1. Department of Chemistry, University of Wisconsin−Milwaukee, P.O. Box 413, Milwaukee, Wisconsin 53201, Department of Pharmacology & Toxicology, University of Arkansas for Medical Sciences/Mail Slot 611, 4301 West Markham Street, Little Rock, Arkansas 72205, and Department of Biochemistry, Merck Sharpe and Dohme Research Laboratories, Terlings Park, Eastwick Road, Harlow, Essex CM20 2QR, U.K.