Preferential sulfonylation of methyl 2,6-di-O-mesyl-α-D-glucopyranoside

When equimolar ratios of mesyl chloride and methyl 2,6-di-O-mesyl-α-D-glucopyranoside were allowed to react in pyridine and the product resolved by preparative t.l.c., the 2,6-di-, 2,3,6-tri-, 2,4,6-tri-, and 2,3,4,6-tetra-mesyl esters were obtained in (0.5–0.6):1:(4–5):(1-2-1.4) molar ratio. Benzoylation of either the isolated 2,4,6-tri-O-mesyl ester or, more conveniently, the mixture from monomesylation gave the crystalline methyl 3-O-benzoyl-2,4,6-triO-mesyl-α-D-glucopyranoside (8). As both of these trimesyl esters (7 and 8) are unreported, isolation of the benzoate established the 2,4,6-ester arrangement, and the 2,3,6-triester was prepared by standard methods. Treating methyl α-D-glucopyranoside with 3 molar equivalents of mesyl chloride and, subsequently, with 1 molar equivalent of benzoyl chloride, proved a convenient method for preparing the 3-O-benzoyl derivative in moderate yield. Monotosylation of methyl 2,6-di-O mesyl-α-D-glucopyranoside was not so definitive as mesylation, but a molar ratio of 1:2.8 for the 3-O-tosyl:4-O-tosyl product was derived from n.m.r. data. This work, when combined with literature reports, establishes that, in methyl α-D-glucopyranoside, the reactivity toward sulfonylation is 6-OH>2-OH>4-OH>3-OH.