1. Departments of Chemistry and Molecular Biology and Skaggs Institute for Chemical Biology, Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, CA 92037, USA.
2. Materials and Process Simulation Center, Division of Chemistry and Chemical Engineering (MC 139-74), California Institute of Technology, Pasadena, CA 91125, USA.
3. Laboratorium für Organische Chemie, Eidgenössische Technische Hochschule (ETH) Zürich, Universitätstrasse 16, CH-8092 Zürich, Switzerland.