The leaves of Ceriops tagal (Figure 1) were collected in Haikou City, Hainan Province of China, and authenticated by Prof. Weidong Han (College of Agriculture, Guangdong Ocean University). A voucher specimen (No. 20130513) was deposited in the Guangdong Key Laboratory for Research and Development of Natural Drugs, Guangdong Medical College, Zhanjiang, China.

Human carcinoma cell lines (CNE-2, A549, HepG2 and HCT-116) all came from ATCC. Column chromatographies (CC) were carried out byD101 macroporous resin (Beilianchem, China), silica gel (Qingdaohaiyang, China) and reversed-phase C₁₈ silica gel (YMC, Japan). 1D and 2D NMR spectra were acquired on Bruker AV- 400 spectrometer.HR-ESI-MS was recorded by an Agilent 6210 LC/MSD TOF mass spectrometer. Analytical high-performance liquid chromatography (HPLC) were carried out on a Agilent 1200 series and a C₁₈ reversed- phase column (Cosmosil, 4.6 mm × 250 mm, 5.0 μm). Preparative HPLC were carried out on a Gilson 305 pump, a Varian Prostar 345 UV detector and a C₁₈ reversed-phase column (Cosmosil, 20 mm × 250 mm, 5.0 μm).

The air dried leaves of Ceriops tagal (10.0 kg) were powdered and extracted three times (24 h for each) with 95% EtOH at room temperature (3 × 30 L). The tannins were precipitated and filtrated after keeping the solution standing for 48 hours at room temperature. Crude extract (800 g) was yielded by concentrating the filtration under vacuum. The residue was suspended in water, and then partitioned with EtOAc. After removing the solvent, the EtOAc extract (300 g) was separated by D101 macroporous resin column chromatography (CC) using gradient ethanol aqueous solutions (60%, 80% and 95%) as eluants to give three fractions (A-C). Fraction B (80% ethanol, 100 g) was then subjected to silica gel CC eluting with n-hexane-ethyl acetate (100:0→10:1) to yield seven subfractions B1-B7. Subfraction B4 (H:E 9:1, 7.0 g) was chromatographed on ODS column eluting with CH₃CN-H₂O (1:9→7:3) to afford eight subfractionsB4a-B4h. B4b(CH₃CN:H₂O 3:7, 0.6 g) was purified repeatedly by prepared HPLC (CH₃CN:H₂O 4:6) to yield compound 1 (13.0 mg), 2 (14.8 mg), 3 (65.1 mg), 4 (28.3 mg), 5 (50.1 mg) and 6 (61.3 mg) (Figure 2).

The cytotoxicity effects of compounds 1-6 were tested against four human carcinoma cell lines (CNE-2, A549, HepG2 and HCT-116) by the MTT method as described previously [7] . Generally, the cell suspensions were platedinto 96-well plates and cultured in RPMI-1640 at 37˚C, with 5% CO₂ in incubator overnight. The test compound solutions (in 0.1% DMSO) at different concentrations were added to the corresponding wells. After exposure for 68 h, MTT was added to each well and the plates were incubated for 4 h. Finally, the supernatant was discarded and 200 μL of DMSO was added to the well to dissolve the blue-violet crystal, then the opticaldensity (OD) values were read on the microplate reader at 570 nm. All tests and analyses were carried out in triplicate. DMSO and doxorubicin were applied as the blank control and positive control, respectively.

