Naturally Occurring Thiophenes: Isolation, Purification, Structural

Phytochem Rev (2016) 15:197–220 DOI 10.1007/s11101-015-9403-7

Naturally occurring thiophenes: isolation, puriﬁcation, structural elucidation, and evaluation of bioactivities

Sabrin R. M. Ibrahim • Hossam M. Abdallah • Ali M. El-Halawany • Gamal A. Mohamed

Received: 26 December 2014 / Accepted: 17 March 2015 / Published online: 22 March 2015 Ó Springer Science+Business Media Dordrecht 2015

Abstract Thiophenes are a class of heterocyclic are generally composed of one to five thiophene rings aromatic compounds based on a five-membered ring that are coupled together through their a-carbons, and made up of one sulfur and four carbon atoms. The carry alkyl chains on their free ortho-positions. thiophene nucleus is well established as an interesting Thiophene-containing compounds possess a wide moiety, with numerous applications in a variety of range of biological properties, such as antimicrobial, different research areas. Naturally occurring thio- antiviral, HIV-1 protease inhibitor, antileishmanial, phenes are characteristic secondary metabolites nematicidal, insecticidal, phototoxic and anticancer derived from plants belonging to the family Aster- activities. This review focuses on naturally occurring aceae, such as Tagetes, Echinops, Artemisia, Bal- thiophene derivatives; their sources, physical and samorhiza, Blumea, Pluchea, Porophyllum and spectral data, and biological activities. Eclipta. Furthermore, naturally occurring thiophenes Keywords Thiophenes Á Biosynthesis Á NMR data Á Anti microbial Á Cytotoxic S. R. M. Ibrahim Department of Pharmacognosy and Pharmaceutical Chemistry, Faculty of Pharmacy, Taibah University, Al Madinah Al Munawwarah 30078, Kingdom of Saudi Arabia Introduction S. R. M. Ibrahim Department of Pharmacognosy, Faculty of Pharmacy, Thiophenes are a class of heterocyclic aromatic Assiut University, Assiut 71526, Egypt compounds based on a five membered ring containing one sulfur and four carbon atoms with a molecular H. M. Abdallah Á A. M. El-Halawany Á G. A. Mohamed Department of Natural Products and Alternative formula of C4H4S. The word ‘thiophene’ is derived Medicine, Faculty of Pharmacy, King Abdulaziz from the Greek words ‘theion’ and ‘phaino’, which University, Jeddah 21589, Kingdom of Saudi Arabia mean sulfur and shining, respectively. Thiophene derivatives make up a significant proportion of the H. M. Abdallah Á A. M. El-Halawany (&) Department of Pharmacognosy, Faculty of Pharmacy, organosulfur-containing compounds found in petro- Cairo University, Cairo 11562, Egypt leum, as well as several other products derived from e-mail: [email protected] fossil fuels, and are formed as the by-products of petroleum distillation (Chaudhary et al. 2012; Mishra G. A. Mohamed Department of Pharmacognosy, Faculty of Pharmacy, et al. 2011). Natural thiophenes are characteristic Al-Azhar University, Assiut Branch, Assiut 71524, Egypt secondary metabolites of plants belonging to the 123 198 Phytochem Rev (2016) 15:197–220 family Asteraceae, including the following genera: chemical shift values in d ppm), 13C NMR (spec- Tagetes, Echinops, Artemisia, Balsamorhiza, Blumea, trometer frequency, solvent, chemical shift in d Pluchea, Porophyllum, and Eclipta. Thiophene values), plant source (family), molecular formula, derivatives isolated from natural sources can be calculated molecular weight and reference(s). The 1H classified according to the number of thiophene rings and 13C NMR data have been rounded to two and one in their structure, including thiophenes (one ring), decimal places, respectively. The molecular weight bithiophenes (two rings), terthiophenes (three rings) data have been rounded to four decimal places. The and quinquethiophenes (five rings) (Fig. 1). Thio- NMR data have been listed on each structure because phene and its derivatives are produced as part of the of the differences in the systems used to number the chemical defense mechanism in numerous plant different structures. The principle aim of this review is species, which involve the manufacture and storage to provide a reference for researchers that they can use of organic substances in different parts of the plants. for the rapid identification of isolated thiophenes These compounds can behave as repellents, act as through a comparison of their physical and spectral toxic substances or have anti-nutritional effects on data. The highlighted bioactivities of these compounds herbivores (Gil et al. 2002). Natural thiophenes are may also be of interest to synthetic and medicinal derived from polyacetylenes, which can be stored in chemists for the design of new drugs using known plant tissues or released into the soil (Tang et al. 1987). thiophenes as raw materials. The thiophenes described These compounds can also act as toxins that are in this review have been arranged in five different activated by sunlight or UV irradiation (300–400 nm). groups according to the number of thiophene rings in These compounds are toxic towards numerous patho- their structure, including group I-thiophene, group II- gens, including nematodes, insects, fungi, and bacteria bithiophenes, group III-terthiophenes, group IV-quin- (Champagne et al. 1984; Gil et al. 2002). quethiophenes, and group V-miscellaneous thio- A recent review of the available literature revealed phenes (Tables 1, 2, 3, 4, 5). that there are currently no reviews pertaining to the biosynthesis, isolation and biological activity of naturally occurring thiophenes. Herein, we have listed Thiophene biosynthesis the thiophenes that have been reported in the literature over the past few decades and provided a summary of The first naturally occurring thiophene derivative, a- their biological activities, physical constants, spectral terthiophene, was isolated in 1947 from Tagetes erecta data, plant sources, and associated references. These (Zechmeister and Sease 1947). Since then, more than data have been listed in the following order for each 150 thiophene-based natural products comprising one, compound: name, structure, melting point (°C), opti- two or three thiophene rings and side chains bearing a cal rotation (concentration, solvent), UV (solvent, variable number of double or triple bonds (Bohlmann kmax nm, log e), IR (medium, absorption band in and Zdero 1985; Kagan 1991) had been characterized cm-1), 1H NMR (spectrometer frequency, solvent, from Asteraceae and fungi (Bohlmann 1988; Sorensen

