Synthesis of Cyclocaric Acid A and Comparison to Material from

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Synthesis of Cyclocaric Acid A and Comparison to Material from Cyclocarya paliurus Matthew E. Wright,†,‡ Jonathan Byrd,†,‡ Chunnian He,§ and Norma Dunlap*,†,‡ †

Department of Chemistry and ‡Tennessee Center for Botanical Medicine Research, Middle Tennessee State University, Murfreesboro, Tennessee 37132, United States § Institute of Medicinal Plant Development, Chinese Academy of Medical Science, Peking Union Medical College, Beijing, People’s Republic of China S Supporting Information *

ABSTRACT: Components previously reported from Cyclocarya paliurus include the oleananes cyclocaric acids A and B, with cyclocaric acid A possessing an oxetane ring. Isolation of cyclocaric acid A from the plant extract and comparison to the literature report show that the compound originally reported as cyclocaric acid A is, in fact, hederagenin. This was confirmed by independent synthesis of the oxetane and indicates that cyclocaric acid A may not actually be a natural product.

Cyclocarya is a flowering plant in the family Juglandaceae. The Cyclocarya genus comprises a single species, Cyclocarya paliurus (wheeled wingnut), an endemic tree growing in southern China, commonly known as the “sweet tea tree”.1 Interest in the tree comes from biological activities including hypoglycemic, hypolipidemic, and antihypertensive, and the leaves of C. paliurus have long been a source for treatment of hypertension and diabetes.2−4 Numerous components have been isolated from the leaves, including polysaccharides, flavonoids, phenols, steroids, and triterpenoids, with most attention focused on the polysaccharides. One triterpenoid previously reported is the oleanane cyclocaric acid A, with subsequent reports of the isolation of cyclocaric acid A referring back to the data in the original 1996 article.5,6 One exception is a reference to sapindic acid from Sapindus laurifolius, assigned the same structure in 1968.7 The ethanol extract of dried plant material from C. paliurus was purified to afford two pentacyclic triterpenoids belonging to the oleanane series, which were initially thought to be cyclocaric acids A and B. Spectroscopic analysis, followed by independent synthesis, shows that the structure initially reported as cyclocaric acid A is actually hederagenin (Figure 1). A crude ethanol extract of C. paliurus was first defatted with hexanes and then extracted sequentially with CHCl3, EtOAc, and n-BuOH. Purification of the CHCl3 fraction by gravity column chromatography on silica gel afforded seven fractions. Fractions 1−3 were combined and further purified by flash column chromatography to provide two pure compounds. In 1996, Zhong et al. had reported the isolation of the oleananes cyclocaric acids A and B.5,8 The structure of the higher Rf compound in our extract was originally assigned as the oxetane cyclocaric acid A, and the lower as cyclocaric acid B. The NMR data for the compound assigned as cyclocaric acid A was identical to the original report. However, on closer investigation © XXXX American Chemical Society and American Society of Pharmacognosy

Figure 1. Structures of oleananes.

of the 1H and 13C NMR data from that report, particularly the coupling constants for the C-23 methylene protons of cyclocaric acid A, it became evident that the compound was in fact not the oxetane, but was hederagenin. This is a common oleanane found in ivy (Hedera helix) and has been isolated from many different plants.9,10 Although the NMR data of the high Rf plant material was identical to that reported in 1996, HRMS data using ESI showed a molecular ion of m/z 473.3625 [M + H] that corresponds to hederagenin. There is also an [M − 18] ion at m/z 455.3491 that could easily be confused for the oxetane structure. In the original report, using EIMS data, it is Received: July 18, 2014

A

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presumed that the true molecular ion was not seen, and the [M − 18] ion was taken as the molecular ion, which would match that of the oxetane structure. In order to confirm these speculations, a semisynthesis of the oxetane was carried out, beginning with hederagenin. Conversion of the acid to the allyl ester was followed by conversion of the primary alcohol into tosylate 1.11 Oxidation of the 3β-alcohol with PCC afforded ketone 2. Reduction with NaBH4 afforded primarily the 3β-alcohol; however, reduction with L-selectride afforded the oxetane 3 as the major product and minor amounts of the 3α-alcohol. Deprotection of the ester to the acid afforded cyclocaric acid A, as shown in Scheme 1.

