1. Divisions of Discovery Chemistry, Oncology Drug Discovery and Pharmaceutical Development, The Bristol-Myers Squibb Pharmaceutical Research Institute, P.O. Box 4000, Princeton, New Jersey 08543-4000, and Gesellschaft für Biotechnologische Forschung mbH, Abt. Naturstoffchemie, Mascheroder Weg 1, D-38124 Braunschweig, Germany