1. Department of Pharmaceutical Sciences, University of Salerno, I-84084 Fisciano, Italy, Departments of Molecular Pharmacology and Chemistry, Brown University, Providence, Rhode Island 02912, Menarini Ricerche, I-50131 Firenze, Italy, and Biopolymer Research Center, CNR, Department of Organic Chemistry, University of Padova, I-35131 Padova, Italy