1. School of Pharmaceutical Sciences, University of Geneva, University of Lausanne, Quai Ernest-Ansermet 30, CH-1211 Geneva 4, Switzerland, Department of Organic Chemistry, University of Geneva, Quai Ernest-Ansermet 30, CH-1211 Geneva 4, Switzerland, and Departamento de Quimica, Facultad de Ciencas, Universidad de Chile, Casilla 653, Santiago, Chile