Two New Pyranoxanthones from Mesua beccariana (Guttiferae)

Our recent study on the stem bark extracts of Mesua beccariana has led to the isolation of two new pyranoxanthones, mesuarianone (1) and mesuasinone (2), two anthraquinones, 4-methoxy-1,3,5-trihydroxyanthraquinone (3) and 2,5-dihydroxy-1,3,4-trimethoxyanthraquinone (4), one coumarin, mammea A/AB (5) and three common triterpenes, stigmasterol (6), friedelin (7) and betulinic acid (8). Structural elucidations of these compounds were achieved using 1D and 2D NMR and MS techniques. This is the first report on the phytochemistry of Mesua beccariana.


Introduction
Mesua is a flowering genus consisting of about 48 species indigenous to south tropical Asia [1]. It is commonly known to the locals in Malaysia as ironwood due to its very hard and heavy wood, which is used for railroad ties and structural timber. Although there are some phytochemical reports on the genus Mesua, focused on Mesua ferrea [2][3][4][5], and a few on Mesua racemosa [6], Mesua thwaitesii [7] and Mesua daphnifolia [8,9], no study has been carried out on the chemistry of Mesua beccariana. These reports revealed the presence of xanthones, coumarins, biflavanoids and triterpenoids. This paper reports our recent phytochemical discovery of two new pyranoxanthones -mesuarianone (1) and mesuasinone (2)
The HMBC spectrum demonstrated the 2 J and 3 J multiple bond 1 H-13 C correlations in the molecule. The sharp singlet in the downfield region at δ 13.05 indicated a chelated hydroxyl group. Its multiplebond correlations to δ 99.6 (C-2), 103.4 (C-9a) and 163.3 (C-1) in the HMBC spectrum confirmed its position at C-1. Moreover, 3 J correlations between δ 7.44 (H-8) and C-9 (δ 180.5) and C-5a (δ 145.3) were clearly seen. Meanwhile, the singlet at δ 6.23 (H-2) gave a 3 J correlation to δ 103.4 (C-9a) and a 2 J correlation to δ 160.7 (C-3). This confirmed the position of these two aromatic methines to be at C-8 and C-2, respectivel. 5.56 (J = 10.1 Hz, H-11) and the carbon signal at δ 78.4 (C-12) respectively were also seen. These data together with the COSY spectrum analysis suggested the existence of a pyrano ring (ring A; Figure 2). Meanwhile, HMBC correlations of H-19 (δ 1.79) and H-18 (δ 1.48) to C-17 (δ 81.7) and the two ortho coupled vinylic protons H-15 and H-16 (δ 6.46 and 5.65) to C-17 (δ 81.7) were also observed ( Figure  3). The latter correlation pattern is similar to that of ring A, thus implying another pyrano ring (ring B) in the molecule. However, the latter pyrano ring carries only one methyl group. Two methylene hydrogens which gave a multiplet signal at δ 2.10 (H-20) demonstrated a 2 J correlation to the carbon signal at δ 41. 8 (C-19). This information allowed the placement of the prenyl moiety at C-19 which is attached to the pyrano ring B. The HMBC spectrum confirms the attachment of C-19 to C-17. 2 J and 3 J linkages of H-19 to C-17 and C-16 respectively were observed.
Compound 2, mesuasinone (Figure 1) 13 C and DEPT NMR spectra of compound 2 showed some similarity to that of 1, suggesting 2 also has a pyranoxanthone structure.
Moreover, the 1 H-NMR spectra also supported the existence of a pyrano ring. Two vinylic protons were observed at δ 6.83 ( The 1 H NMR spectra also gave a sharp singlet at δ 13.02 indicating a chelated hydroxyl group, while the broad singlet at δ 5.73 was due to the free hydroxyl group. In the HMBC spectra, the chelated hydroxyl proton (δ 13.02) showed cross-peaks with three quaternary carbon signals at δ 103.0 (C-9a, 3 J), 111.8 (C-2, 3 J) and 160.6 (C-1, 2 J). The deshielded carbon signal at 160.6 (C-1) was found to be attached to the hydroxyl group.

General
Infrared spectra were measured using the universal attenuated total reflection (UATR) technique on a Perkin-Elmer 100 Series FT-IR spectrometer. EIMS were recorded on a Shimadzu GCMS-QP5050A spectrometer. NMR spectra were obtained using a Unity JEOL 400 MHz FTNMR spectrometer using CDCl 3 as solvent and tetramethylsilane (TMS) as internal standard. Ultraviolet spectra were recorded in EtOH on a Shimadzu UV-160A, UV-Visible Recording Spectrophotometer.

Plant Material
The stem bark of Mesua beccariana was collected from the Sri Aman district in Sarawak, Malaysia. The plant material was identified by Associate Professor Dr Rusea Go, Biology Department, Faculty of Science, Universiti Putra Malaysia.

Conclusions
Two new pyranoxanthones, mesuarianone (1) and mesuasinone (2), along with two anthraquinones, a coumarin, and three triterpenes were isolated from the stem bark of Mesua beccariana. Future work on the biological activities such as anti-cancer and anti-oxidant properties will be carried out in due course.