Table 1. NMR Spectroscopic Data (500 MHz, Pyridine-d5) of Cyclocaric Acid A and Hederagenin cyclocaric acid A

Scheme 1. Synthesis of Cyclocaric Acid A

position

δC, type

1

35.3, CH2

2 3 4 5 6

24.8, 86.4, 42.5, 51.8, 20.6,

7

32.6, CH2

8 9 10 11 12 13 14 15

A more lengthy synthesis of the methyl ester of cyclocaric acid A had been reported in 1976 as part of an investigation of oleananes, although it had not been given the name cyclocaric acid A at that time.12 Although the MS data indicate that the structure of cyclocaric acid A, originally reported as a component of C. paliurus, is indeed hederagenin, the NMR data are also conclusive. Table 1 collates the 1H and 13C NMR data of hederagenin (from C. paliurus) and the synthetic cyclocaric acid A. Data from the original report are identical to hederagenin and are listed in the Experimental Section for comparison. One clear indication of whether the oxetane is present or not is the coupling constant of the C-23 methylene protons. These protons show a similar pattern (two doublets) in both hederagenin and the oxetane. However, in the seco form, the J value is 10.3 Hz, whereas in the cyclic form it is 4.6 Hz. The smaller J value is typical for oxetanes and comparable to values found in the earlier oxetane synthesis.12 Generally, in syntheses involving a ring-closure to give an oxetane, the J value decreases and the signals are deshielded.13 The differences are evident in Figure 2, which shows an overlay of the plant isolate, hederagenin (purchased), and the synthetic cyclocaric acid A. The plant isolate and hederagenin are clearly identical and do not have the oxetane structure of cyclocaric acid A. The MS data of the synthetic cyclocaric acid A showed a molecular ion at m/z 455.3511 [M + H]+ as well as a fragment from loss of CO2H at m/z 406.2161. It is also important to address some confusion regarding the C-3 configuration. For the oxetane of cyclocaric acid A, the

40.0, 45.4, 35.5, 23.9, 122.8, 144.7, 41.3, 28.3,

CH2 CH C CH CH2

C CH C CH2 CH C C CH2

16

23.8, CH2

17 18 19

46.8, C 42.2, CH 46.4, CH2

20 21 22

31.0, C 34.3, CH2 33.4, CH2

23

82.7, CH2

24 25 26 27 28 29 30

13.5, 19.5, 17.4, 26.0, 180.2, 33.3, 23.9,

CH3 CH3 CH3 CH3 C CH3 CH3

hederagenin δH (J in Hz)

α 1.36, m β 1.51, m 1.61, m 4.66, d (4.0) 1.86, m α 1.16, m β 1.31, m 1.53, m

1.24, m 1.95, m 5.55, t (2.9)

α 1.21, m β 2.13, m 2.15, m

3.34, dd (13.8, 4.1) α 1.33, m β 1.83, m 1.23, m α 1.81, m β 2.04, m α 3.95, d (4.6) β 4.30, d (4.6) 0.67, s 0.93, s 1.06, s 1.27, s 0.98, s 1.03, s

δC

δH (J in Hz)

38.8

α 1.07, m β 1.59, m 1.89, m 4.23, dd (11.2, 5.1)

27.7 73.4 42.9 48.6 18.6 33.0 39.8 48.2 37.3 23.9 122.6 144.9 42.4 28.4 23.7 46.7 42.0 46.5 31.0 34.3 33.1 68.0 13.2 16.0 17.5 26.2 180.2 33.3 23.8

1.55, m α 1.48, m β 1.69, m α 1.32, m β 1.67, m 1.80, m 1.95, m 5.51, t (2.9)

α 1.18, m β 2.13, m α 1.99, m β 2.13, m 3.32, dd (13.8, 4.0) α 1.33, m β 1.81, m 1.45, 1.22, m α 1.80, m β 2.03, m α 3.74, d (10.3) β 4.21, d (10.3) 1.07, s 0.99, s 1.07, s 1.26, s 0.95, s 1.02, s

oxygen is reported as 3β, which would equate to a trans-fused oxetane moiety; however, it is drawn as the 3α-orientation in most references. Although a 3β oxygen may make more sense as a hederagenin derivative, there are very few reports of transfused bicyclo[4.2.0]oxetanes.14 In fact, Tsuda and co-workers were unable to synthesize trans-fused oxetanes of oleananes and recovered only retro-aldol, ring-opened products.12 Similar results were obtained in the current investigation, with attempts to form a trans-oxetane from the tosylate 1, resulting in multiple decomposition products and no oxetane. In conclusion, two components of C. paliurus have been isolated, and independent semisynthesis confirms that “cyclocaric acid A” was not previously isolated from this plant and the compound originally reported as an isolate is, in fact, hederagenin. The similarity of the NMR data, combined with the MS techniques available at that time, shows how this mistake could easily have been made based on the available information. Although the same structure was reported in 1968 as sapindic acid, this was done without NMR data, and the B

