Glycosyl imidates, 73. Synthesis of ganglioside gm 1 via a GA 1 intermediate

The synthesis of GM 1 pentasaccharide 36 via GA 1 ‐intermediate 24 was based on lactose building block 11 , Galβ(1–3)‐GalN 3 building block 23 , and sialyl donors 32 . For the synthesis of 11 , tetra‐ O ‐acetyl galactosyl trichloroacetimidate 3 or 2,3‐di‐ O ‐acetyl‐4,6‐ O ‐benzylidene‐galactosyl trichloroacetimidate 5 as donors were allowed to react with 4‐ O ‐unprotected glucose derivative 4 as acceptor to afford lactose derivatives 6 and 7 , respectively, in high yields. They were readily transformed via 8 into 2b,3b‐ O ‐MPM‐protected 9 ; reductive opening of the dioxane ring with DIBAH furnished regioselectively the desired 4b‐ O ‐unprotected lactose derivative 11 . From donor 3 and 6‐ O ‐benzoyl‐protected 2‐azidoglucose 19 as acceptor Galβ(1–3)GalN 3 disaccharide 20 was synthesized which was readily converted into per‐ O ‐acylprotected trichloroacetimidate 23 . Reaction of 23 with 11 afforded upon Sn(OTf) 2 catalysis the desired β‐linked tetrasaccharide 24 in high yield. Removal of the MPM‐protective groups with DDQ furnished 2b,3b‐ O ‐unprotected derivative 31 , which gave by treatment with sialyl donors 32a and 32b GM 1 ‐pentasaccharide intermediate 33 in good yield. 2b‐ O ‐Acetylation and then hydrogenolysis in the presence of Pearlman's catalyst and ensuing peracetylation afforded O ‐acyl‐protected GM 1 ‐pentasaccharide 36 which was already previously converted into GM 1 .