1. Department of Chemistry, Laboratory of Organic Chemistry, University of Athens, Panepistimiopolis, 15771 Athens, Greece, Instituto Universitario de Bio-Orgánica “Antonio González”, Universidad de La Laguna, Carretera de La Esperanza, 2, 38206 La Laguna, Tenerife, Spain, and Department of Pharmaceutical Chemistry, The School of Pharmacy, University of London, 29-39 Brunswick Square, London, WC1N 1AX, UK