1. Department of Chemistry, Durham University, Sciences Laboratories, South Road, Durham DH1 3LE, U.K., Chemistry Department, College of Science, King Saud University, PO Box 2455, Riyadh 11451, Saudi Arabia, and GlaxoSmithKline Pharmaceuticals, New Frontiers Science Park, Third Avenue, Harlow, Essex, CM19 5AW, U.K.