Glycosyl imidates. 49. Synthesis of the octasaccharide moiety of the dimeric Lex antigen

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Abstract
A versatile synthesis of the basic Galβ (1→4)[Fucα(1→3)]‐GlcNAc trisaccharide building block of the Lex antigen family based on two important findings could be developed. Firstly, selective 3‐O‐allylation of the readily available azidolactose derivative 3 led to an efficient synthesis of the 3‐O‐unprotected fucosyl acceptor 9. Secondly, α‐fucosylation of 9 with O‐fucosyl trichloroacetimidate 11 as donor exhibited a dramatic increase in yield when carried out by an “inverted procedure”, i,e, the donor was added to an acceptor/catalyst solution. The obtained trisaccharide building block 12 could be readily transformed into acceptor 13 and donor 15, respectively. Thus, the dimeric hexasaccharide 16 was obtained, which was then transformed into donor 19. Its reaction with the 3′‐O‐unprotected lactose acceptor 20 provided the desired ocetasaccharide 21, which gave upon azido group reduction, acetylation of the amino groups, and complete O‐deprotection the desired target molecule 2 characterized as its fully O‐acetylated product 24.

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