LAETANINE, ICARISIDE E3, (-)-PINORESINOL AND MANTOL O-Β-D-GLUCOPYRANOSIDE FROM AQUEOUS EXTRACT OF PHOEBE TAVOYANA (MEISSN) HOOK

Phoebe tavoyana (Meissn) Hook often grows scatterly in tropical rain forest in Viet Nam. The plant is used in traditional medicine to treat acne and the bark is used to prevent rheumatism. In continous course of study on chemical composition of Phoebe genus, this paper describes the extracion and structrure evaluation of four compounds including laetanine (1), icariside E3 (2), (-)-pinoresinol (3) and maltol O-β-D-glucopyranoside (4) from an aqueous extract of the leaves of Phoebe tavoyana (Meissn) Hook. These compounds were isolated based on thin layer and column chromatographies. Their chemical structures were determined by analyses of their ESI-MS, 1D-NMR and 2D-NMR spectral data, and compared with those reported in the literature. This is the first report of compound 2 and 4 from the genus Phoebe.


INTRODUCTION
Phoebe is a genus of tall, flowering plants belonging to the Laurel family, Lauraceae, which contains about 200 species, mainly distributed in Asia and America. They are found in large numbers in the Borneo peninsula and Malaysia. In Viet Nam, the genus Phoebe includes about 12 species and distributed throughout the country. Phoebe is widely used in folk remedies such as healing wounds or sores, eliminating pimples and rheumatism. Phytochemical studies revealed attractive alkaloids, particularly, oxoaporphine, and aporphine alkaloids; terpenoids, lignans were recognized as main compounds of this genus as well as in P. tavoyana [1 -3]. In Viet Nam, seven alkaloids including corydydine, N-methylaurolitsine, N-methyllaurotetanine, pronuciferine, stepharine, norcorydine and anonaine, some steroids and lignans such as phoebenoside A, phoebenoside B, dendranthemoside A, lyoniresinol, (+)-3-O-Lrhamnopyranoside-5-methoxyisolariciresinol, and (+)-lyoniresinol 3α-O-β-D-glucopyranoside were isolated from P.tavoyana [1 -3]. To continue the phytochemical studies on Phoebe, this paper reports the isolation and characteration of 4 compounds from P. tavoyana.

Plant material
The leaves of Phoebe tavoyana (Meisn.) Hook. were collected at Me Linh, Vinh Phuc province, Viet Nam in May, 2016 and identified by Dr. Nguyen The Cuong, Institute of Ecology and Biological Resources, Vietnam Academy of Science and Technology (VAST). A voucher specimen (NCCT-P59) was deposited at the Biological Resources Research Department, Institute of Marine Biochemistry, VAST.

RESULT AND DISCUSSION
Compound 1 was separated as the white amorphous powder. The 1 HNMR showed two methoxyl protons at δ H 3.90 (3H, s) and 3.61 (3H, s), respectively. Seven aliphatic protons were observed at the high chemical shift from δ 2.60 to 3.75 ppm, expected of protons of H-4, H-5, H-6a, and H-7. A singlet proton at δ 6.59 was determined to be the signal at H-3, confirming that C-1 and C-2 were substituted. Two singlet signals seen at δ 8.03, 6.74 were assigned to the protons at H-11 and H-8. These values were typical of a 9,10 substitution pattern of aporphine moiety. The H-5 proton appeared at the lower field compared to H-4 due to the neighboring Natom adjacent to C-5.
The 13 C-NMR showed 18 carbon signals, including three aromatic carbons, four aliphatic carbons, nine quaternary carbons, and two methoxyl groups.
Compound 4 was obtained as a white amorphous powder. The 1 H-NMR showed the signal of 2 aromatic protons at δ H 6.46 (1H, d, 6.0), 8.02 (1H, d, 6.0), located at H-5 and H-6, confirming that the other carbons of the ring is substituted. 13 C-NMR spectrum of compound 1, showing a total of 12 carbon signals, of which, there is one carbon signal of ketone group at C-4 δ C 177.2, 3 signals of the quaternary carbon at δ C 164.2; 157.1; 143.6. These values indicated the appearance of a maltol ring in the molecule. Six typical carbon signals of glucopyranose molecule at δ C 105.5; 78.6; 78.0; 75.4; 71.2; 62.6.
The HSQC spectrum showed that the anomeric proton is at the chemical displacement range of the solvent, which had direct interaction with C-1' (105.5). On the other hand, the HMBC spectrum, there was an interaction between anomeric proton and C-3 (143.6), confirmed that the sugar moiety linked to C-3 of the aglycone. HMBC interaction between H-5 (6.46) and C-4 (177.2), C-6 (157.1), C-3 (143.6) allowed determining the position and chemical shift of C-4 and C-6. HMBC interaction of proton H-6 (8.02) to C-2 (164.2), C-5 (117.3), C-4 (177.2). Besides, the interaction between -CH3 (2.49) and C-3, C-2, determining the position and chemical shift of C-5 and C-2. The proton coupling constant of H-1' revealed this proton was axial orientation. All the NMR data of 4 were consistent with the corresponding data of maltol O-β-D-glucopyranoside. Furthermore, the ESI-MS of 4 exhibited an ion peak at m/z 289 [M+H] + corresponding to the molecular formula of C 12 H 16 O 18 . Hence, the structure of 4 was determined as maltol O-β-D-glucopyranoside [7]. This compound was firstly isolated from Phoebe genus.