1. Department of Chemistry, Imperial College London, Exhibition Road, London SW7 2AZ, U.K., Departamento de Química Orgánica, Facultad de Ciencias, Universidad Autónoma de Madrid, 28049 Cantoblanco, Spain, and Institute of Chemical Research of Catalonia, Avenida Països Catalans 16, Tarragona 43007 and Catalan Institution for Research and Advanced Studies (ICREA), Spain