The total synthesis of ganglioside GM3

Previous syntheses of ganglioside GM3 (NeuAc alpha3Gal beta4Glc beta1Cer) are reviewed, and both chemoenzymatic and chemical total synthetic approaches were investigated. In a chemoenzymatic approach, (2S,3R,4E)-5'''-acetyl-alpha-neuraminyl-(2''' --> 3'')-beta-galactopyranosyl-(1'' --> 4')-beta-glucopyranosyl-(1' <--> 1)-2-azido-4-octadecene-1,3-diol (azidoGM3) was readily prepared utilizing recombinant beta-Gal-(1'' --> 3'/4')-GlcNAc alpha-(2''' --> 3'')-sialyltransferase enzyme, and was evaluated as a synthetic intermediate to ganglioside GM3. The chemical total synthesis of ganglioside GM3 was performed on one of the largest scales yet reported. The highlights of this synthesis include minimizing the steps necessary to prepare the lactosyl acceptor as a useful anomeric mixture, which was present in excess for the highly regioselective and fairly stereoselective sialylation with a known neuraminyl donor to give the protected GM3 trisaccharide. The synthetic methodology maximized convergence by a subsequent glycosidic coupling of the well-characterized GM3 trisaccharide trichloroacetimidate derivative with protected ceramide. The ganglioside GM3 was nearly homogeneous as the two glycosidic couplings utilized preparative HLPC purifications, and variations in the sphingosine base and fatty acyl group were under 0.1 and 0.2%, respectively.