Kokosanolide D: A New Tetranortriterpenoid from Fruit Peels of Lansium domesticum Corr. cv Kokossan

: A novel tetranortriterpenoid named kokosanolide D has been isolated from fruit peels of Lansium domesticum . The structure of kokosanolide D was elucidated primarily on the basis of spectroscopic data including infrared, 1D and 2D-NMR, as well as high resolution mass spectroscopy analysis and comparison with related compounds previously reported.

During our previous research on triterpenoid compounds from Indonesian Meliaceae plants, we have isolated two tetranortriterpenoid compounds (kokosanolide A and C) from the fruit seeds and one onoceranoid type triterpenoid compound (kokosanolide B) from the bark of L. domesticum cv Kokossan [12]. In the present study, we isolated a new tetranortriterpenoid compound from the methanol extract of fruit peels of L. domesticum Corr. cv Kokossan which is named kokosanolide D (1) (Figure 1). The chemical structure of compound 1 is determined by spectroscopic data, including infrared, 1D and 2D-NMR and HRMS.

Discussion
Kokosanolide D (1) is obtained as colorless and amorphous from chloroform acetone. The molecular formula is determined to be C 27 H 32 O 10 by LC-ESI-MS data (m/z 517.2076, [M + H] + ), which is combined with 1 H and 13 C-NMR spectral data (Table 1) with twelve degrees of unsaturation. The IR (KBr) spectrum shows bands which can be assumed to be derived from the hydroxyl groups (v max : 3441 cm −1 ), ketones (v max : 1760 cm −1 ), unsaturated ketones (v max : 1726 cm −1 ) isolated double bonds (v max : 1698 cm −1 ) and gem-dimethyl (v max : 1440 cm −1 and 1392 cm −1 ). 1 H-NMR ( Figure S1) showed three singlet signals (δ 0.98, 1.26, and 1.37) from the tertiary methyl group and one doublet from the secondary methyl group at δ 1.16 which correlated with H-10 (δ 3.29, 1H, d, 7). The singlet signal also appears in the lower field area (δ 3.67), which is thought to be the C-27 methoxy proton signal. A more refined analysis of the 1 H-NMR spectrum reveals the signal characteristics of the tetranortriterpenoid skeleton in the presence of β-substituted furan signals (δ 7.48, 7.39 and 6.43) and olefinic signals of α, β-unsaturated ketone (δ 6.46, 1H, s). 13 [17]. The correlation of the partial structure was further explained by the HMBC correlation spectral data (Figures 2 and S6).  5). An α,β-unsaturated δ-lactone ring system is determined by the correlation of H-15 (δ 6.46) which connected with C-8 (δ 67.8), C-13 (δ 38.4) and C16 (δ 165.7). The correlations arising from two methyl (δ 0.98 and 1.37) to C-3 (δ 209.7), C-4 (δ 47.8) and C-5 (δ 55.9) indicate that gem-dimethyl is bound to C-4. Another correlation between the carbomethoxyl (δ 3.67) and H-6 (δ 4.82) signals to the carbonyl ester (δ 172.0) determines the position of the ester group attached to C-6. Basically, the NMR data of compound 1 were similar to those of kokosanolide A [12]. The only difference lies in C-8. Kokosanolide A has C-8 (δ 34.6), which is a metine, while in compound 1, C-8 (δ 67.8) is a quaternary carbon that binds the hydroxyl group. The absence of a C-8 correlation with any protons in the HMQC data supports this suggestion. A NOESY spectrum shows H-5/ H-6/H20 correlation. This NOESY correlation is also found in kokosanolide A [12]. It can be assumed that kokosanolide A and compound 1 have the same stereochemical configuration and indicated that hydroxyl group C-8 should have α-orientation. Thus, the structure of compound 1 is determined to be a new tetranortriterpenoid from kokosanolide group named kokosanolide D.

General Experimental Procedures
Mass spectra were measured with a waters Xevo QTOFMS (Waters, Milford, MA, USA). IR spectra were measure on a One Perkin Elmer infrared-100. NMR data were recorded on a JNM-ECZ500R/S1 spectrometer at 500 MHz for 1 H and 125 MHz for 13 C using TMS as an internal standard. Chromatographic separations were carried out on silica gel G60 (Merck, Darmstadt, Germany) and RP18 (Merck). TLC plates were pre-coated with silica gel GF254 (Merck, 0.25 mm), and detection was achieved by spraying with 10% H 2 SO 4 in ethanol, followed by heating.

Plant Material
The fruit peels of L. domesticum were collected from Cililin, West Java, Indonesia in April 2018. The plant was identified and deposited in the Herbarium Laboratory of the Department of Biology, Faculty of Mathematics and Natural Sciences, Universitas Padjadjaran, Indonesia (Identification Number: 195/HB/08/2018).

Conclusions
A new tetranortriterpenoid was isolated from the methanol extract of fruit peels of L. domesticum Corr. cv kokossan, which is named kokosanolide D.