Lactam Triterpenoids from the Bark of Toona sinensis

Nat. Prod. Bioprospect. (2016) 6:239–245 DOI 10.1007/s13659-016-0108-4 ORIGINAL ARTICLE Lactam Triterpenoids from the Bark of Toona sinensis Qian-Qian Meng . Xing-Rong Peng . Shuang-Yang Lu . Luo-Sheng Wan . Xia Wang . Jin-Run Dong . Rui Chu . Lin Zhou . Xiao-Nian Li . Ming-Hua Qiu Received: 29 July 2016 / Accepted: 22 September 2016 / Published online: 18 October 2016 Ó The Author(s) 2016. This article is published with open access at Springerlink.com Abstract Three new limonoid-type triterpenoids, namely toonasins A–C (1–3) with a rare lactam E ring, along with six known compounds (4–9) were isolated from the barks of Toona sinensis. The structures of new compounds were elucidated by interpretation of spectroscopic data, and the relative configuration of compound 1 was further characterized by X-ray crystallographic analyses. The isolated compounds were evaluated for their cytotoxic activities against five human tumor cell lines (HL-60, SMMC-7721, A-549, MCF-7 and SW480), and compounds 3 and 5 showed weak cytotoxicities. Keywords Toona sinensis  Limonoids  Lactam triterpenoids  Cytotoxicity 1 Introduction Toona sinensis is a shrub of Meliaceae distributed widely in Asian countries [1]. The leaves of T. sinensis, which contain a distinct flavor, are very popular in vegetarian cuisine and have long been used as a nutritious food in Qian-Qian Meng and Xing-Rong Peng have contributed equally to this work. Electronic supplementary material The online version of this article (doi:10.1007/s13659-016-0108-4) contains supplementary material, which is available to authorized users. Q.-Q. Meng  X.-R. Peng  S.-Y. Lu  L.-S. Wan  X. Wang  J.-R. Dong  R. Chu  L. Zhou  X.-N. Li  M.-H. Qiu State Key Laboratory of Phytochemistry and Plant Resources in West China, Kunming Institute of Botany, Chinese Academy of Sciences, Kunming 650201, People’s Republic of China Q.-Q. Meng  S.-Y. Lu  X. Wang  J.-R. Dong  R. Chu  M.-H. Qiu University of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China X.-R. Peng  L.-S. Wan  L. Zhou  X.-N. Li  M.-H. Qiu (&) Yunnan University of Traditional Chinese Medicine, Kunming 650500, People’s Republic of China e-mail: China and Malaysia [2]. In folk, almost every part of T. sinensis, including seeds, bark, root bark, petioles, and leaves, can be used to treat cold, rheumatic pain, stomach pain, and diarrhea without any irreversible side effects [3, 4]. Modern pharmacological researches also demonstrated that this plant showed wide spectrum of biological activities, such as antioxidant [5], anti-diabetes [6], antiinflammatory [7], antimicrobial [8], antinociceptive [9] and anti-tumor [10], due to its plentiful chemical constituents (limonoids, flavonoids, phytols, coumarins and norcyteine derivatives) [11–14]. Limonoids were mainly identified from Meliaceae species and possessed fascinating structures [15] and various bioactivities [16–19]. Our previous phytochemical investigation of the Meliaceae species (T. ciliata and Swietenia mahagoni) led to the isolation of structurally diverse limonoids with anti-cancer and anti-bacterial effects [15, 20, 21]. In our continuing search for structurally interesting and biologically important chemical constituents, the bark of the title plant was investigated and three new limonoids, namely toonasins A–C (1–3), along with six known compounds, including one limonoid, photogedunin (4) [22], one tirucallane triterpenoid, bourjotinolone