1. Laboratory for Medicinal Chemistry (FFW), University of Gent, Harelbekestraat 72, 9000 Ghent, Belgium, Unité de Chimie Organique, URA CNRS2128, Institut Pasteur, Paris 15, France, and Laboratory for Medicinal Chemistry and Laboratory of Virology and Chemotherapy, Rega Institute for Medical Research, Minderbroedersstraat 10, B-3000 Leuven, Belgium