1. Department of Biochemistry, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, Laboratorium für Organische Chemie, Swiss Federal Institute of Technology (ETH), Universitätstrasse 16, CH-8092 Zürich, Switzerland, and The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037