Department of Chemistry, Chemistry Research Laboratory, University of Oxford, Mansfield Road, Oxford, OX1 3TA, U.K., Syngenta, Jealott's Hill International Research Centre, Bracknell, Berks, RG42 6EY, U.K., and AstraZeneca Pharmaceuticals, Mereside, Alderley Park, Macclesfield, Cheshire, SK10 4TG, U.K. (At 2006-02-04)