Oleodaphnoic Acid and Coriaceol, Two New Natural Products from the Stem Bark of Wikstroemia coriacea

Fractionation of the chloroform extract of Wikstroemia coriacea led to the isolation of two new compounds, oleodaphnoic acid (1), a guaiane-type sesquiterpenoid, and coriaceol (2), an 1,5-diphenyl-1-pentanone analogue, together with nine known compounds. The structures of 1 and 2 were elucidated by extensive spectroscopic data analysis. The known compounds were oleodaphnal (3), indicanone (4), (5R,8R,8aR)-3,8-dimethyl-4,5,6,7,8,8a-hexahydro-5-(1-methylethenyl)-2(1H)-azulenone, (5), 1,5 diphenyl-1-pentanone (6), (+)-3-hydroxy-1,5-diphenyl-1-pentanone (7), umbelliferone (8), daphnoretin (9), β-sitostenone (10) and (−)-hinokinin (11).


Results and Discussion
A total of eleven compounds were identified from the stem bark of W. coriacea. A new sesquiterpenoid oleodaphnoic acid (1, Figure 1) was isolated as a colorless powder. Its molecular formula C 15 H 18 O 3 was assigned on the basis of HR-ESI-MS (m/z 247.1326 [M+H] + , calcd. 247.1329), implying seven degrees of unsaturation. The IR spectrum displayed significant bands for an unsaturated ketone (1681 cm −1 ) and an α,β,, unsaturated carboxylic acid (3373 and 1646 cm −1 ). The H-NMR spectrum showed two tertiary methyl groups as singlets ( 1.85 and 1.77), and an exo-methylene resonance as two broad singlets ( 4.76 and 4.75) ( Table 1). The 13 C-DEPTQ NMR spectrum exhibited 15 carbon resonances, including two methyls, four methylenes, one exo-methylene, one ketone, one carboxylic function and two double bonds (Table 1). From the COSY and HSQC spectra, the occurrence of both partial structures, CH 3 C=CH 2 and CH 2 CH 2 CHCH 2 , was suggested. In the HMBC diagram, cross-peak observed between the methyl protons H-13 and the methine carbon C-7 was indicative of the combination of the above partial structures. Further analysis of the other significant long-range 1 H-13 C correlations ( Figure 2) suggested that NMR data were typical of a guaiane-type skeleton [21].
NMR data close similarity regarding the C-7 carbons of oleodaphnal (3) [22], indicanone (4) [6] and (5R,8R,8aR)-3,8-dimethyl-4,5,6,7,8,8a-hexahydro-5-(1-methylethenyl)-2(1H)-azulenone (5) [23], respectively, allowed to infer the R stereochemistry of C-7. Finally, the carboxylic group was assigned to be at the C-10 position based on the HMBC relationship between H-9 and C-14. From the above results the structure of oleodaphnoic acid was formulated as 1.   The IR spectrum of 2 suggested the presence of hydroxyl group (3440 cm −1 ), conjugated ketone carbonyl (1683 cm −1 ) and aromatic rings (1603-1480 cm −1 ). The 13 C-NMR spectrum gave a total of 17 separate resonances and the 13 C-DEPTQ sequence showed three methylene, eleven methine groups and three quaternary carbons, including a conjugated ketone carbonyl at 203.2 ( Table 1). The 1 H-NMR spectrum exhibited two AA'MM'X spin systems, typical of two monosubstituted aromatic rings and an oxygen-bearing methine signal at  5.10 (dd, J = 8.4, 3.3 Hz) ( Table 1). With the aid of COSY experiments, a -OCH(CH 2 ) 3 subunit was identified by further analysis of the remaining 1 H resonances. Finally, the location of the two phenyl moieties was supported by the HMBC correlations observed between the ortho H-2' (d 7.90) and H-2'' (d 7.13) signals and carbons C-1 and CH 2 -5, respectively. From the above spectral data, the structure of coriaceol (2) was established as 2-hydroxy-1,5diphenylpentan-1-one. This compound, which was previously reported with no NMR data as a synthetic intermediate [24], had not yet previously isolated from a natural source. Attempt to determine the stereochemistry at C-2 was not successful due to the decomposition of the compound.

General
HPLC was performed using an Agilent 1100 pump equipped with a Varian Dynamax Microsorb Si column (250 × 21.4 mm i.d., 5 μm, 100 Å) and a Varian Dynamax Microsorb C18 column (250 × 10 mm i.d., 5 μm, 100 Å), respectively, a refractomeric and a Diode Array Detector (DAD) detector. Optical rotations were measured on a Perkin-Elmer model 241 polarimeter equipped with a sodium lamp (589 nm) and a 1 dm cell. HRMS experiments were performed with a QStar Elite mass spectrometer (Applied Biosystems SCIEX) equipped with an ESI source operated in the positive ion mode. IR spectra were obtained with cell composed of two calcium fluoride windows separated by 0.21-mm thick PTFE spacer A145 using a Bruker FTIR Vertex 70 spectrometer. NMR spectra were recorded at 300 K for ~1 mg samples using a Bruker Avance DRX 500 spectrometer, equipped with a Bruker Cryoplateform and a 5 mm TXI cryoprobe. NMR spectra were referenced to CDCl 3 ( H = 7.26 ppm and  C = 77.16 ppm) or to CD 3 OD ( H = 3.31 ppm and  C = 49.00 ppm) [51]. Standard Bruker pulse sequences were used for homonuclear and heteronuclear two-dimensional experiments.

Plant Material
The stem bark of W. coriacea was collected from Nuku Hiva, Marquesas Islands, and was identified by Dr Jean-François Butaud. A voucher specimen (CM 1725) has been deposited in the Herbarium of French Polynesia [52].

Conclusions
This work is part of our ongoing phytochemical studies on Polynesian endemic plants aiming at a better knowledge of Polynesian plant biodiversity. We report herein the first phytochemical assessment of the stem bark of W. coriacea with the occurrence of two new natural compounds oleodaphnoic acid (1) and and coriaceol (2), a 1,5-diphenyl-1-pentanone analogue, beside nine known ones. The identified components belong to different secondary metabolite classes including guaiane-type sesquiterpenoids (compounds 1, 3, 4, 5), triterpenoids (10), phenylphenalenones (2, 6, 7), coumarins (8,9) and a lignan (11) which raises questions about the complexity of the biosynthetic routes to yield such less common chemodiversity exhibited by the same plant. Most of the isolated compounds are known for their relevant biological activities, which add more interest to this endemic Polynesian plant. We will follow up phytochemical analysis along with phylogenetic studies of all endemic species belonging to Wikstroemia genus grown in Polynesia aiming at a biodiversity assessment regarding insular plant adaptation and evolution.