A Structural-Reporter Group to Determine the Core Conformation of Sialyl Lewis^x Mimetics

The d-GlcNAc moiety in sialyl Lewis^x (sLe^x, 1) acts predominantly as a linker to position the d-Gal and the l-Fuc moieties in the bioactive spatial orientation. The hypothesis has been made that the NHAc group of GlcNAc pushes the fucose underneath the galactose and, thus, contributes to the stabilization of the bioactive conformation of the core of sLe^x (1). To test this hypothesis, GlcNAc mimetics consisting of (R,R)-1,2-cyclohexanediols substituted with alkyl and aryl substituents adjacent to the linking position of the fucose moiety were synthesized. To explore a broad range of extended and spatially demanding R-groups, an enzymatic approach for the synthesis of 3-alkyl/aryl-1,2-cyclohexanediols (3b-n) was applied. These cyclohexanediol derivatives were incorporated into the sLe^x mimetics 2b-n. For analyzing the relationship of affinity and core conformation, a ¹H NMR structural-reporter-group concept was applied. Thus, the chemical shift of H-C5^Fuc proved to be a sensitive indicator for the degree of pre-organization of the core of this class of sLe^x mimetics and therefore could be used to quantify the contribution of the R-groups.