1. Department of Organic Chemistry “U. Schiff”, University of Firenze, Via della Lastruccia 13, I-50019 Sesto Fiorentino, Italy, Department of Clinical Physiopathology, Endocrinology Unit, University of Firenze, Viale G. Pieraccini 6, I-50134 Firenze, Italy, Serono International S.P.R.I., 12, Chemin des Aulx, CH-1228 Plan-les Ouates, Geneve, Switzerland, and Department of Pharmaceutical Sciences, University of Bologna, Via Belmeloro 6, I-40126 Bologna, Italy