1. Department of Chemistry, King's College London, Strand, London WC2R 2LS, U.K., Dipartimento di Chimica “G. Ciamician” dell'Universita di Bologna, Via Selmi 2, 40126 Bologna, Italy, and Institute für Organische Chemie, Universität Würzburg, Am Hubland, D-8700 Würzburg, FRG