garegg, K.-J. Johansson, P. Konradsson, andB. Lindberg, Anomerization of methyl glycosides by acid-catalyzed methanoly sis Trapping of intermediates,/. Carbohydr. Chem., 18 (1999) 31-40. 

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