1. Department of Chemistry, University of Potsdam, Karl-Liebknecht-Strasse 24-25, D-14476 Potsdam, Germany, Department of Organic Chemistry, University of Debrecen, P.O. Box 20, H-4010 Debrecen, Hungary, and Department of Chemistry, University of Siegen, Adolf Reichwein Strasse 2, D-57068 Siegen, Germany