1. Contribution from the Dipartimento di Chimica Organica e Industriale, Universita' di Parma, Italy, Centre for Chemical Biology, Krebs Institute for Biomolecular Science, Department of Chemistry, University of Sheffield, Sheffield S3 7HF, UK, and Departament de Química, Universitat de les Illes Balears, 07122 Palma de Mallorca, Illes Balears, Spain