1. Contribution from the Department of Chemistry, King's College London, Strand, London WC2R 2LS, U.K., Institut für Organische Chemie, University of Würzburg, Am Hubland, D-97074, Würzburg, Germany, and Istituto di Chimica Organica, Università di Siena, Via Aldo Moro, 53100 Siena, Italy