Glycosyl Imidates, 62. – Synthesis of the Heptasaccharide Moiety of Ganglioside BGM 1

The synthesis of the O ‐acetyl protected heptasaccharide moiety ( 2 ) of BGM 1 was performed according to the following reaction sequence: Reaction of 2,3‐di‐ O ‐acetyl‐4,6‐ O ‐benzylidenegalactosyl trichloroacetimidate 4 (as donor) with 3‐ O ‐unprotected 2‐azidogalactose 5 (as acceptor) gave β(1→3)‐connected disaccharide 6 . Subsequent O ‐deacetylation followed by reaction with galactosyl donor 8 afforded regioselectively trisaccharide 9 which was converted into tetrasaccharide 12 by treatment with fucosyl donor 11 . Transformation of 12 via acid‐catalyzed O ‐deisopropylidenation, O ‐acetylation, anomeric O ‐desilylation, and then base‐catalyzed treatment with trichloroacetonitrile afforded trichloroacetimidate 16 as tetraosyl donor. Reaction of 16 with the known 4b‐ O ‐unprotected sialyllactose derivative 17 gave in acetonitrile at −40°C in the presence of TMSOTf as the catalyst the desired heptasaccharide 18 . Azido group reduction with propanedithiol, N ‐acetylation, hydrogenolytic O ‐debenzylation, and O ‐debenzylidenation under acidic conditions followed by O ‐acetylation afforded the target molecule 2 . The structural assignments were based on the 1 H‐NMR data.