1. Organic Chemistry Department, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium, Department of Chemistry, Warsaw University, Pasteura 1, 02-093 Warsaw, Poland, Medical Research Centre, Polish Academy of Sciences, Pawinskiego 5, 02-106 Warsaw, Poland, and Laboratory of Chemical Biology and Peptide Research, Clinical Research Institute of Montreal, Montreal, PQ H2W 1R7, Canada