1. Dipartimento di Scienze Chimiche, Università di Camerino, Via S. Agostino, 1, 62032 Camerino, Italy, Laboratory for Experimental Oncology, Indiana University School of Medicine, Indianapolis, Indiana 46202, Department of Biochemistry and Biophysics, University of Rochester Medical Center, Rochester, New York 14642, and Laboratoire de Chimie Bio-organique, URA CNRS 488, Université de Montpellier II, 34095 Montpellier Cédex 5, France