Tetrasaccharide synthesis, hydroxyl protecting groups

The de novo Achmatowicz approach to the tris-rhamno portion of the anthrax tetrasaccharide began with the synthesis of disaccharide 115 from pyranone ent-44 and benzyl alcohol (Scheme 1.21). After glycosylation and postglycosylation transformations to install the rhamno-stereochemistry (ent-44 to 111), the 1,2-traus-diol of 111 was then protected with the Ley-spiroketal to provide monosaccharide 112 with a free C-2 hydroxyl group. After a similar three-step glycosylation (112 and 113) and postglycosylation sequence, 113 was converted into disaccharide 114, which in a one-pot ortho-ester protocol was protected to give disaccharide 115 with a free C-3 alcohol. 

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