O-Benzylated thio sugars. Derivatives of 2,3,6-tri-O-benzyl-1-thio-d-galactopyranose suitable for use in oligosaccharide synthesis

The 4-O-benzoyl (15a) 4-O-p-nitrobenzoyl (15b) derivatives of 2,3, 6-tri-O-benzyl-1-thio-d-galactopyranose were synthesized from allyl 2,6-di-O-benzyl-α-d-galactopyranoside (1). In the first stage of the synthesis the 3-position of 1 was benzylated by an indirect route, and also by the direct reaction (preferred) of benzyl bromide with the 3,4-O-dibutylstannylene intermediate 7. The product 6 was sequentially isomerized (allyl → 1-propenyl), acylated at the 4-position, and hydrolyzed. The free sultars 11a and 11b were converted into the thio sugars by a standard sequence involving formation of the glycosyl halides 13a and 13b and the reaction of these with appropriate sulfur nucleophiles. A third derivative (29) of 2,3,6-tri-O-benzyl-1-thio-d-galactopyranose, having a 4-O-allyl protecting group, was similarly made from the corresponding normal sugar 25. The key intermediate 22, precursor to 25, was prepared by two routes from methyl 2,3,6-tri-O-benzoyl-α-d-galactopyranoside (17).