1. Katholieke Universiteit Leuven, Department of Chemistry, Celestijnenlaan 200 F, B-3001 Leuven, Belgium, Universität Würzburg, Institut für Organische Chemie, Am Hubland, D-97074 Würzburg, Germany, and Eindhoven University of Technology, Laboratory of Macromolecular and Organic Chemistry, P.O. Box 513, 5600 MB, The Netherlands