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90% DCM) afforded hederagenin (7.1 mg) and cyclocaric acid B (16.6 mg). Allyl Hederagenate.11 To a solution of hederagenin (0.314 g, 0.66 mmol) in DMF (8 mL) were added K2CO3 (0.132 g, 0.96 mmol) and allyl bromide (83 μL, 0.96 mmol). The solution was stirred at room temperature for 24 h, extracted with EtOAc, and washed with H2O. The organic layer was dried over MgSO4 and filtered, and the solvent evaporated. The crude product was chromatographed on a 25 × 120 mm silica gel column eluting with a gradient of 1:9 to 1:1 EtOAc−hexanes to afford 313 mg of the known allyl ester (92%) as an amorphous solid: 1H NMR (500 MHz, CDCl3) δ 0.72 (s, 3H, H-26), 0.88 (s, 3H, H-24), 0.90 (s, 3H, H-29), 0.93 (s, 3H, H-30), 0.95 (s, 3H, H-25), 1.13 (s, 3H, H-27), 2.88 (dd, J = 14.0, 4.0 Hz, 1H, H-18), 3.42 (d, J = 10.9 Hz, 1H, H-23a), 3.64 (t, J = 8.0 Hz, 1H, H-3), 3.70 (d, J = 10.8 Hz, 1H, H-23b), 4.52 (m, 2H, CH2O-allyl), 5.20 (dd, J = 10.3, 1.2 Hz, 1H, vinyl-CH2), 5.28 (t, J = 3.4 Hz, 1H, H-12), 5.32 (dd, J = 17.1, 1.7 Hz, 1H, vinyl-CH2), 5.90 (m, 1H, CH-vinyl); 13C NMR (125 MHz, CDCl3) δ 11.4 (C-24), 15.7 (C-25), 17.0 (C-26), 18.5 (C6), 23.0 (C-16), 23.4 (C-11), 23.6 (C-30), 25.9(C-27), 26.6 (C-2), 27.6 (C-15), 30.7 (C-20), 32.4 (C-22), 32.5 (C-7), 33.1 (C-29), 33.9 (C-21), 36.9 (C-10), 38.1 (C-1), 39.3 (C-8), 41.3 (C-18), 41.7 (C14), 41.7 (C-4), 45.9 (C-19), 46.7 (C-17), 47.6 (C-9), 49.7 (C-5), 64.8 (CH-allyl), 71.8 (C-23), 77.0 (C-3), 117.7 (vinyl-CH2), 122.4 (C12), 132.5 (CH-vinyl), 143.7 (C-13), 177.4 (C-28). 23-Tosyloxy Allyl Hederagenate (1). To a solution of hederagenin allyl ester (0.313 g, 0.61 mmol) in pyridine (5 mL) was added p-toluenesulfonyl chloride (0.233 g, 1.22 mmol). The mixture was stirred at room temperature for 24 h and then poured into 1 M HCl, extracted with EtOAc, washed with NaHCO3 and brine, dried with MgSO4, and filtered, and the solvent was evaporated. The crude product was chromatographed on a 25 × 120 mm silica gel column eluting with a gradient of 1:9 to 1:5 EtOAc−hexanes to afford 240 mg of the monotosylate 1 (59%) as an amorphous solid: [α]25D +58 (c 0.0006, CHCl3); 1H NMR (CDCl3) δ 0.67 (s, 3H), 0.68 (s, 3H), 0.89 (s, 3H), 0.92 (s, 3H), 0.93 (s, 3H), 1.12 (s, 3H), 2.45 (s, 3H, tosylCH3), 