1. Dyson Perrins Laboratory, South Parks Road, Oxford, OX1 3QY, UK, Department of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, UK, AstraZeneca Pharmaceuticals, Mereside, Alderley Park, Macclesfield, Cheshire, SK10 4TG, UK, and GlaxoSmithKline, New Frontiers Science Park, Third Avenue, Harlow, Essex, CM19 5AW, UK