1. Departament of Organic Chemistry, University of Geneva, 30 Quai Ernest Ansermet, 1211 Genève 4, Switzerland, Department of Organic Chemistry, Stockholm University, Arrhenius Laboratoriet, 106 91 Stockholm, Sweden, Organic Chemistry II, Dormund University of Technology, Otto-Hahn-Strasse 6, D-44227 Dortmund, Germany, and Departament de Química Física i Inorgànica, Universitat Rovira i Virgili,C/ Marcel·lí Domingo s/n, 43007 Tarragona, Spain