1. Department of Chemistry, Institute of Chemistry, University of Sao Paulo, Av. Prof. Lineu Prestes, 748, CP 26077, CEP 05513-970 Sao Paulo SP, Brazil, Departamento de Fisiologia e Farmacologia, Universidade Federal do Ceará, P.O. Box 3157, 60430-270, Fortaleza, Ceará, Brazil, and Department of Chemistry, University of Basel, St. Johanns-Ring 19, CH-4056 Basel, Switzerland