Fig. 1 Classes of naturally occurring thiophenes

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Table 1 Naturally occurring thiophene: group-I: thiophene 1. 3-(4,8,12,16-Tetramethylheptadeca-3,7,11,15-tetraenyl)-thiophene-1-oxide

-1 Pale yellowish oil; UV kmax (CH3OH) (log e): 218 (4.42) nm; IR (Nujol) cmax: 2925, 1642, 1230, 1025 cm ; EIMS m/z (rel. int.): 386 [M]? (10), 371 (17), 315 (18), 293 (7), 285 (15), 272 (5), 217 (8), 204 (15), 175 (10), 161 (12), 149 (17), 147 (10), 135 (27), 123 (22), 95 (27), 81 (94), 69 (100); HREIMS m/z: 386.2643 (calcd. for C25H38OS, 386.2645); NMR data (CDCl3, 500 and 125 MHz); The marine sponge Xestospongia sp. (Pedpradab and Suwanborirux 2011) 2. Xanthopappin A; 2-(E)-Hept-5-ene-1,3-diynylthiophene diol

? ? Brown oil; UV kmax (CH3OH) (log e): 206 (4.31), 252 (4.22), 313 (4.08) nm; EIMS m/z (rel. int.): 172 [M] (47), 171 [M–H] (34), ? ? 144 [M–C2H4] (32); HRTOFMS m/z: 173.0428 [M?H] (calcd. for C11H9S, 173.0424); NMR data (CDCl3, 500 and 125 MHz); The stems and roots of Xanthopappus subacaulis C. Winkl (family: Asteraceae) (Tian et al. 2006) 3. 10,11-Threo-xanthopappin D; 2-Hept-5,6-threo-dihydroxy-1,3-diynylthiophene

