1. Contribution from Biomolecular Architecture, Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3001 Leuven (Heverlee), Belgium, Laboratory of Medicinal Chemistry, Rega Institute, Katholieke Universiteit Leuven, Minderbroederstraat 10, B-3000 Leuven, Belgium, and Department of Haematology, University of Cambridge, Cambridge Institute for Medical Research, Hills Road, Cambridge CB2 2XY, U.K.