Ne. Nifantev et al., SELECTIN RECEPTORS .4. SYNTHESIS OF TETRASACCHARIDES SIALYL-LEWIS-A AND SIALYL-LEWIS-X CONTAINING A SPACER GROUP, Journal of carbohydrate chemistry, 15(8), 1996, pp. 939-953
Synthesis of two isomeric tetrasaccharides, namely Neu5Ac alpha(2-->3)
Gal beta(1-->3)[Fuc alpha(1-->4)]GlcNAc beta (sLe(a)) and Neu5Ac alpha
(2-->3)Gal beta(1-->4)[Fuc alpha(1-->3)]GlcNAc beta (sLe(x)) as 3-amin
opropyl glycosides is described. Preparation of these compounds was pe
rformed by sialylation of selectively protected trisaccharides Le(a) a
nd Le(x) which contain three unsubstituted OH groups at positions 2, 3
and 4 of Gal residue. Glycosylation of Le(x) trisaccharide with ethyl
thio sialoside under promotion by NIS and TfOH in acetonitrile was eff
ective and regio- and stereoselective to give sLe(x) derivative in 81%
yield. In contrast, sialylation of the Le(a) acceptor was accompanied
by a variety of undesirable by-processes, namely, N-thioethylation of
the GlcNAc residue, beta-sialylation, and lactonisation. In order to
improve the yield of sLe(a) tetrasaccharide the glycosylation of Le(a)
acceptor by sialyl donors of ethyl and phenyl thioglycoside (promoted
by NIS-TfOH or NBS-Bu(4)NBr), xanthate (promotion by NIS-TfOH mixture
or MeOTf) and phosphite (promoted by TMSOTf) types was also studied.
Among the reactions investigated the glycosylation by phenyl thioglyco
side sialoside promoted by NIS-TfOH gives the best yield (39%) of sLe(
a) tetrasaccharide product.