Chem. Pharm. Bull. 53(1) 125—127 (2005)

ring in Viburnum awabuki and V. odoratissimum from a phytochemical point of view. More than 30 kinds of vibsanetype diterpenes have been found so far, and they can be successfully categorized into three sub-classes. As part of our studies on biologically active compounds in Viburnum species, we have examined chemical components in the leaves of V. luzonicum, an evergreen shrub widely distributed in Taiwan. Since one paper reported that a 70% acetone extract of this plant inhibited KB cell growth, our attention was directed to exploration of the cytotoxic principles, resulting in the isolation of four new iridoid aldehydes 1—4 named luzonials A and B, luzonidials A and B. In this paper, we report the structure elucidation of these new compounds and their inhibitory activity against HeLa S3 cancer cells. The methanol extract of the leaves of V. luzonicum was fractionated on a silica gel column into fractions 1—17. Fractions 3 and 10 were purified by a combination of silica gel chromatography and preparative HPLC, which yielded the four new iridoids 1—4. Luzonial A (1) had a [M] ion peak at m/z 360.1204 in high-resolution (HR)-EI-MS, corresponding to the molecular formula C19H20O7 and its IR spectrum displayed absorptions due to the presence of hydroxyl (3338 cm ) and carbonyl (1683 cm ) groups, and an aromatic (1587, 1515 cm ) moiety. The UV (313 nm) and NMR data of 1 (Table 1) showed the presence of an (E)-p-coumaroyl group [d 6.80 (2H, d, J 8.8 Hz), 7.47 (2H, d, J 8.8 Hz), 6.37 (1H, d, J 15.9 Hz), 7.64 (1H, d, J 15.9 Hz); d 168.7], which was supported by the observation of prominent fragment peaks at m/z 147 and 164 in the EI-MS. The remaining NMR spectral data (Table 1) of 1 indicated the presence of an aldehyde [d 9.58 (s); d 196.0] and an exo-methylene [d 6.24 (s), 6.55 (d, J 1.1 Hz); d 134.8, 153.4], which formed a conjugated system on the basis of the correlation of the exo-methylene signals with the aldehyde carbonyl in the H-detetced heteronuclear multi-bond correlation (HMBC), as well as of an acetal moiety [d 5.34 (d, J 0.8 Hz); d 104.6] and an isolated oxymethylene [d 3.93 (d, J 9.5 Hz), 4.22 (d, J 9.5 Hz); d 73.5]. The routine analyses of H–H shift correlation spectroscopy (COSY) and H-detected heteronuclear correlation through multiple quantum coherence (HMQC) indicated the presence of a partial structure (bold line), as shown in Fig. 2. This partial structure was found to contain the conjugated exo-methylene aldehyde unit at the C-5 position and the pcoumaroyl group at the C-7 position according to the HMBC correlations of H-5 and H-7 resonating at d 2.99 and 5.22 with the exo-methylene carbons C-4 and C-1 and the ester carbonyl of a p-coumaroyl group, respectively. In the HMBC as summarized in Fig. 2, the H-10 oxy-methylene signals showed correlations with the acetal carbon C-1, whereas the H-1 acetal and H-9 signals had cross peaks to the C-8 quaternary oxygen-bearing carbon at d 90.7, which further correlated with H-7. These spectral data indicated that C-8 was connected to the p-coumaroyl-bearing oxycarbon C-7 and the methine C-9 was involved in the partial unit, and also that C10 was bonded to C-8 as well as to C-1 through an ether bond, resulting in the formation of an iridoid structure fused with two five-membered rings, as shown in Fig. 1. In addition, stirring a methanol solution of 1 in the presence of Amberlyst 15E gave 1a, which confirmed the presence of a cyclic acetal ring in 1. The relative stereochemistry of 1 was elucidated on the basis of the following nuclear Overhauser and exchange spectroscopy (NOESY) correlations as shown in Fig. 2: H-5/H-9, H-5/H-6b , H-7/H-10a , and H-10b /H-9, which can account for a p-coumaroyl group at C-7 and a conjugated aldehyde moiety at C-5 taking a band an a-configuration, respectively. However, the configuration on the C-1 acetal carbon was unable to be determined by the NOESY because H-5 observes NOE for H-1a or H-1b . The most stable conformation for 1 obtained by a MM2 calculation using Marcromodel indicates that a dihedral angle between H-5 and H-1a , and H-5 and H-1b is 82° and 39°, respectively. Considering the small J value (0.8 Hz) observed for H-1, the OH located at C-1 should take a b configuration. Accordingly, luzonial A (1) was represented as 1. Luzonial B (2) was assigned the same molecular formula C19H20O7 as 1, obtained from HR-EI-MS at m/z 360 [M] , and exhibited physical and NMR data (Table 1) very similar to those of compound 1 except for the NMR data assignable to a p-coumaroyl group. Namely, a small J value (12.6 Hz) was observed for two vicinal olefinic protons, indicating the presence of a (Z)-p-coumaroyl group. It was also evident from the HMBC correlation of the H-7 signal at d 5.19 with the ester carbonyl at dC 167.8 that this p-coumaroyl group was located at the C-7 position. Other NMR data including HMBC and NOESY were consistent with those of 1. Thus the structure of luzonial B (2) was determined to be 7-O-(Z)January 2005 Notes Chem. Pharm. Bull. 53(1) 125—127 (2005) 125