1. Contribution from the Department of Chemistry, Box H, Brown University, Providence, Rhode Island 02912-9108, Department of Organic Chemistry, Wageningen Agricultural University, Dreijenplein 8, 6703 HB Wageningen, The Netherlands, Plant Research International, P.O. Box 16, 6700 AA Wageningen, The Netherlands, and Roy and Diana Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323