Colourless oil; [a]D -20 (c 0.5, acetone); UV kmax (CH3OH): 306, 290, 232 nm; IR (KBr) cmax: 3367 (OH), 2924 (CH3), 2233 -1 ? (C:C) cm ; HRESIMS m/z: 229.0292 [M?Na] (calcd. for C11H10O2SNa, 229.0294); NMR data (CDCl3, 600 and 150 MHz); Whole plant of Xanthopappus subacaulis C. Winkl (family: Asteraceae) (Zhang et al. 2014) 4. 10,11-Erythro-xanthopappin D; 2-Hept-5,6-erythro-dihydroxy-1,3-diynylthiophene

Colourless oil; [a]D ?20 (c 0.5, acetone); UV kmax (CH3OH): 304, 289, 232 nm; IR (KBr) cmax: 3345 (OH), 2924 (CH3), 2219 -1 ? (C:C) cm ; HRESIMS m/z: 435.0687 [2M?Na] (calcd. for 2(C11H10O2S) Na, 435.0695); NMR data (CDCl3, 600 and 150 MHz); Whole plant of Xanthopappus subacaulis C. Winkl (family: Asteraceae) (Zhang et al. 2014)

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Table 1 continued 5. N-Isobutyl-6-(2-thiophenyl)-2,4-hexadienamide

? HRESIMS m/z: 272.1072 ([M?Na] , (calcd. for C14H19NOS); NMR (CDCl3, 300 and 75 MHz); Leaves of Chrysanthemum coronarium L. (family: Asteraceae) (Ragasa et al. 1997) 6. Amplectol; (3,4-Dihydroxy-8-[50-methyl-thiophen-20-yl]-1,5-octadien-7-yne)

-1 ? Colorless oil; UV kmax (CH3OH): 295 nm; IR mmax: 3620, 3565 (OH), 2200 (C:C) cm ; HREIMS m/z (rel. int.): 234.072 [M] ? ? (calcd. for C13H14O2S, 234.073) (6), 216 [M–H2O] (9), 177 [(M–CH(OH)CH=CH2)] ; NMR data (CDCl3, 400 MHz); Aerial parts of Blumea amplectens DC var. arenaria (family: Asteraceae) (Pathak et al. 1987) 7A. Echinoynethiophene A; 7,10-Epithio-7,9-tridecadiene-3,5,11-triyne-1,2-diol

Yellow needles (acetone), mp. 122–123 °C; IR (KBr) cmax: 3328 (br), 3104, 2956, 2923, 2872, 2150, 1778, 1451, 1322, 1186, 1080 -1 ? (s), 1022, 946, 864, 805, 688 cm ;C13H10O2S; EIMS m/z (rel. int.): 230 [M] (90), 199 (100), 171 (33), 170 (32), 169 (33), 145 (22), 139 (20), 127 (50); NMR data (Acetone-d6, 500 and 125 MHz); Roots of Echinops grijissii Hance (family: Asteraceae) (Liu et al. 2002) 7B. Echinoynethiophene A; 7,10-Epithio-7,9-tridecadiene-3,5,11-triyne-1,2-diol

Yellow amorphous powder; [a]D ?92.2 (c 0.1 CH3OH); UV kmax (e): 237 (7682), 245 (10,293), 251 (10,293), 273 (7728), 275 -1 (7935), 280 (8556), 324 (17,917), 341 (15,755) nm; IR mmax: 3321, 2912, 2863, 2222, 1634, 1446, 1416, 1385, 1090, 798 cm ; EIMS m/z (rel. int.): 230 [M]? (53), 212 (10), 199 (100), 183 (6), 170 (30), 169 (24), 149 (6), 139 (9), 127 (18), 93(9); HREIMS 123 Phytochem Rev (2016) 15:197–220 201

