1. Department of Organic Chemistry, Faculty of Agricultural and Applied Biological Sciences, Ghent University, Coupure Links 653, B-9000 Ghent, Belgium, Institute of Pharmaceutical Chemistry, University of Szeged, H-6701, Szeged, POB 121, Hungary, and Organic Chemistry Department, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium