1. Natural Products Laboratory, Division of Medicinal Chemistry and Natural Products, School of Pharmacy, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, Biotech Research Laboratories, 3 Taft Court, Rockville, Maryland 20850, Boston Biomedica, Inc., 375 West Street, West Bridgewater, Massachusetts 02379, and Department of Surgery, Duke University Medical Center, Durham, North Carolina 27710