2.88 (dd, J = 14.0, 4.0 Hz, 1H, H-18), 3.61 (m, 1H, H-3), 3.71 (d, J = 9.7 Hz, 1H, H-23a), 3.98 (d, J = 9.7 Hz, 1H, H-23b), 4.52 (m, 2H, allyl-CH2O), 5.20 (dd, J = 10.6, 1.2 Hz, 1H, vinyl-CH2), 5.27 (t, J = 3.4 Hz, 1H, H-12), 5.31 (dd, J = 17.2, 1.7 Hz, 1H, vinyl-CH2), 5.89 (m, 1H, vinyl-CH), 7.35 (d, J = 8.0 Hz, 2H, aryl-H), 7.81 (d, J = 8.6 Hz, 2H, aryl-H); 13C NMR (CDCl3) δ 11.7 (C-24), 15.8 (C-25), 16.9 (C-26), 17.9 (C-6), 21.7 (tosyl CH3), 23.0 (C-16), 23.3 (C-11), 23.6 (C-30), 26.0 (C-27), 26.5 (C-2), 27.6 (C-15), 30.7 (C-20), 32.1 (C22), 32.4 (C-7), 33.1 (C-29), 33.9 (C-21), 36.9 (C-10), 37.8 (C-1), 39.3 (C-8), 41.3 (C-18), 41.7 (C-14), 42.3 (C-4), 45.9 (C-19), 46.7 (C-17), 46.9 (C-9), 47.5 (C-5), 64.8 (allyl-CH2O), 71.1 (C-3), 71.7 (C-23), 117.7 (allyl-CH2), 122.2 (C-12), 127.9 (aryl C), 129.9 (aryl C), 132.6 (allyl-CH), 133.0 (aryl C), 143.8 (C-13), 144.8 (aryl C), 177.4 (C-28); HRESIMS m/z (C40H59O6S) calcd for [M + 1]+ 667.4027; found 667.4049. 3-Didehydro-23-tosyloxy Allyl Hederagenate (2). To a solution of hederagenin monotosylate (0.235 g, 0.35 mmol) in DCM (7 mL) was added pyridinium chlorochromate (0.304 g, 1.41 mmol). The mixture was stirred for 2 h at room temperature, the reaction mixture was filtered through Celite, and the solvent was evaporated. The crude material was chromatographed on a 25 × 120 mm silica gel column with a gradient of 1:9 to 1:5 EtOAc−hexanes to afford 194 mg of the ketone 2 (83%) as an amorphous solid: [α]25D +33 (c 0.003, CHCl3); 1H NMR (CDCl3) δ 0.77 (s, 3H), 0.91 (s, 3H), 0.92 (s, 3H), 0.94 (s, 3H), 1.04 (s, 3H), 1.15 (s, 3H), 2.46 (s, 3H, tosyl-CH3), 2.90 (dd, J = 13.0, 4.0 Hz, 1H, H-18), 3.84 (d, J = 9.2 Hz, 1H, H-23a), 4.05 (d, J = 9.2 Hz, 1H, H-23b), 4.53 (m, 2H, allylCH2O), 5.21 (dd, J = 10.6, 1.2 Hz, 1H, allyl-CH2), 5.31 (t, J = 3.4 Hz, 1H, H-12), 5.32 (dd, J = 17.2, 1.7 Hz, 1H, allyl-CH2O), 5.90 (m, 1H, CH-vinyl), 7.35 (d, J = 8.0 Hz, 2H, aryl-H), 7.77 (d, J = 8.6 Hz, 2H, aryl H); 13C NMR (125 MHz, CDCl3) δ 14.9 (C-24), 17.0 (C-25), 17.5 (C-26), 19.1 (C-6), 21.7 (tosyl-CH3), 23.0 (C-16), 23.4 (C-11), 23.6 (C-30), 25.9 (C-27), 27.6 (C-15), 30.7 (C-2), 31.7 (C-20), 32.4 (C-22), 33.1 (C-7), 33.9 (C-29), 34.8 (C-21), 36.3 (C-10), 37.1 (C1), 39.3 (C-8), 41.4 (C-18), 41.8 (C-14), 45.8 (C-19), 46.4 (C-17),

