1. Department of Applied BioSciences, Swiss Federal Institute of Technology (ETH), Winterthurerstrasse 190, CH-8057 Zürich, Switzerland, Institut für Organische Chemie und Biochemie, Albert-Ludwigs-Universität, Albertstrasse 21, D-79104 Freiburg im Breisgau, Germany, and Laboratory of Medicinal Chemistry, Division of Basic Sciences, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892