A Synthesis of Methyl 3-O-(β- d -Mannopyranosyl)-α- d -mannopyranoside from Sulfonate Intermediates

Methyl 3-O-(β-d-mannopyranosyl)-α-d-mannopyranoside was synthesized by the reaction of 3,4,6-tri-O-benzyl-2-O-benzylsulfonyl-1-O-tresyl-d-mannopyranose with methyl 2,4,6-tri-O-benzyl- and 2,4,6-tri-O-(p-bromobenzyl)-α-d-mannopyranoside, followed by deprotection. The corresponding 1-O-(β-tolylsulfonyl)derivative gave better yield and stereoselectivity in the reaction with methanol, but was not sufficiently reactive to couple with a hindered secondary alcohol. The 2-O-benzylsulfonyl group is comparable in β-directing effect to 2-O-methylsulfonyl group, but its stereoselectivity is dependent on the structure of the reactant alcohol. It can be selectively removed by the reaction with sodium amide in N,N-dimethylformamide.