1. Institute of Pharmaceutical Chemistry, University of Szeged, H-6720 Szeged, Eötvös u. 6, Hungary, Department of Medical Chemistry, University of Szeged, H-6720 Szeged, Dóm t. 8, Hungary, and Institute of Chemistry, Department of Organic Chemistry, Eötvös Loránd University, H-1117 Budapest, Pázmány P. s. 1/A, Hungary