1. Contribution from the Department of Chemical Technology, Twente University, P.O. Box 217, NL-7500 AE Enschede, The Netherlands, Laboratory of Organic Chemistry, Eindhoven University of Technology, P.O. Box 513, NL-5600 MB Eindhoven, The Netherlands, and Organische Chemie III/Makromolekulare Chemie, Universität Ulm, D-89069 Ulm, Germany