1. Chemistry Research Laboratory, University of Oxford, Mansfield Road, Oxford OX1 3TA, U.K., Université Claude Bernard Lyon I, 43 bd du 11 Novembre, 1918 Villeurbanne 69622, France, and GlaxoSmithKline Medicines Research Centre, Gunnels Wood Road, Stevenage, Herts SG1 2NY, U.K.