1. Contribution from the Laboratory of Medicinal Chemistry, Center for Cancer Research, National Cancer Institute at Frederick, 376 Boyles Street, Frederick, Maryland 21702, Laboratory for the Structure of Matter, Naval Research Laboratory, Washington, D.C. 20375, and The National Institute for Biotechnology and Department of Virology, and Department of Clinical Pharmacology, Faculty of Health Sciences, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel