1. Department of Medicinal and Structural Chemistry, G. D. Searle and Company, 700 Chesterfield Parkway North, St. Louis, Missouri 63198, Department of Analytical Chemistry, Monsanto Corporate Research, Monsanto Company, 800 North Lindbergh Boulevard, St. Louis, Missouri 63167, and Department of Molecular Biology and Pharmacology, Washington University School of Medicine, St. Louis, Missouri 63110