1. Institut für Organische Chemie, Georg-August-Universität Göttingen, D-37077 Göttingen, Germany, Laboratory of Bioorganic Chemistry, National Institute of Diabetes, and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892-0820, and Developmental Therapeutics Program, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892-7448