Studies on selectin blockers. 7. Structure-activity relationships of sialyl Lewis X mimetics based on modified ser-glu dipeptides

We have previously found that heterochiral fucodipeptides, L-Ser-D-Glu (3a) and D-Ser-L-Glu (3b), exhibited up to 20-100 times more potency than a sialyl Lewis X (sLe(X), 1) and a 3'sulfated Lewis X analogue (2) toward E- selectin binding and have also proposed, from molecular dynamics calculation, that their strong activities would depend on a possible formation of the type II and/or type II' β-turn of compounds 3a,b (Tsukida, T.; Hiramatsu, Y.; Tsujishita, H.; Kiyoi, T.; Yoshida, M.; Kurokawa, K.; Moriyama, H.; Ohmoto, H.; Wada, Y.; Saito, T.; Kondo, H. J. Med. Chem. 1997, 40, 3534-3541). To clarify our hypothesis, we synthesized several analogues of compounds 3a,b and investigated their structure-activity relationships. As a result, it was indicated that the type II and/or type II' β-turn conformation would be a comparatively tight form and would play important roles in favorable binding to E-selectin. These findings indicate that sLe(X) mimetics with type II and type II' β-turn dipeptides could be a useful methodology for the design of an active selectin blocker.