1. Contribution from the Institute of Organic Chemistry, Justus-Liebig-University, Heinrich-Buff-Ring 58, 35392 Giessen, Germany, Departamento de Química Orgánica y Unidad Asociada al CSIC, Facultade de Química, Universidade de Santiago de Compostela, 15706 Santiago de Compostela, Spain, and Department of Chemistry, Center for Computational Chemistry, The University of Georgia, Athens, Georgia 30602-2556