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Constituents of the Roots of Dichapetalum Pallidum and Their Anti-Proliferative Activity

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Constituents of the Roots of Dichapetalum Pallidum and Their Anti-Proliferative Activity Article Constituents of the Roots of Dichapetalum pallidum and Their Anti-Proliferative Activity Dorcas Osei-Safo 1,*, Godwin Akpeko Dziwornu 1,†, Regina Appiah-Opong 2, Mary Anti Chama 1, Isaac Tuffour 2, Reiner Waibel 3, Richard Amewu 1 and Ivan Addae-Mensah 1 1 Chemistry Department, School of Physical and Mathematical Sciences, College of Basic and Applied Sciences, University of Ghana, P.O. Box LG 56, Legon, Accra, Ghana; [email protected] (G.A.D.); [email protected] (M.A.C.); [email protected] (R.A.); [email protected] (I.A.-M.) 2 Department of Clinical Pathology, Noguchi Memorial Institute for Medical Research, College of Health Sciences, University of Ghana, P.O. Box LG 581, Legon, Accra, Ghana; [email protected] (R.A.-O); [email protected] (I.T.) 3 Department of Chemistry and Pharmacy, Friederich Alexander University of Erlangen-Nurnberg, Schuhstrasse 19, 91052 Erlangen, Germany; [email protected] * Correspondence: [email protected] or [email protected]; Tel.: +233-208-169376 † Present address: Department of Chemistry, University of Cape Town, Rondebosch 7701, South Africa. Academic Editor: David J. Newman Received: 27 February 2017; Accepted: 23 March 2017; Published: 27 March 2017 Abstract: As part of our search for bioactive compounds from the Dichapetalaceae, repeated chromatographic purification of the roots of a hitherto unexamined species, Dichapetalum pallidum, led to the isolation of the newly occurring 7-hydroxydichapetalin P (1) and the known dichapetalins A (2) and X (3). Also isolated were the known compounds friedelin-2,3-lactone (4), friedelan-3-one (6), friedelan-3β-ol (7) and pomolic (8), as well as the dipeptide aurantiamide acetate (5). The compounds were characterized by direct interpretation of their IR, 1D NMR and 2D NMR spectral data and by comparison of their physico-chemical data, including their chromatographic profiles, with the literature and authentic samples in our compound library for the genus Dichapetalum. The compounds were assayed for their anti-proliferative activities against the human T-lymphocytic leukemia (Jurkat), acute promyelocytic leukemia (HL-60) and T-lymphoblast-like leukemia (CEM) cell lines. Overall, dichapetalin X showed the strongest (3.14 μM) and broadest cytotoxic activities against all the leukemic cell lines tested, exhibiting even stronger activities than the standard compound, curcumin. Keywords: Dichapetalum pallidum; 7-hydroxydichapetalin P; dichapetalin A; dichapetalin X; spiroketal; leukemia 1. Introduction Dichapetalum Thouars (Dichapetalaceae) is a rich source of triterpenoids and several species of the genus have produced a variety of this class of secondary metabolites. Although the genus was initially noted for being lethal to livestock and even humans, recent research has focused on the less toxic plant species and has led to the discovery of a novel class of phenylpyranotriterpenoid compounds, the dichapetalins. This unique class of triterpenoids in which a 2-phenylpyrano moiety is annellated to ring A of a 13,30-cyclodammarane-type triterpenoid is believed to be biogenetically derived from the condensation of the triterpenoid with a C6–C2 moiety, which is probably derived from the shikimic acid pathway. The main distinguishing feature of the dichapetalins is the nature of the side chain on the cyclopentane ring D which may comprise a lactone, methyl ester, spiroketal, lactol, acetyl and furan moieties. Originally known for their cytotoxic activity, dichapetalins with a spiroketal side chain have exhibited the most potent antineoplastic properties. Moreover, they have Molecules 2017, 22, 532; doi:10.3390/molecules22040532 www.mdpi.com/journal/molecules Molecules 2017, 22, 532 2 of 11 also been reported to have antioxidant, antihelminthic, antifeedant, nematicidal, antifungal, anti- HIV, nitric oxide– and acetylcholinesterase-inhibitory activities [1–7]. Dichapetalum pallidum (Oliv.) Engl. is distributed in west and central Africa. It has hitherto not been examined, neither for its chemical constituents nor for bioactivity. The main documented ethnobotanical applications are the use of the seeds as food in Togo and the leaves in the treatment of diarrhea [8]. As part of our on-going studies on the less common and less toxic species of the genus, in our search for new and more potent cytotoxic dichapetalins, we report the isolation and characterization of three dichapetalins from the roots of D. pallidum, namely 7-hydroxydichapetalin P (1), dichapetalin A (2) and the recently reported dichapetalin X (3) from D. filicaule [7]. Also isolated were friedelin-2,3-lactone (4), the dipeptide aurantiamide acetate (5), friedelan-3-one (6), friedelan- 3β-ol (7) and pomolic acid (8) (Figure 1). Although dichapetalin P has been reported from D. zenkeri [5], its 7-hydroxy derivative (1) has hitherto not been reported as a naturally occurring compound. The dichapetalins exhibited better anti-proliferative activity against the human T-lymphocytic leukemia (Jurkat), acute promyelocytic leukemia (HL-60) and T-lymphoblast-like leukemia (CEM) cell lines than curcumin. The results further substantiate this class of dammarane-type triterpenoids as potential antitumor agents. Figure 1. Structures of compounds 1–8. 