1. Contribution from the University Chemical Laboratory, Lensfield Road, Cambridge CB2 1EW, U.K., SmithKline Beecham Pharmaceuticals, New Frontiers Science Park (North) Third Avenue, Harlow, Essex CM19 5AW, U.K., and SmithKline Beecham Pharmaceuticals, 1250 South Collegeville Road, P.O. Box 5089, Collegeville, Pennsylvania 19426-0989