B (5) [23], three pentacyclic triterpenoids, betulinic acid (6) [24], betulin (7) [24], and erythrodiol (8) [25], as 123 240 well as one diterpenoid, gossweilone (9) [26] (Fig. 1) were isolated. Among them, compounds 1–3 had a unique lactam E ring moiety and were first isolated from this species. The structures of new isolates were elucidated on the basis of the 1D, 2D NMR, and MS spectra. The structure of 1 was further confirmed by the X-ray crystallographic analyses. Subsequently, their cytotoxic activities were evaluated by MTT method. 2 Results and Discussion Compound 1 was assigned a molecular formula of C28H33NO8 by HRESIMS and 1D NMR spectroscopic data (Table 1), which required 13 degrees of unsaturation. The IR spectrum revealed the presence of NH (3485 cm-1), carbonyl (1721 cm-1) and amide (1651 cm-1) groups. The 1 H NMR spectrum of compound 1 showed five singlet methyls (dH 1.07, 1.08, 1.18, 1.23 and 1.26), a typical singlet methyl signal at dH 2.11 for acetyl, two oxymethines (dH 4.55, br s, H-7; dH 3.53, s, H-15), and three aromatic/olefinic methines (dH 7.12, d, J = 10.2 Hz, H-1; dH 5.90, d, J = 10.2 Hz, H-2; dH 6.72, s, H-22). The 13CDEPT spectra of 1 displayed twenty-eight carbon resonances. Apart from an acetyl group, the remaining signals were assigned as five methyls, three methylenes, an a, bunsaturated carbonyl (dC 157.0, C-1; dC 126.2, C-2; dC 204.2, C-3), a d-lactone ring with a 14,15-epoxy fraction (dC 69.7, C-14; dC 56.8, C-15; dC 166.7, C-16; dC 75.4, Q.-Q.Meng et al. C-17), and two lactam carbonyls (dC 169.6, C-21; dC 168.8, C-23), which were further supported by its HSQC, HMBC and 1H-1H COSY experiments (Fig. 2). Meanwhile, the HMBC correlations of H-7 to the acetyl, C-5, C-6, C-8, C-9, and C-14, and of H-5 and H-6 with C-7, along with the 1 H-1H COSY correlations of H-5/H-6/H-7 illustrated that the acetoxyl was located at C-7. Above information suggested that 1 was a limonoid-type triterpenoid and resembled photogedunin [22] except that they had a different substituent at C-17. The presence of a maleimide moiety at C-17 in 1 was established by the HMBC correlations of NH (dH 7.83, s) to C-20, C-21, C-22, and C-23, of H-22 (dH 6.72, s) to C-17, C-20, C-21, and C-23 (Fig. 2). The observed ROESY correlations (Fig. 2) of H-7/H3-19/H3-30, and of H-9/H3-18 allowed the assignment of 7-OAc and H-9 as a-oriented. To further confirm its skeleton of 1, a single crystal was cultivated (Fig. 3). Based on above information, the structure of 1 was determined and named as toonasin A (1). Compound 2 was obtained as a colorless needle with a molecular ion peak at m/z 592.2269 [M ? Na]? (calcd 569.2261) in the HRESIMS, coincided with the molecular formula of C30H35NO10. The IR spectrum exhibited absorption bands for NH (3435 cm-1), carbonyl (1745 cm-1) and amide (1657 cm-1) groups. Detailed comparison of the 1D NMR data between 1 and 2 (Table 1) showed that their main difference was an additional acetyl group [dC 170.1 (s), 21.2 (q)] in 2 instead of a methylene in 1. Furthermore, the HMBC correlations of H-6 (dH 5.27, Fig. 1 Structures of compounds 1–9 isolated from the bark of Toona sinensis 123 Lactam Triterpenoids from the Bark of Toona sinensis 241 Table 1 1H (600 MHz) and 13C NMR (150 MHz) data of compounds 1–3 in CDCl3 Position 1 2 3 dC dH (J in Hz) dC dH (J in Hz) dC dH (J in Hz) 1 157.0, CH 7.12, d (10.2) 155.9, CH 7.08, d (10.1) 157.3, CH 7.13, d (10.2) 2 126.2, CH 5.90, d (10.2) 126.7, CH 5 (...truncated)