Isolation and Crystal Structure of Marcanine A from Polyalthia plagioneura

Marcanine A was isolated from the stems of Polyalthia plagioneura as light yellow crystals. The molecular and crystal structures have been determined by 1D,2D-NMR and X-ray diffraction analysis. It crystallizes in the triclinic system, space group P-1 with a = 5.2140(5)Å, b = 10.1871(11)Å, c = 11.0709(13)Å, α = 110.452(2)º, β = 103.376(2)°, γ = 90.1870(10)°, V = 533.74(10)Å3, Z = 2. There are three intermolecular hydrogen bonds in a unit cell. It displays some inhibitory activities towards four kinds of human tumor cells, including BEL-7402, K562, SPCA-1and SGC-7409.


Introduction
Polyalthia plagioneura (Annonaceae) is a typical medium-sized tree in P.R. China, occurring mainly in Hainan, Guangdong, Guangxi and Yunnan provinces [1]. In a previous study only two compounds, howiicin A and plagionicin A were isolated from P. plagioneura [2,3]. In the present paper, we report marcanine A ( Figure 1) isolated from this plant as light yellow crystals and its singlecrystal structure determination by X-ray diffraction analysis. Marcanine A was isolated from Polyalthia genus for the first time.

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It is reported that marcanine A showed several biological activities. Soonthornchareonnon et al. [4] reported the cytotoxicity of marcanine A, which displayed IC 50 values between 80 nM and 2.1 μM against several human tumor cells, including A-549, HT-29, MCF7, RPMI and U251. Ichino et al. [5] found marcanine A had in vitro antimalarial activity against the drug-resistant K1 strain of Plasmodium falciparum.

Crystal structure determination
The molecular structure of the title compound was elucidated on the basis of physicochemical properties and spectral data including IR,1D-NMR and 2D-NMR. And the molecular formula of the crystal is C 14 H 9 NO 3 (Mr = 239.22). Its molecular structures is shown in Figure 2, and the hydrogen bonding diagram and packing diagram in a unit cell are shown in Figure 3, respectively. A summary of the crystal data and refinements is listed in Table 1. The thermal parameters of non-hydrogen atoms are given in Table 1. The selected bond lengths, bond angles and hydrogen bond lengths and angles are given in Tables 2 and 3, respectively.
114.28 (15)    From Table 2, it can be seen that all of bond angles are larger than 110° and less than 125°. The molecule contains three six-membered rings. In the amide ring (ring A) the bond distances and angles around N1 and O1 are in keeping with the geometric parameters found in intra-amides which are conjugated with a carbon-carbon double bond. All atoms in ring A are nearly coplanar ,C(1), C(2), C(3) C(4) and C(13) are almost in a plane controlled by the three double bonds [C(1)=O(1),C(2)=C(3) and C(4)=C(13)]. In the six-membered ring of the benzoquinone group (ring B), all atoms are strictly coplanar, that is C(4), C(5), C(6) C(11), C(12) and C(13) are also almost in a plane controlled by the four double bonds [C(5)=O(2), C(12)=O(3),C(6)=C(11) and C(4)=C(13)], respectively. Table 3 gives the hydrogen-bonding geometry. Owing to the peculiar spatial arrangement of the crystal, a few noticeable intermolecular hydrogen bonds are formed. Two molecules are connected by three intermolecular hydrogen bonds in a unit cell. A N-H group at N(1) forms an intermolecular hydrogen bond. The oxygen atom at C(1) has the C=O group as its acceptor. The value suggests that all the six member ring atoms are parallel in packing. Indeed, the stacking interaction also exhibits a planar molecular array (Figure 3). These intermolecular conventional and unconventional interactions link the molecules into an infinite two-dimensional supramolecular network structure and play key roles in stabilizing the crystal packing.

Cytotoxicity
From Table 4, we can learn that the title compound has some inhibitory activity towards four kinds of tumor cells. The IC 50 were less than 12 μM, which works most effective on SGC-7409 (IC 50 = 1.53 μM), and relatively weak on K562 (IC 50 = 11.78 μM).

Plant material
The stems of P. plagioneura were collected from BaWangling mountain in Hainan Province, P.R. China in May 2008, and identified as Polyalthia plagioneura by vice-professor Qiongxin Zhong from the College of Life Science in Hainan Normal University. A voucher specimen has been preserved in the Key Laboratory of Tropical Medicinal Plant Chemistry of Ministry of Education, Hainan Normal University.

Extraction and separation
Air-dried stems of P.plagioneura (20 kg) were ground and percolated (4 × 3 h) with 75% EtOH at 60 ºC, which was suspended in 5 L water and then successively partitioned with chloroform, ethyl acetate and n-BuOH, yielding a chloroform extract, an ethyl acetate extract and a n-BuOH extract, respectively. The chloroform extract was subjected to a silica gel CC column using petroleum ether as first eluent and then increasing the polarity with EtOAc, to afford 33 fractions. Fraction 6 was further