Note pubs.acs.org/jnp
Synthesis of Alocasin A Se Hun Kim and Jonathan Sperry* School of Chemical Sciences, University of Auckland, 23 Symonds Street, Auckland 1142, New Zealand S Supporting Information *
ABSTRACT: Herein is reported a synthesis of alocasin A (1), an alkaloid component of Alocasia macrorrhiza, a herbaceous plant used in folk medicine throughout southern Asia. A double Suzuki−Miyaura cross-coupling reaction between a 3-borylindole and 2,5-dibromopyrazine was used to assemble the heteroaromatic framework of the natural product. Removal of the protecting groups gave a synthetic sample of 1, the spectroscopic data of which matched those in the isolation report of this compound.
T
Scheme 1. Synthesis of Alocasin A (1)
he herbaceous plant Alocasia macrorrhiza (L.) Schott (Araceae) is widespread throughout southern Asia. Extracts from the roots are used in folk medicine to treat a variety of conditions ranging from headaches and flu to hemorrhoids and pulmonary tuberculosis.1 An aqueous extract of this species has been shown to possess cytotoxic properties.2 Despite the obvious potential medicinal benefits associated with A. macrorrhiza, it was not until 2012 that a detailed evaluation of the constituents of the plant was performed, revealing the presence of several indole alkaloids including the pyrazinelinked bisindole alocasin A (1).3,4 Alocasin A (1) exerted weak antiproliferative activity against Hep-2 and Hep-G2 cell lines (IC50 values of 151 and 85 μM, respectively). A synthesis of alocosin A (1) was initiated since it aligns with our ongoing interests in the synthesis of bisindole alkaloids5 and in natural products present in traditional therapeutic preparations.6
reflux in a mixture of aqueous hydrobromic acid and acetic acid,9 a pure synthetic sample of 1 was obtained after passage over an ion-exchange column followed by flash chromatography on silica gel. The spectroscopic data for 1 were in complete agreement with the isolation report (Table 1).3 In summary, a short synthesis and structural confirmation of alocasin A (1) is described, an alkaloid component of the herbaceous plant A. macrorrhiza, a folk medicine used to treat a number of ailments.
The synthesis of alocasin A (1) was modeled on our recent synthesis of scalaridine A.7,8 We have previously shown that a directed C−H borylation can be used to access the 3borylindole 2,7 which was subjected to a double Suzuki− Miyaura cross coupling with 2,5-dibromopyrazine to give 3, containing the heteroaromatic framework of alocasin A (1) (Scheme 1). Removal of both Boc groups was straightforward, giving alocosin A dimethyl ether (4). The attempted demethylation of 4 failed with boron tribromide, which served only to degrade the substrate. However, when 4 was heated to © XXXX American Chemical Society and American Society of Pharmacognosy
Received: September 25, 2015
A
DOI: 10.1021/acs.jnatprod.5b00853 J. Nat. Prod. XXXX, XXX, XXX−XXX
Journal of Natural Products
Note
NMR (400 MHz, CDCl3) δ 9.07 (2 H, s, ArH), 8.19 (2 H, s, ArH), 8.12 (2 H, d, J 9.0, ArH), 7.94 (2 H, d, J 2.4, ArH), 7.03 (2 H, dd, J 9.0, 2.6 Hz, ArH), 3.94 (6 H, s, OMe), 1.71 (18 H, s, Me); 13C NMR (100 MHz, CDCl3) δ 156.7 (C), 149.6 (C), 146.9 (C), 141.6 (CH), 131.0 (C), 129.1 (C), 125.7 (CH), 118.0 (C), 116.2 (CH), 114.2 (CH), 104.5 (CH), 84.6 (C), 56.0 (Me), 28.4 (Me); HREIMS m/z 593.2370 [M + Na+] (calcd for C32H34N4O6Na 593.2371). 