Sikokianin D, A New C-3/C-3"-Biflavanone from the Roots of Wikstroemia indica

A new 3,3′′-biflavanone, sikokianin D (1), was isolated from the roots of Wikstroemia indica, together with two known compounds. Their structures were elucidated by chemical evidence and spectral analyses, including HR-ESI-MS, and 1D- and 2D-NMR techniques.


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and sikokianin A (3) (Figure 1) was isolated from the roots of Wikstroemia indica. This paper describes the isolation and structure elucidation of these compounds.   (Table 1), the spectra showed its structural fragments to include two sets of typical 5,7-dioxygenated A rings (δ H 5.74, 5.77, each 1H, d, J = 2.0 Hz; δ H 5.78, 5.98, each 1H, d, J = 2.0 Hz), and two sets of para-oxygenated B rings (δ H 7.22, 6.90, each 2H, d, J = 8.5 Hz; δ H 6.93, 6.63, each 2H, d, J = 8.5 Hz). From the 13 C-NMR data (Table 1), two carbonyl groups (δ C 198.5, 196.1) were also observed. These structural fragments were connected to form the given carbon framework of 1 as a dimer of flavanonol derivatives. The partial (-CH-CH-CH-CH-) structure inferred from the 1 H-1 H COSY spectrum (bold line in Figure 2) suggested that the linkage of the two flavanones was possible only at the C-3 and C-3" positions, which was supported by the comparison of the 1 H-and 13 C-NMR data of 1 with those of known 3,3"-biflavanones [4,6,8,10], and further confirmed by the HMBC correlations of H-2 (δ H 5.57) with C-3" (δ C 51.0). The B ring could be located at C-2, based on the observation of the clear cross-peaks of H-2' and H-6' (δ H 7.22) with C-2 (δ C 81.2). In the same way, linkage of the B' ring to C-2" of the C' ring was deduced by the correlations of H-2"'and H-6"'(δ H 6.93) with C-2" (δ C 83.3). The HMBC cross-peak between the methoxyl group and C-4' on the B ring indicated that the methoxyl group was connected to C-4'.

Results and Discussion
The stereochemistry at the C-2/C-3 and C-2"/C-3" positions in 1 was determined as cis-trans by comparison of the J values (J H-2 = 5.0 Hz and J H-2" = 9.5 Hz) with those of the known 3,3"-biflavanones. The key NOESY correlations between H-2" (δ H 5.19) with H-2'(H-6') (δ H 7.22) further confirmed the conclusion above. The relative stereochemistry of compound 1 was confirmed as shown in Figure 1 and the compound named sikokianin D.
Compound 2 was first reported as sikokianin B of which the location of MeO group was unsettled [8], and the exact configuration was elucidated by Nunome [4]. Sikokianin B and sikokianin C were determined by comparing their 1 H-and 13 C-NMR and MS data with published values.

General
Melting points were measured on a Thermal Values analytical microscope and are uncorrected. Optical rotations were recorded on a Perkin-Elmer 341 polarimeter. IR spectra were recorded on a Nicolet FI-IR 200SXY spectrophotomer. The spectra of high resolution-electrospray ionization-mass spectrometry (HR-ESI-MS) were acquired with a Micromass Q-TOF mass spectrometer (Waters Corporation USA). 1 H-and 13 C-NMR spectra were measured in CD 3 OD with TMS as the internal standard on a Bruker DMX-500 NMR instrument. Silica gel G 254 and H (Qingdao Sea Chemical Factory, China) were used for TLC and column chromatography, respectively.

Plant Material
The roots of Wikstroemia indica were purchased from a Chinese medicine pharmacy in Guangzhou, China, in September, 2011. The authentication process was carried out by Le Cai (Yunnan University). A voucher specimen was deposited in the Zhejiang University City College.