1. Department of Chemistry, Imperial College London, Exhibition Road, South Kensington, London SW7 2AZ, U.K., The Chemistry Laboratory, University of Sussex, Falmer, Brighton, East Sussex BN1 9QJ, U.K., and GlaxoSmithKline Medicines Research Centre, Gunnels Wood Road, Stevenage, Hertfordshire SG1 2NY, U.K.