1. Laboratory for Medicinal Chemistry (FFW), Ghent University, Harelbekestraat 72, 9000 Gent, Belgium, Unité de Chimie Organique, Institut Pasteur, 28, rue du Dr Roux, 75724 Paris Cedex 15, France, and Laboratory for Medicinal Chemistry, Rega Institute, Catholic University of Leuven, Minderbroedersstraat 10, B-3000 Leuven, Belgium