1. Laboratory for Organic Chemistry, Swiss Federal Institute of Technology, Universitätstrasse 16, CH-8092 Zürich, Switzerland, Centro de Biologia Molecular ”Severo Ochoa”, Facultad de Ciencias, Universidad Autonoma de Madrid, E-28049 Madrid, Spain, and Department of Pharmacy, Swiss Federal Institute of Technology, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland