Synthesis of ganglioside GM1 containing a thioglycosidic bond to its labeled ceramide(s). A facile synthesis starting from natural gangliosides

Capitalizing on the readily available ganglioside, GM1, we have devised a simple synthesis of labeled GM1 analogues with sulfur in place of oxygen in their linkage to the ceramide residue (SGM1). The sugar moiety of ganglioside GM1 was released by ozonolysis and subsequent alkaline fragmentation in good yield. During acetylation of the ganglioside sugar, the carboxyl group of the sialic acid residue lactonized with the 2-hydroxyl group of the inner galactose moiety (galactose II). The resulting sialoyl-II2-lactone of pentadeca-O-acetyl-monosialogangliotetraose could be readily transformed into the α-glycosyl bromide. Subsequent treatment of this glycosyl bromide with potassium thioacetate afforded the sialoyl-II2-lactone of tetradeca-O-acetyl-1-S-acetyl-1-thio-β-monosialogangliotetraose. The latter could be condensed with (2R,3R,4E)-3-O-benzoyl-2-dichloroacetamido-1-iodo-4-octadecen-3-ol in methanolic sodium acetate to afford a protected lyso-SGM1 derivative. One-step removal of the protecting groups under alkaline conditions gave β-monosialogangliotetraosyl thiosphingosine. This lyso-SGM1 was converted into labeled analogues of SGM1 using the N-succinimidoyl derivative of radiocarbon-labeled octanoic and octadecanoic acid, respectively. Subsequent actions of GM1-β-galactosidase, β-hexosaminidase A, sialidase and again GM1-β-galactosidase on these labeled analogues of SGM1 in the presence of taurodeoxycholate produced the respective analogues of GM2, GM3, lactosylceramide and glucosylceramide, respectively.