Ganglioside GAA-7 (1) derived from the starfish Asterias amurensis versicolor showed neuritogenic and growth-inhibitory activities towards the mouse neuroblastoma cell line (Neuro2a). On the other hand, the structure of its oligosaccharide moiety is very unique, since 1 possesses two sialic acids which directly linked to the GalNAc residue. We are conducting the reconstitution of the biologically active ganglioside 1, and now report the synthetic studies of the demethyl compound (2) of 1. The strategy for the synthesis of 2 is shown in chart 1. 1 possesses the less available N-glycolyl type sialic acid (NeuGc), therefore we prepared the glycosyl donor of NeuGc 5 from the N-acetyl type sialic acid (NeuAc) in 6 steps. For examination of its donor property, 5 was glycosylated with two lactose derivatives 8 and 11. As a result, the NeuGc-containing oligosaccharides 12 and 13 were obtained in good yields. In the preparation of the asialotrisaccharide 6, three GalNAc donors, bromide, trichloro-acetimidate and thioglycoside were examined. As a result, the thioglycoside of GalNAc 7 gave the highest chemical yield and regioselectivity. For examination of the disialylation reaction, tetra O-acetyl lactose derivative 17 was prepared and glycosylated with the NeuGc donor 5 in CH_3CN in the presence of N-iodosuccinimide and trifluoromethanesulfonic acid. The disialylated lactose derivative 18 was obtained in 15% yield together with some monosialylated lactosides. The disialylation of the NeuGc donor 5 towards the asialooligosaccharide 6 was achieved to give 3 by one step in moderate chemical yield. Ceramide moiety was prepared from the cerebrosides derived from the starfish Acanthaster planci. The extracted cerebroside molecular species were hydrolyzed by the glycosidases (Charonia lampas), and the resultant ceramides were separated by flash chromatography to yield pure ceramide 21. The isolated ceramide 21 was easily converted to the synthetically useful ceramide 4. Glycosylation of the sialopentasaccharide 3 and the ceramide 4 thus obtained is in progress.
