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Derivatives 24-34 bearing both a 4-ketone and a 27-silyl ether group were prepared from compound 23 by silylation at C-27 with different silyl chloride reagents as shown in Scheme 4 (Supporting Information, S41-S46). Scheme 4. Synthesis of 4-Oxo-withaferin A-silyl Ether Analogues 24-34 a a Reagents and conditions: (i) R 1 R 2 R 3 SiCl, imidazol, DMAP, CH 2 Cl 2 , rt.

Seven new (1-4 and 7-9) sesquiterpenes with a dihydro-beta-agarofuran skeleton, along with four known compounds (5, 6, 10, and 11), have been isolated from the leaves of Maytenus jelskii. The structures of the new compounds were elucidated by means of spectroscopic data analysis, including 1D and 2D NMR techniques, and their absolute configurations were determined by circular dichroism and chemical correlations. The compounds have been tested for their inhibitory effects on Epstein-Barr virus early antigen (EBV-EA) activation induced by 12-O-tetradecanoylphorbol-13-acetate (TPA). Compound 10 was found to be an effective antitumor-promoting agent and also showed a potent chemopreventive effect in an in vivo two-stage carcinogenesis model.