2,5-Bis(5-methoxyindol-3-yl)pyrazine (4). A solution of bisindole 3 (34 mg, 0.06 mmol) in dichloromethane (2 mL) was cooled to 0 °C. Trifluoroacetic acid (0.09 mL, 1.2 mmol, 20 equiv) was added, and the reaction mixture was stirred at room temperature for 19 h. The solution was concentrated in vacuo, and the crude residue purified by flash chromatography on silica gel eluting with dichloromethane−methanol (96:4) gave compound 4 (20 mg, 0.05 mmol, 90%) as a yellow solid: mp 100−105 °C; νmax 3228, 1673, 1527, 1488, 1441, 1283, 1192, 1131, 1031, 949, 919, 799, 723 cm−1; 1H NMR (400 MHz, acetone-d6) δ 10.61 (2 H, br s, NH), 9.10 (2 H, s, ArH), 8.14 (2 H, d, J 2.9, ArH), 8.09 (2 H, d, J 2.5, ArH), 7.41 (2 H, d, J 8.8, ArH), 6.88 (2 H, dd, J 8.8, 2.5, ArH), 3.89 (6 H, s, OMe); 13C NMR (100 MHz, acetone-d6) δ 156.0 (C), 148.2 (C), 141.0 (CH), 133.3 (C), 127.3 (C), 126.1 (CH), 114.5 (C), 113.4 (CH), 113.2 (CH), 104.6 (CH), 56.1 (Me); HREIMS m/z 371.1503 [M + H+] (calcd for C22H19N4O2 371.1503). Alocasin A (1).3 To a solution of bisindole 4 (10 mg, 0.027 mmol) in acetic acid (0.2 mL) was added aqueous hydrogen bromide (48%, 0.2 mL), and the resulting mixture heated under reflux for 18 h. After cooling, the solvent was removed in vacuo. The residual solid was passed through a column packed with an ion-exchange resin (Dowex 66, 100% methanol) and then concentrated in vacuo. The crude residue was purified by flash chromatography on silica gel eluting with ethyl acetate to give compound 1 (4 mg, 0.012 mmol, 43%) as a yellow solid: mp 250−253 °C (dec) (lit.3 mp 243−244); νmax 3369, 2926, 1548, 1458, 1364, 1196, 1155, 1026, 923, 808, 673 cm−1; 1H and 13 C NMR data, see Table 1; HREIMS m/z 343.1178 [M + H+] (calcd for C20H15N4O2 343.1190).
Table 1. NMR Data for Synthetic and Natural Alocasin A (1)3 (CD3OD) synthetic 1a position
δC
2/5 3/6 2′/2″ 3′/3″ 4′/4″ 5′/5″ 6′/6″ 7′/7″ 8′/8″ 9′/9″
148.4 141.5 126.6 113.9 106.2 152.8 113.3 113.2 133.6 127.4
δH (J in Hz) 8.93, s 7.90, s 7.73, d (2.4) 6.79, dd (8.6, 2.4) 7.30, d (8.6)
alocasin A (1)b δC 148.4 141.5 126.7 113.9 106.1 152.8 113.3 113.2 133.6 127.3
δH (J in Hz) 8.94, s 7.90, s 7.72, d (2.4) 6.79, dd (8.7, 2.4) 7.30, d (8.7)
a1 H and 13C NMR run at 400 and 125 MHz, respectively. b1H and 13C NMR run at 300 and 75 MHz, respectively.
■
EXPERIMENTAL SECTION
General Experimental Procedures. Commercially available reagents were used throughout without purification unless otherwise stated. Anhydrous solvents were used as supplied. Diethyl ether, tetrahydrofuran, and dichloromethane were dried using an LC Technology Solutions Inc. SP-1 solvent purification system under an atmosphere of dry nitrogen. All reactions were routinely carried out in oven-dried glassware under a nitrogen or argon atmosphere unless otherwise stated. Melting points were recorded on an Electrothermal melting point apparatus and are uncorrected. Infrared spectra were obtained using a PerkinElmer spectrum One Fourier transform infrared spectrometer as thin films between sodium chloride plates. Absorption maxima are expressed in wave numbers (cm−1). NMR spectra were recorded as indicated on either a Bruker Avance III 400 NMR spectrometer operating at 400 MHz for 1H nuclei and 100 MHz for 13C nuclei or on a Bruker Avance III HD 500 NMR spectrometer operating at 500 and 125 MHz for 1H and 13C nuclei, respectively. Chemical shifts are reported in parts per million (ppm) relative to the tetramethylsilane peak recorded as δ 0.00 ppm in CDCl3/TMS solvent or the residual chloroform (δ 7.26 ppm), DMSO (δ 2.50 ppm), or acetone (δ 2.05 ppm) peaks. The 13C NMR values were referenced to the residual chloroform (δ 77.1 ppm), DMSO (δ 39.5 ppm), or acetone (δ 29.8 ppm) peaks. 