1. Department of Chemistry, Section of Organic Chemistry and Biochemistry, University of Ioannina, Ioannina GR-451 10, Greece, Institut Curie, Section de Recherche, URA 1387 CNRS, Centre Universitaire, 91405 Orsay, France, Department of Chemistry, University College London, 20 Gordon Street, London WC1H OAJ, U.K., and Department of Chemistry, Queen Mary and Westfield College, Mile End Road, London E1 4NS, U.K.