Synthesis of GM1-Ganglioside Inner Ester

A new procedure is described for preparing a GM1-ganglioside (GM1) inner ester, in which the sialic acid carboxyl group esterifies with the hydroxyl group at carbon-2 of the inner galactose unit. The reaction of lactonization consists of treatment of GM1 with dicyclohexylcarbodiimide in anhydrous dimethylsulfoxide. Under these conditions, GM1 inner ester is produced with almost quantitative yield (97%) and recovered from the reaction mixture by simple acetone precipitation. The involvement of a sialic acid carboxyl group in an ester linkage was proved by ammonolysis experiments, which gave rise to the amide of sialic acid, and by column chromatography on an ionexchange resin which retained GM1 but not the GM1 inner ester. The GM1 inner ester was analyzed for carbohydrate, long chain base and fatty acid composition by chemical and spectrometric methods. The inner galactose unit, involved in the ester linkage, was recognized by positive and negative ion fast Atom Bombardment Mass Spectrometry and the hydroxyl group at carbon-2, involved in the linkage, by proton Nuclear Magnetic Resonance spectroscopy.