Short and Effective Synthesis of a Thiophene Analogue of (\pm)-4-Ipomeanol and Its Biological Evaluation

Turk J Chem 30 (2006) , 451 – 454. c TÜBİTAK
Short and Effective Synthesis of a Thiophene Analogue of (±)-4-Ipomeanol and Its Biological Evaluation Jürgen KRAUSS∗, Doris UNTERREITMEIER, Franz BRACHER Department of Pharmacy. Ludwig-Maximilians-University, Butenandtstr. 5-13, 81377 Munich-GERMANY e-mail: Received 06.01.2006 The thiophene analogue of (±)-4-ipomeanol (1) was prepared in 2 steps starting from 3-iodothiophene and pent-4-yn-2-ol. The key steps were a Sonogashira reaction, followed by HgO catalysed hydratisation of the triple bond. The resulting thiophene was tested in the MTT assay for its cytotoxic activity against HL 60 cells. Key Words: Sonogashira reaction, 4-ipomeanol, cytotoxic activity, MTT assay. Introduction (±)-4-Ipomeanol (1) is a natural cytotoxin, first isolated from Fusarium solani infected sweet potatoes, Ipomoea batata, in 1972 by Boyd and coworkers.1,2 4-Ipomeanol (1) is a stress metabolite in response to microbial infection, with an LD50 of 20-70 mg/kg.2 The mechanism of action is closed to the lung Clara cells, which leads to a bioactivation of the compound by cytochrome P-450 monooxygenase to a highly reactive alkylating furan epoxide as recently published by Baer et al.3 Because of its specific lung toxicity (±)-4-ipomeanol (1) is being tested as a new drug for the treatment of lung carcinoma. On the other hand, (±)-4-ipomeanol (1) is metabolised by liver cells as well, and so it was recently tested in a phase II clinical trial on patients with hepatocellular carcinoma,4−7 but the results were not encouraging. O OH O 1 Figure 1. (±)-4-Ipomeanol (1). In continuation of our work on 4-ipomeanol derivatives we developed the first synthesis of a thiophene analogue. The described synthesis can also be used for the synthesis of (±)-ipomeanol (1). ∗ Corresponding author 451 Short and Effective Synthesis of a Thiophene Analogue of..., J. KRAUSS, et al., Chemistry The target compound 3 was synthesised by a Sonogahira reaction of the commercially available 3-iodothiophen and pent-4-yn-2-ol under catalysis of PdCl2 (PPh3 )2 and CuI in ethyldimethylamine (EDMA) to give the alkyne 2 in almost quantitative yield.8 The triple bond of 2 was regioselective hydrated under HgO catalysis to give the aromatic ketone 3.9 OH S S HgO OH a I b 2 S OH O 3 Scheme. a: CuI, EDMA, PdCl2 (PPh3 )2 . b: HgO, methanol, H2 SO4. Compound 3 was tested in the MTT assay for its cytotoxic activity against HL 60 cells using the method described by Mosman10 . The compound showed only weak cytotoxic activity. Cell growth inhibition potential of 3 120 percent of control 100 80 60 40 20 -3 -2 -1 0 0 1 2 10 x concentration [µM] 3 4 5 Figure 2. MTT-assay of thioipomeanol (3). Discussion As diagrammed in Figure 2, the thiophene analogue showed only weak cytotoxic activity against HL 60 cells (IC50 = 102 µM). Work is in progress to test the cytotoxicity after cytochrome P450 activation as described 452 Short and Effective Synthesis of a Thiophene Analogue of..., J. KRAUSS, et al., for (±)-4-ipomeanol (1). The described synthesis might also be used for the synthesis of (±)-ipomeanol (1), when starting from 3-iodofuran. The synthesis allowed us to build up enantiomeric pure products by starting from enantiomeric pure pent-4-yn-2-ol. Experimental IR-spectra: Perkin-Elmer FT-IR Paragon 1000; MS: Hewlett Packard MS-Engine, electron ionisation (EI) 70 eV, chemical ionisation (CI) with CH4 (300 