Figure 2. Overlay of 1H NMR (δ 3.6−4.8) of cyclocaric acid A (top), plant isolate (middle), and hederagenin (bottom), showing the C-23 hydrogens and the C-3 hydrogen.

assignment was based on chemical tests and elemental analysis. Thus, it is unclear whether or not that isolate was actually the same structure as that assigned to cyclocaric acid A.

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EXPERIMENTAL SECTION

General Experimental Procedures. The NMR data were obtained on a 500 MHz FT-NMR model ECA-500 JEOL (Peabody, MA, USA) purchased with funding provided by the National Science Foundation. Coupling constants (J values) are recorded in hertz (Hz). All signal assignments are based on COSY, HMQC, and DEPT data. Polarimetry was performed using an Autopol III polarimeter (Rudolph Research, Fairfield, NJ, USA). High-resolution ESIMS was performed at Notre Dame University, Notre Dame, IN, USA. Solvents and chemicals were purchased from Fisher Scientific (Pittsburgh, PA, USA) or Sigma-Aldrich (Milwaukee, WI, USA). Hederagenin was purchased from Biopurify Phytochemicals (Chengdu, Sichuan, China). Plant Material. The leaves of Cyclocarya paliurus (Batal.) Iljinskaja (CPs) were collected in Zhangpu County, Fujian Province, China, in May 2012, and identified by one of the authors (C.-N.H.). A voucher specimen (No. 2012 cP003) has been deposited at the Herbarium of the Institute of Medicinal Plant Development, Chinese Academy of Medical Sciences and Peking Union Medical College. Guangxi Botanical Garden of Medicinal Plants (Nanching, China) provided the crude ethanol extract from C. paliurus. The dried leaves of C. paliurus (1.0 kg) were pulverized and extracted successively four times with 95% EtOH (each 10 L, each 24 h) at room temperature. The combined EtOH extract was concentrated under reduced pressure at 60 °C to afford a dark brown residue (100 g). Method A. The crude EtOH extract (11.2 g) was partitioned by liquid/liquid extraction to provide five fractions: hexanes, EtOAc, CHCl3, n-BuOH, and H2O. The CHCl3 fraction was then purified by gravity column chromatography on 70−230 mesh silica gel, eluting with a CHCl3−MeOH gradient system (1:0 to 0.1:2), to afford seven fractions. Fractions 1−3 were combined and chromatographed by flash column chromatography eluting with an EtOAc−hexanes−MeOH gradient (50:50:0−100), affording six fractions. Fractions 2 and 3 were combined and purified further by flash column chromatography to afford 1.1 mg of hederagenin (originally assigned as cyclocaric acid A) and 12.4 mg of cyclocaric acid B. Method B. The crude EtOH extract (21.2 g) was dissolved in a small amount of CHCl3, and hexanes were added to form a precipitate. The precipitate was filtered and washed with hexanes. The precipitate (1.6 g) was dissolved in CHCl3 and suspended on silica gel. Flash column chromatography, eluting with CHCl3−acetone (0−40), afforded a mixture of hederagenin and cyclocaric acid B. Further purification by flash column chromatography (isocratic, 10% acetone− C

