Nine sesquiterpenes from Solanum torvum

Abstract Three new sesquiterpenes, namely 3β,11-dihydroxy-4,14-oxideenantioeudesmane (1), 1β,10β,12,14-tetrahydroxy-allo-aromadendrane (2) and 1β,10β,13,14-tetrahydroxy-allo-aromadendrane (3), along with six known sesquiterpenes (4–9), were isolated from the roots of Solanum torvum. Compound 4 and 5 are epimers, their main difference lies in the C-11 configulation. Normally, epimers do not make a huge difference in C NMR spectra, but in this kind of structure of A, B, C rings, and C ring is sterically strained structure, stericall effects influence strongly the 13C NMR chemical shifts, when C-11 configulation changed, it makes a huge difference in the three ring of structure, such as C-6, C-7, C-11. New compound 2 and 3 are epimers and similar to compound 4 and 5, their just increase a hydroxy in C-1 and have a same regular pattern in C NMR spectra, otherwise, compound 5 was firstly confirmed by single-crystal X-ray diffraction.


Introduction
Solanum torvum is widely distributed in Thailand and is commonly known as 'Turkey berry' (Agrawal et al. 2010). Its fruits are used as vegetable in the south Indian population's diet. Its roots are used as folk medicine for the treatment of gasteralgia, furuncle, trauma and fever in the south of China. In the genus of Solanum, sesquiterpenes have been previously reported (Nie et al. 2014;Zhang et al. 2015). Although, many chemical components, including steroidal glycosides (Yahara et al. 1996;Lu et al. 2009;Shu et al. 2013;Li et al. 2014), triterpenoids (Zhou et al. 2011) and alkaloids (Moreira et al. 2010) have been isolated from this plant. Sesquiterpenes have never been reported from S. torvum. In this paper, we describe the isolation and structure elucidation of three new sesquiterpenes (1)(2)(3) (Figure 1), namely 3β, 11-dihydroxy-4,14-oxideenantioeudesmane (1), 1β,10β,12,14-tetrahydroxy-allo-aromadendrane (2) and 1β,10β,13,14-tetrahydroxy-allo-aromadendrane (3), along with six known ones (4-9) (Figure 1), isolated from the roots of S. torvum for the first time. Their structures were established based on 1d and 2d NMR spectral data and mass spectrometry. Interestingly, all aromadendrane sesquiterpenes from S. Torvum by us found to be epimers. Normally, epimers have no a distinctive difference in C NMR spectra, but in this kind of structure, when C-11 configulation changed, it makes a huge difference in the three ring of structure, such as C-6, C-7, C-11 (Table 1). This is due to three membered rings are sterically strained structures. It was well known that stericall effects influence strongly the 13 C chemical shifts.
Compound 3 was obtained as colourless powder and had the molecular formula C 15 h 26 o 4 by hReSIMS (m/z 293.1728 [M + Na] + , calculated for 293.1729). The NMR spectral data ( Table 1) of 3 showed similar NMR spectral features with 2. The 1 h-1 h CoSY and hMBC spectra showed that 2 and 3 had the same planar structure. however, the chemical shifts of C-12 and C-13 in 3 had distinctive differences with those in compound 2; thus, 2 and 3 should have different configurations. The relative configuration of 3 was deduced from the observed Noe correlations in 3 (Figure 4), in which Noe correlation signals were found between h- 6 and h-7, h-15, h-13; h-7 and h-8a; h-8a and h-14; h-5 and h-4, h-12, oh-1, oh-10. Therefore, compared with 2, C-11 configuration in 3 was switched. The structure of compound 3 was thus determined as 1β,10β,13,14-tetrahydroxy-allo-aromadendrane (Figure 4).
Compound 5 was colourless crystals, this is the first time that the absolute configuration of 5 were performed by single-crystal X-ray crystallographic analysis ( Figure 5).

General experimental procedures
optical rotations were determined by a Perkin-elmer 341 polarimeter. The IR spectra were obtained on a Nicolet-Magna-750-FTIR spectrophotometer with KBr pellets (Thermo Fisher, Madison, uSA). NMR spectra were recorded on a Bruker ARX-400 spectrometer. eSI-MS and hR-eSI-MS analyses were performed on a Finnigan LCQ deca XP equipped with an electrospray ionisation source mass ion-trap spectrometer and Waters Micromass Q-ToF ultima Globe spectrometer, respectively. Chromatographies were made on silica gel (100-200 and 300-400 mesh, Qingdao Marine Chemical Group, Co., Qingdao, China), Sephadex Lh-20 gel (Pharmacia Biotech AB, uppsala, Sweden).

Plant material
The roots of S. torvum were collected in 2011 from Guangxi Province, China, and identified by Professor Zhili Zhao of the Shanghai university of Traditional Chinese Medicine. A voucher sample (20111101) was deposited at the Pharmacy School of Shanghai university of Traditional Chinese Medicine, Shanghai, China.