1. Department of Organic Chemistry, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, B-9000, Ghent, Belgium, Institute of Pharmaceutical Chemistry, University of Szeged, H-6701 Szeged, P.O. Box 427, Hungary, and Department of Chemistry, University of Jyväskylä, Fin-40351, Jyväskylä, Finland