Reinvestigation of the synthesis of 4-methylcoumarin-7-yl 5-acetamido-3,5-dideoxy-α- d- glycero- d- galacto-2-nonulopyranosidonic acid, a fluorogenic substrate for neuraminidase

A crystalline tetrabutylammonium salt of 7-hydroxy-4-methylcoumarin was prepared and shown to contain two coumarin residues for each ammonium group. Condensation of this salt with the glycosyl chloride of methyl 5-acetamido-4,7,8,9-tetra- O-acetyl-3,5-dideoxy-β- d- glycero- d- galacto-2-nonulopyranosonate in dry acetonitrile at room temperature gave the corresponding α-glycoside in higher yield and purity than previously reported methods. Removal of the acetyl and methyl ester blocking-groups gave the free glycoside, which was shown to have the α configuration by n.m.r. spectroscopy. In contrast, the reaction of the free coumarin derivative with the chloro sugar in refluxing, dry toluene in the presence of cadmium carbonate as acid acceptor gave none of the above glycoside, but gave the corresponding glycal in good yield.