13C NMR values are reported as chemical shifts δ, multiplicity, and assignments. 1H NMR shift values are reported as chemical shifts δ, relative integral, multiplicity (s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet), coupling constant (J in Hz), and assignments. Assignments are made with the aid of DEPT 135, COSY, NOESY, HMBC, and HSQC experiments. Highresolution mass spectra were obtained by electrospray ionization in positive ion mode at a nominal accelerating voltage of 70 eV on a Bruker micrOTOF-QII high-resolution mass spectrometer. Analytical TLC was performed using silica gel plates, and compounds were visualized at 254 and/or 360 nm ultraviolet irradiation followed by staining with either alkaline permanganate or ethanolic vanillin solution. Di-tert-butyl 3,3′-(Pyrazine-2,5-diyl)bis(5-methoxyindole-1carboxylate) (3). To a solution of degassed 1,4-dioxane (1 mL) in a sealed tube were added tetrakis(triphenylphosphine)palladium(0) (6.2 mg, 0.005 mmol, 10 mol %), cesium carbonate (87.3 mg, 0.27 mmol, 5 equiv), 2,5-dibromopyrazine (12.9 mg, 0.05 mmol, 1 equiv), and 3-borylindole 27 (50 mg, 0.13 mmol, 2.5 equiv). The reaction mixture was stirred at 100 °C for 18 h under a blanket of argon. Upon cooling to room temperature, ethyl acetate and water were added, and the whole was washed with brine. The organic phase was dried (MgSO4), filtered, and concentrated in vacuo. Purification by flash chromatography on silica gel eluting with light petroleum−ethyl acetate (9:1) gave compound 2 (21 mg, 0.037 mmol, 69%) as a yellow solid: mp 218−220 °C; νmax 2924, 1726, 1588, 1565, 1471, 1366, 1287, 1247, 1108, 1057, 1057, 1022, 902, 857, 802, 764 cm−1; 1H
■
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jnatprod.5b00853. 1 H and 13C NMR spectra for all new compounds (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*Tel: +64 (0) 9 923 8269. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We thank the Royal Society of New Zealand for the award of a Rutherford Discovery Fellowship (J.S).
■
REFERENCES
(1) Lu, X. M.; Huang, G. J.; Jiang, G. H.; Yang, Y. J. Sichuan Tradit. Chin. Med. 2005, 23, 44−45. (2) Fang, S.; Lin, C.; Zhang, Q.; Wang, L.; Lin, P.; Zhang, J.; Wang, X. J. Ethnopharmacol. 2012, 141, 947−956. (3) Zhu, L.-H.; Chen, C.; Wang, H.; Ye, W.-C.; Zhou, G.-X. Chem. Pharm. Bull. 2012, 60, 670−673. (4) Prior to the isolation of the indole alkaloids outlined in ref 3, only two other compounds had been identified from Alocasia macrorrhiza, namely, an antifungal protein and trypsin inhibitor, alocasin, and a ceramide, alomacrorrhiza A; see: (a) Wang, H. X.; Ng, T. B. Protein Expression Purif. 2003, 28, 9−14. (b) Tien, N. Q.; Ngoc, P. H.; Minh, B
DOI: 10.1021/acs.jnatprod.5b00853 J. Nat. Prod. XXXX, XXX, XXX−XXX
Journal of Natural Products
Note
P. H.; Van Kiem, P.; Van Minh, C.; Kim, Y. H. Arch. Pharmacal Res. 2004, 27, 1020−1022. (5) (a) Wang, C.; Sperry, J. Chem. Commun. 2013, 49, 4349−4351. (b) Boyd, E. M.; Sperry, J. Org. Lett. 2014, 16, 5056−5059. (c) Boyd, E. M.; Sperry, J. Org. Lett. 2015, 17, 1344−1346. (6) (a) Davison, E. K.; Sperry, J. Org. Biomol. Chem. 2015, 13, 7911− 7914. (b) Calvert, M. B.; Sperry, J. Chem. Commun. 2015, 51, 6202− 6205. (7) Kim, S. H.; Sperry, J. Tetrahedron Lett. 2015, 56, 5914−5915. (8) For a similar approach to diazine-bridged bisindoles, see: Tasch, B. O. A.; Merkul, E.; Müller, T. J. J. Eur. J. Org. Chem. 2011, 2011, 4532−4535. (9) Attempts to conduct both deprotection steps in a single pot were unsuccessful.
C
DOI: 10.1021/acs.jnatprod.5b00853 J. Nat. Prod. XXXX, XXX, XXX−XXX