eV); NMR (400 MHz): Jeol GSX 400 (1 H: 400 MHz, 13 C: 100 MHz); GLC-MS: Shimadzu GC 17 A; flash column chromatography (FCC): silica gel 60 (230-400 mesh, E. Merck, Darmstadt, Germany). (±)-5-Thiophen-3-ylpent-4-yn-2-ol (2). First 950 mg (4.5 mmol) of 3-iodothiophen and 800 mg (9.5 mmol) of (±)-pent-4-yn-2-ol were dissolved in 30 mL of EDMA; then 190 mg (1.0 mmol) of CuI and 150 mg (0.2 mmol) of PdCl2 (PPh3 )2 were added and the mixture was stirred for 12 h at room temperature under N2 atmosphere. The solvent was evaporated and the residue was dissolved in 50 mL of 5% aqueous Na2 S2 O3 solution, extracted with diethyl ether (3 × 50 mL) and the combined organic layers were dried over Na2 SO4 . The organic solvent was evaporated and the residue purified by FCC (n-hexane / ethyl acetate 10:1) to give 710 mg (95%) of 2 as a brown oil. 1 H-NMR (CDCl3) δ (ppm) = 1.32 (d, J= 6.2 Hz, 3 H, CH3 ), 2.01 (s, 1 H, OH), 2.53 (dd, J = 6.8 Hz, J = 16.7 Hz, 1 H, CH2 ), 2.61 (dd, J = 6.8 Hz, J = 16.7 Hz, 1 H, CH2 ), 4.04 (m, 1 H, CH), 7.09 (dd, J = 1.1 Hz, J = 5.0 Hz, 1 H, aromat. CH), 7.25 (dd, J = 3.2 Hz, J = 5.0 Hz, 1 H, aromat. CH), 7.39 (dd, J = 3.2 Hz, J = 1.1 Hz, 1 H, aromat. CH). 13 C-NMR (CDCl3 ) δ (ppm) = 22.41 (CH3 ), 30.01 (CH2 ), 66.52 (CH), 78.07 (quart. C), 85.68 (quart. C), 122.27 (quart. C), 125.20 (aromat. CH), 128.25 (aromat. CH), 129.95 (aromat. CH). MS (CI): m/z (%) = 333 (2 x M+ +1, 100), 167 (M+ +1, 20). IR (KBr): ν [cm−1 ] = 3390, 3106, 2970, 2926, 2905, 1114, 1084. C9 H10 OS (166.24) Calcd: C: 65.03 H: 6.06 S: 19.29. Found: C: 64.54. H: 6.06. S: 18.41. (±)-4-Hydroxy-1-thiophen-3-yl-pentan-1-one (3). First 250 mg (1.5 mmol) of 3 was dissolved in 20 mL of methanol, and 10 mL of 5% H2 SO4 and 800 mg (3.7 mmol) of yellow HgO were added. The solution was stirred for 12 h at room temperature. The methanol was evaporated and residue was quenched with 20 mL of saturated Na2 CO3 solution and extracted with diethyl ether (3 × 30 mL). The combined organic layers were dried over Na2 SO4 and the residue was purified by FCC (n-hexane/ethyl acetate 5:1) to give 70 mg (25%) of 3 as a colourless oil. 1 H-NMR (CDCl3 ) δ (ppm) = 1.25 (d, J = 6.2 Hz, 3 H, CH3 ), 1.82 (m, 1 H, CH2 ), 1.94 (m, 1 H, CH2 ), 3.06 (t, J = 6.9 Hz, 2 H, CH2 ), 3.89 (m, 1 H, CH), 7.32 (ddd, J = 0.6 Hz, J = 2.9 Hz, J = 5.1 Hz, 1 H, aromat. CH), 7.56 (ddd, J = 5.1 Hz, J = 1.2 Hz, J= 0.7 Hz, 1 H, aromat. CH), 8.10 (dd, J = 1.2 Hz, J = 2.9 Hz, 1 H, aromat. CH). 13 C-NMR (CDCl3 ) δ (ppm) = 23.85 (CH3 ), 33.04 (CH2 ), 36.20 (CH2 ), 67.51 (CH), 126.37 (aromat. CH), 126.96 (aromat. CH), 132.10 (aromat. CH), 142.17 (quart. C), 195.12 (CO). MS (CI): m/z (%) = 185 (M+ +1, 58), 167 (100), 111 (20). MS (EI): m/z (%) = 184 (M+ , 3), 167 (16), 126 (56), 111 (100). IR 453 Short and Effective Synthesis of a Thiophene Analogue of..., J. KRAUSS, et al., (KBr): ν [cm−1 ] = 3433, 3106, 2965, 2925, 1669, 1510, 1411, 913, 743. C9 H12 O2 S(184.26). HR-MS: Calcd.: 184.0558. Found: 184.0562. References 1. M.R. Boyd and B.J. Wilson, J. Agric. Food. Chem. 20, 428-430 (1972). 2. L.T. Burka, L. Kuhnert, B.J. Wilson and T.M. Harris, J. Am. Chem. Soc., 99, 2302- 2305 (1977). 3. B.R. Baer, A.E. Rettie and K.R. Henne, Chem. Res. Toxicol. 18, 855-864 (2005). 4. M.C. Christian, R.E. Wittes, B. Leyland-Jones, T.L. McLemore, A.C. Smith, C.K. Grieshaber, B.A. Chabner and M.R. Boyd, J. Natl. Cancer Inst., 81, 1133-1143 (1989). 5. S.M. Smiley-Jewell and C.G. Plopper, Toxicol. Appl. Pharm. 192, 69-77 (2003). 6. S. Lakhanpal, R.C. Donehower and (...truncated)