2. Results 2.1. Characterization of Compounds Repeated silica gel column chromatographic separation of the ethyl acetate extract of the roots of D. pallidum afforded a total of seven triterpene derivatives and a dipeptide, aurantiamide acetate (5). Among the triterpenoids were dammarane-type 7-hydroxydichapetalin P (1), which is reported for the first time, dichapetalin A (2) and the recently reported dichapetalin X (3) from D. filicaule [7]. Molecules 2017, 22, 532 3 of 11 The remaining triterpenoids were the friedelane-type compounds friedelin-2,3-lactone (4), friedelan- 3-one (6) and friedelan-3β-ol (7), and the ursane-type pomolic acid (8). Compound 1 was obtained as a white crystalline solid. Five methyl protons were identified in the 1H-NMR spectrum within the range δH 0.92 to δH 2.03, the latter being assigned to an acetoxymethyl. The 1H-NMR spectrum exhibited signals due to five aromatic protons resonating at δH 7.24 (m, H-4″), δH 7.33 (m, H-3″, 5″) and δH 7.35 (m, H-2″, 6″). The 13C-NMR spectrum showed 38 signals. Two ester carbonyl functionalities were inferred by chemical shifts at δC 173.9 and δC 170.4 and supported by absorptions at 1780 and 1745 cm−1 in the IR spectrum. The intensity of the olefinic/aromatic signals at δC 125.8 and δC 128.4 appeared to be twice that of the other signals, indicating the presence of two sets of equivalent carbons. The HSQC data revealed 11 quaternary carbons interspersed within both the sp2- and sp3-hybridized regions, nine methylene, 13 methine and five methyl carbons. Thus, the expected total number of carbons present in 1 was 40. Six 1H-1H COSY spin systems were observed (Figure 2), while some key HMBC correlations enabled the complete assignments of the 1H and 13C resonances of 1 which were consistent with previously accumulated data in our compound library of a dichapetalin carbon arrangement. For example, the 2-phenylpyrano moiety was corroborated from a mono-substituted phenyl ring comprising the four sp2 carbon signals C-1″ (δC 142.6), CH-2″, 6″ (δC 125.8, δH 7.35, m), CH-3″, 5″ (δC 128.4, δH 7.33, m) and CH-4″ (δC 127.5, δH 7.24, m), together with HMBC correlations from H-2″, 6″ to C-6′ (δC 81.8) and H-2′ to C-6′ and C-3 (δC 140.0) (Figure 2). The presence of the cyclopropane moiety, comprising C-13, -14 and -30, was supported by HMBC correlations from H-12 and H-17. The positions of the two isolated double bonds (C-2/C-3 and C-11/C-12), arising from four sp2 carbon signals at δC 117.8 (C-2), δC 124.1 (C-11), δC 129.1 (C-12), δC 140.0 (C-3) with corresponding olefinic proton multiplets at δH 5.42 (1H, dd, J = 1.8, 7.1 Hz, H-2), δH 5.46 (1H, dd, J = 2.6, 10.2 Hz, H-11), and δH 6.20 (1H, dd, J = 3.1, 10.2 Hz, H-12), were assigned based on HMBC correlations, for example from H2-1, H-2′ and H-9. In addition to an acetal at δC 111.3 (C-23), compound 1 revealed a spiroketal side chain at C-17 bearing an acetoxy group identical to those of dichapetalins M [4] and P [5] (Figure 3). However, the parent structure of the compound differed from those of both compounds, evident from an obvious absence of a C-7 ketocarbonyl in the 13C-NMR. This was replaced with a 7-OH group, supported by HSQC cross-peaks of C-7 (δC 72.3) to H-7 (δH 3.95), the 1H-1H COSY spin system involving H-5, H2-6 and H-7, and HMBC correlations to C-7 (δC 72.3) and C-8 (δC 36.4) from the methine proton H-5. Figure 2. COSY (bold bonds) and some key HMBC (curved arrows) correlations of compound 1 (Supplementary Materials S4 and S6). Molecules 2017, 22, 532 4 of 11 Figure 3. Structures of compound 1 and dichapetalins P and M. Thus, compound 1 was unequivocally identified as 7-hydroxydichapetalin P, the reduced form of dichapetalin P, which has hitherto not been reported as a naturally occurring dichapetalin. Moreover, 1 exhibited peaks in its IR spectrum and characteristic chromatographic profile with respect to its reaction with vanillin reagent as previously reported for the dichapetalins [5]. Based on the close similarity of its NMR data to previously reported congeners, compound 1 is most likely to share a relatively similar stereochemistry as its congeners. Compounds 2 and 3 were identified as dichapetalins A and X, respectively, by comparison of their physico-chemical and spectroscopic data with published results [1,7] as well as by co-TLC with authentic samples in several solvent systems.
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