1. Laboratory of Medicinal Chemistry, Division of Basic Sciences, and Experimental Retrovirology Section, Medicine Branch, Division of Clinical Sciences, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892, and Center for Advanced Biotechnology and Medicine and Rutgers University Chemistry Department, 679 Hoes Lane Piscataway, New Jersey 08854, and Institute of Organic Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, D-20146 Hamburg, Germany