General Experimental Procedures. Melting points were measured on a Fisher-Johns apparatus and are uncorrected. IR spectra were obtained a Perkin-Elmer 599 B spectrometer. UV spectra were recorded on a Shimazu 160A instrument. NMR spectra were recorded using a Bruker DRX 500 NMR spectrometer in CDCl3. Chemical shifts are presented on the δ (ppm) scale using TMS as internal standard. EIMS and HREIMS data were obtained by Finnigan-450 and MAT-711 mass spectrometers, respectively. Column chromatography was performed with Si gel 60 (Qingdao Marine Chemical Co. Qingdao, People’s Republic of China), 100-200 mesh. VP-16 was purchased from Sigma Chemical Co