Compound 1, obtained as pale yellow amorphous powder in methanol, had the molecular formula C₁₈H₂₈O₂ as established by its HR-ESI-MS at m/z 299.1992 [M + Na]⁺ (calcd for C₁₈H₂₈O₂Na: 299.1982), which implied that 1 had five degrees of unsaturation. The ¹³CNMR and DEPT-135 spectra of 1 displayed a carbonyl carbon, and two olefinic carbons, as well as 15 aliphatic carbons including four methyls, five methylenes, three methines, and three quaternary carbons. The ¹HNMRspectrum of 1 showed fourmethyls at δ_H 0.87 (3H, s), 0.92 (3H, s), 1.03 (3H, d, J = 5.2 Hz), 1.31 (3H, s), a pair of olefinic protons at δ_H 6.84 (1 H, dd, J = 10.1, 5.9 Hz) and6.13 (1H, d, J = 10.2 Hz), a methine at δ_H 2.8 (1H, dd, 13.3, 6.6)and a set of aliphatic protons ranging from δ_H 1.22 to 1.96. The above spectroscopic features revealed that compound 1 was a dolabrane-type dinorditerpenoid. The ¹³C NMR and ¹H NMR spectra of compound 1 were very similar to those of tagalsin Q (5) [8] except for the positions at 3-5, 18 and 19, which suggested that their B/C rings were the same, while their A rings were different. All the ¹H and C NMR signals of 1 were assigned as shown in Table 1 with the aid of ¹H-¹H COSY, HSQC and HMBC experiments. The bond between C-4 and C-18 changed from olefinic double-bond of tagalsin Q (5) into aliphatic single-bond of 1, which was further supported by the HMBC correlations between H-18 (δ_H 1.03) and C-3 (δ_C202.7)/C-5 (δ_C39.0) (Figure 3). The relative configuration of 1 was proposed on the basis of ROESY correlations (Figure 3). The β-orientation of H-8, H-10, H₃-19 and H₃-17, and the α-orientation of H₃-18 and were H₃-20 deduced from the presence of ROESY interactions between H-8/H-6β, H-6β/H-10, H-10/H₃-19, H-19/H₃-4, H-10/H-6β, H-6β/H-8, H-10/H-8, H-10/H-11β, H-11β/H-17 and H-18/H-6a, and the absence of ROESY interactions between H₃-18/H-19, H₃-18/H-10, H₃-20/H-8 and H₃-20/H-19. On the basis of the above results, compound 1 was identified as (4S*, 5S*, 8S*, 9S*, 10R*)-13S*-hydroxy-15, 16-dinorlabr-1(2)-en-3-one, and named tagals in X.

The known compounds were identified as (5S*, 8S*, 9S*, 10R*, 13S*)-2-hydroxy-16-nor-3-oxodolabr- 1,4(18)-dien-15-oic acid (2) [8] , (5S*, 8S*, 9S*, 10R*, 13S*)-3-hydroxy-16-nor-2-oxodolabr-3-en-15-oic acid (3) [8] , tagalsin P (4) [8] , tagalsin Q (5) [8] and (5S*, 8S*, 9S*, 10R*, 13S*)-3,16-dihydroxydolabr-3-ene-2,15- dione (6) [8] , respectively, on the basis of their ¹H NMR and ¹³C NMR spectra analysis and comparison with those reported data in the related literatures (Figure 2).

^aAssignments were established by interpretation of the ¹H-¹H COSY, HSQC, and HMBC spectra; ^bOverlapped signals are reported without designating multiplicity.

As can be seem from Table 2, compound 2 showed significant cytotoxicity against CNE-2, A549, HepG2 andHCT-116 cell lines with IC₅₀ values of 13.57 ± 1.02, 42.32 ± 2.21, 11.21 ± 1.13 and 15.23 ± 1.42 μM, respectively, while the other five analogues had no obvious effect even with the concentration of 50 μM.

^aAll results are expressed as mean ± SD, n = 3 for each group; ^bPositive control.

Up to now, chemical examinations of the plant Ceriops tagal have resulted in the isolation of 27 dolabrane-type diterpenes [3] [9] , which could be divided into four sub-types: diterpene (twenty), 16-norditerpene (three), 15, 16-dinorditerpene (two) and ring A-seco-diterpene (two). Interestingly, dolabranes were regarded as a small group of natural products as there were only about fifty dolabranes isolated from plants, and as this papermentioned, most of them existed in Ceriops tagal. So we regarded dolabrane as chemoaxonomic marker of Ceriops tagal.