1. Department of Chemistry, South Kensington Campus, Imperial College London, London SW7 2AZ, U.K., Department of Chemistry, University of Fribourg, 9 Ch. du Musee, CH-1700 Fribourg, Switzerland, Department of Organic Chemistry, University of Geneva, 30, quai Ernest Ansermet, CH-1211 Geneva 4, Switzerland, and GSK Tonbridge, Old Powder Mills, Leigh, Tonbridge, Kent TN11 9AN, U.K.