1. Contribution from the Chemistry Department, University College, Gower Street, London, WC1E 6BT, U.K., Institut für Organische Chemie der Georg-August-Universität Göttingen, Tammannstrasse 2, Göttingen 37077, Germany, and Department of Organic Chemistry “A. Mangini”, University of Bologna, Viale Risorgimento 4, Bologna 40136, Italy