1. Department of Pharmaceutical Sciences, University of Bologna, Via Belmeloro, 6, I-40126 Bologna, Italy, Department of Chemistry, University of Basilicata, Via N. Sauro, 85, I-85100 Potenza, Italy, and Pharmaceutical and Medicinal Chemistry, Saarland University, P. O. Box 151150, D-66041 Saarbrücken, Germany