1. Departamento de Química Orgánica I, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, 28040 Madrid, Spain, UCR-CNRS Joint Research Chemistry Laboratory (UMI 2957), Department of Chemistry, University of California, Riverside, California 92521-0403, and Fachbereich Chemie, Philipps-Universitat Marburg, Hans-Meerwein-Strasse, 35032 Marburg, Germany