Synthesis of N-acetylglucosamine containing Lewis A and Lewis X building blocks based on N-tetrachlorophthaloyl protection - Synthesis of Lewis X pentasaccharide

Phenyl 6-O-benzyl-2-deoxy-2-tetrachlorophthalimido-1-thio-beta-D-glucopyranoside (5a) and thexyldimethylsilyl 6-O-benzyl-2-deoxy-2-tetrachlorophthalimido-beta-D-glucopyranoside (5b) gave with O-(2,3,4,6-tetra-O-acetyl-alpha-D-galactopyranosyl)trichloroacetimidate (8) in the presence of BF3. Et2O as catalyst exclusively lactosamine derivatives 7a and 7b, respectively, in high yields. Ensuing reaction with O-(3,4-di-O-acetyl-2-O-benzyl-alpha-L-fucopyranosyl)trichloroacetimidate (9) in the presence of TMSOTf as catalyst afforded Le(X) trisaccharide intermediates 10a,b. With fucosyl donor 9 and 5a,b as accepters in the presence of TMSOTf as catalyst glycosylation either at the 3-O or the 4-O was observed, thus leading to mixtures of disaccharides 11a/12a and 11b/12b, respectively; their reaction with 8 furnished Le(X) trisaccharide intermediates 10a,b and Le(a) trisaccharide intermediates 14a,b. Transformation of 10b into the corresponding trichloroacetimidate 17 and reaction with lactose acceptor 19 in the presence of Zn(OTf)(2) as catalyst gave protected Le(X) pentasaccahride intermediate 21, which on deprotection led to unprotected Le(X) pentasaccharide 1.