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46.7 (C-9), 47.1 (C-4), 50.4 (C-5), 64.8 (allylCH2O), 72.2(C-23), 117.8 (allyl-CH2), 122.0 (C-12), 128.0 (aryl-C), 129.8 (aryl-C), 132.5 (allyl-CH), 132.7 (aryl-C), 143.9 (C-13), 144.8 (aryl-C), 177.4 (C28), 212.8 (C-3); HRESIMS, m/z (C40H57O6S) calcd for [M + 1]+ 665.3970; found 665.3904. Cyclocaric Acid A Allyl Ester (3). To a solution of ketone 2 (0.028 g, 0.042 mmol) at −78 °C in anhydrous THF (1 mL) was added L-selectride (150 μL, 1 M solution). After 10 min the dry ice− acetone bath was removed, and the reaction mixture was allowed to warm to room temperature. After 2 h, 2 M NaOH (1.1 mL) and 30% H2O2 (0.25 mL) were added to the reaction mixture. The reaction mixture was stirred for one additional hour, poured into H2O, and extracted with EtOAc. The organic layer was dried over MgSO4 and filtered, and the solvent evaporated. The crude product was chromatographed on a 25 × 120 mm silica gel column eluting with a gradient of 1:9 to 1:5 EtOAc−hexanes to afford 6 mg of oxetane 3 (29%) as an amorphous solid: [α]25D +40 (c 0.001, CHCl3); 1H NMR (CDCl3) δ 0.78 (s, 3H), 0.80 (s, 3H), 0.91 (s, 3H), 0.93 (s, 3H), 1.03 (s, 3H), 1.17 (s, 3H), 2.90 (dd, J = 13.8, 4.0 Hz, 1H, H-18), 3.87 (d, J = 5.2 Hz, 1H, H-23a), 4.29 (d, J = 5.2 Hz, 1H, H-23b), 4.53 (m, 2H, allyl-CH2O), 4.67 (d, J = 2.9 Hz, 1H, H-3), 5.21 (dd, J = 10.6, 1.2 Hz, 1H, allyl-CH2O), 5.32 (dd, J = 17.5, 1.7 Hz, 1H, allyl-CH2), 5.34 (t, J = 3.4 Hz, 1H, H-12), 5.90 (m, 1H, allyl-CH); 13C NMR (125 MHz, CDCl3) δ 13.5 (C-24), 17.0 (C-26), 19.4 (C-25), 20.3 (C-6), 23.1 (C16), 23.5 (C-11), 23.6 (C-30), 24.3 (C-2), 25.7 (C-27), 27.6 (C-15), 30.7 (C-20), 32.2 (C-22), 32.4 (C-7), 33.1 (C-29), 33.9 (C-21), 34.8 (C-10), 35.2 (C-1), 39.6 (C-8), 41.3 (C-18), 41.6 (C-14), 42.1 (C-4), 44.9 (C-19), 45.8 (C-9), 46.9 (C-17), 51.3 (C-5), 64.8 (allyl-CH2O), 83.1 (C-23), 87.0 (C-3), 117.7 (allyl-CH2), 122.7 (C-12), 132.6 (allylCH), 143.6 (C-13), 177.4 (C-28); HRESIMS m/z (C33H51O3) calcd for [M + 1]+ 495.3833; found 495.3839. Cyclocaric Acid A. To a solution of oxetane allyl ester 3 (0.039 g, 0.079 mmol), triphenylphosphine (0.013 g, 0.048 mmol), and pyrrolidine (0.011 g, 0.16 mmol) in anhydrous THF (1 mL) was added tetrakis(triphenylphosphine) palladium(0) (0.028 g, 0.024 mmol). The solution was stirred at room temperature for 24 h, and then the solvent was evaporated. The crude product was chromatographed on a 25 × 120 mm silica gel column eluting with a gradient of 1:9 to 1:2 EtOAc−hexanes to afford 14.4 mg of cyclocaric acid A (40%) as an amorphous solid: [α]25D +45 (c 0.007, CHCl3); NMR data in Table 1; HRESIMS m/z (C30H46O3) calcd for [M + 1]+ 454.3447; found 454.3451. “Cyclocaric Acid A”. Data reported in the original paper,5 which match those of hederagenin and not the actual cyclocaric acid A, are recorded here for ease of comparison. 1 H NMR (pyridine-d5) δ 0.93 (s, 3H), 0.98 (s, 3H), 1.01 (s, 3H), 1.05 (s, 3H), 1.06 (s, 3H), 1.24 (s, 3H), 3.30 (dd, 1H, H-18), 3.72 (d, J = 10 Hz, 1H, H-23a), 4.18 (d, J = 10 Hz, 1H, H-23b), 4.19 (d, J = 5.16 Hz, 1H, H-3), 5.51 (t, 1H, H-12); 13C NMR (pyridine-d5) δ 13.1 (C24), 17.5 (C-26), 16.1 (C-25), 18.7 (C-6), 23.7 (C-16), 23.9 (C-11), 23.8 (C-30), 27.7 (C-2), 26.2 (C-27), 42.3 (C-15), 31.0 (C-20), 33.1 (C-22), 33.3 (C-7), 33.3 (C-29), 34.3 (C-21), 37.5 (C-10), 38.9 (C-1), 39.8 (C-8), 42.1 (C-18), 42.3 (C-14), 42.9 (C-4), 46.5 (C-19), 48.2 (C-9), 46.7 (C-17), 48.8 (C-5), 68.3 (C-23), 73.7 (C-3), 122.6 (C-12), 144.9 (C-13), 180.2 (C-28).

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Note

ACKNOWLEDGMENTS This project was supported financially by the Tennessee Center for Botanical Medicine Research at Middle Tennessee State University.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

NMR spectra (1H and 13C) are available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 1-615-898-2954. E-mail: [email protected]. Notes

The authors declare no competing financial interest. D

dx.doi.org/10.1021/np500575q | J. Nat. Prod. XXXX, XXX, XXX−XXX