The investigation of Taxus canadensis (Canada yew) needles as a renewable source of taxol and its congeners led to the isolation of a novel taxane derivative, 9-dihydro-13-acetylbaccatin III, which was characterized by spectral analyses and confirmed by single crystal X-ray diffraction studies. Since the discovery of the promising anticancer activity of taxol [2], attention has focused on the acute need for alternate sources of taxol and efficient isolation methods (1-3). The members of the family Taxaceae, the genus Taxus in particular, have been investigated extensively; however, only limited chemical work has been reported on Taxus canadensis Marsh. (4), a species relatively abundant in North America. Canada yew is an evergreen shrub growing in the mixed conifer-hardwood forests of northeastern United States and southeastern Canada (5). Analysis of an EtOH extract of the needles and twigs of this plant by hplc according to literature methods (6) showed the taxol content to be 0.1%. An enriched fraction containing 1.27% of taxol, obtained by solvent extraction, was separated by high speed planetary coil countercurrent chromatography (pccc). The fractions were analyzed by a combination of tlc, ms, and P-388 cytotoxicity. The biological activity identified the taxol-containing fractions which were further purified by pccc to furnish about 60% of the taxol present in the extract. A similar pccc method for the separation of a taxol-containing column chromatographic fraction was reported recently (7). The mass spectral analysis of the fractions indicated the presence of [M]+ ions corresponding to a number of previously reported taxane diterpenoids (8), and their isolation will be described separately. A fraction producing a hitherto undescribed [M]+ ion, at m/z 630, was recrystallized from MeOH to give compound 1. The formula of the novel compound, C33H42O12, determined by hrfabms, was found to differ from that of 13-acetylbaccatin III (9) by two hydrogen atoms. The comparison of the ¹H nmr spectra of 1 and baccatin III [4], recorded in CDCl3, indicated that the two compounds are closely related but differ at C-9 and C-13. Further, the spectra of 1 displayed signals for an additional acetate function. The ¹H signal due to H-10 in 4 appeared as a singlet at 6.32 ppm, while that due to H-10 in 1 appeared as a doublet at 6.2 ppm coupled to a doublet at 4.45 ppm (J=10.9 Hz), suggesting that 1 has a hydroxyl function at C-9 instead of the carbonyl function present in 4. The signal due to H-13 in 4 appeared at 4.48 ppm while that in 1 appeared at 6.17 ppm, indicating that the additional acetate function is at C-13 in the latter compound. The only other significant difference is in the chemical shift of the signal due to H-3. In compound 4 this appeared at 3.9 ppm while in compound 1 this signal has moved up-field to 3.05 ppm. The analysis of one- and three-bond C-H correlations by HMQC (10) and HMBC (11) experiments and nOe information allowed the unambiguous assignment of all ¹H- and ¹³C-nmr signals (Table 1) and the assignment of the structure 9-dihydro-13-acetylbaccatin III for compound 1. Finally, the structure was confirmed by single crystal X-ray crystallography (Figure 1). The antiproliferative activity of compounds and fractions was assessed by MTT reduction as previously described (12). In parallel determination (means±SD, n=6), taxol [2] (IC₅₀=0.05±0.01 μg/ml) was a 400-fold more potent inhibitor of P-388 leukemia cell proliferation than 9-dihydro-13-acetylbaccatin III [1] (IC₅₀=20±1 μg/ml). To the best of our knowledge this is the first report of this compound; however, the isolation of the corresponding diacetate 5 from Taxus baccata has been reported (13). We believe that this compound, one of the major constituents of the T. canadensis extract, could be used as a synthetic precursor of 9-dihydrotaxol analogues that cannot be obtained from taxol itself due to the resistance of the C-9 carbonyl function to chemical reduction (14). The superimposition of both solid state and solution structures of taxol [2] and 1 showed that the presence of the 9-hydroxy function has a minimal effect on the conformation of the taxol skeleton.