1. Contribution from the Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands, Philips Research High Tech Campus 4, 5656 AE, Eindhoven, The Netherlands, and Universität Würzburg, Institut für Organische Chemie, Am Hubland, D-97074 Würzburg, Germany