Table 1 continued

m/z: 230.0403 (calcd. for C13H10SO2, 230.0401); NMR data (CD3OD, 200 and 128.5 MHz); Roots of Balsamorhiza sagittata (Pursch) Nuttall (family: Asteraceae) (Matsuura et al. 1996) 8. 10,11-Cis-xanthopappin B; 5-(2-Chloro-1-hydroxyethyl)-2-(Z)-hept-5-ene-1,3-diynylthiophene

Colourless oil; [a]D ?10 (c 0.5, acetone); UV kmax (CH3OH): 315, 252, 268, 213 nm; IR (KBr) cmax: 3382 (OH), 2918 (CH3), 2200 -1 ? (C:C) cm ; HRESIMS m/z: 251.0302 [M?H] (calcd. for C13H11ClOSNa, 251.0294); NMR data (CDCl3, 600 and 150 MHz); Whole plant of Xanthopappus subacaulis C. Winkl (family: Asteraceae) (Zhang et al. 2014) 9. Xanthopappin B; 5-(2-Chloro-1-hydroxyethyl)-2-(E)-hept-5-ene-1,3- diynylthiophene

? Brown oil; [a]D 0(c 0.377, acetone); UV kmax (CH3OH) (log e): 209 (4.37), 268 (4.44) nm; EIMS m/z (rel. int.): 252 [M?2] (14), ? ? ? ? ? 251 [M?1] (6), 250 [M] (38), 201 [M–CH2Cl] (100), 171 [M-CH2ClCH(OH)] (18); HRTOFMS m/z: 273.0112 [M?Na] (calcd. for C13H11ONaSCl, 273.0116); NMR data (CDCl3, 500 and 125 MHz); The stems and roots of Xanthopappus subacaulis C. Winkl (family: Asteraceae) (Tian et al. 2006) 10. 5-(But-4-chloro-3-hydroxy-1-ynyl)-2-(Z)-pent-3-ene-1-ynylthiophene

Colourless oil; [a]D -10 (c 1.0, acetone); UV kmax (CH3OH): 211, 227, 316, 333 nm; IR (KBr) cmax 3344 (OH), 2924 (CH3), 2219 -1 ? (C:C) cm ; HRESIMS m/z: 273.0115 [M?Na] (calcd. for C13H11ClOSNa, 273.0111); NMR data (CDCl3, 600 and 150 MHz); Whole plant of Xanthopappus subacaulis C. Winkl (family: Asteraceae) (Zhang et al. 2014) 11. 5-(But-4-chloro-3-hydroxy-1-ynyl)-2-(E)-pent-3-ene-1-ynylthiophene

Colourless oil; [a]D ?10 (c 1.0, acetone); UV kmax (CH3OH): 210, 226, 319, 334 nm; IR (KBr) cmax: 3344 (OH), 2924 (CH3), 2180 -1 ? (C:C) cm ; HRESIMS m/z: 273.0115 [M?Na] (calcd. for C13H11ClOSNa, 273.0111); NMR data (CDCl3, 600 and 150 MHz); Whole plant of Xanthopappus subacaulis C. Winkl (family: Asteraceae) (Zhang et al. 2014)

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Table 1 continued 12. 5-(1,2-Dihydroxyethyl)-2-(Z)-hept-5-ene-1,3-diynylthiophene

Colourless oil; [a]D ?30 (c 10.0, acetone); UV kmax (CH3OH): 316, 268, 253, 216 nm; IR (KBr) cmax: 3359 (OH), 2921(CH3), -1 ? 2204 (C:C) cm ; HRESIMS m/z: 255.0457 [M?Na] (calcd. for C13H12O2SNa, 255.0450); NMR data (CDCl3, 600 and 150 MHz); Whole plant of Xanthopappus subacaulis C. Winkl (family: Asteraceae) (Zhang et al. 2014) 13. 5-(But-3,4-dihydroxy-1-ynyl)-2-(Z)-pent-3-ene-1-ynylthiophene

Colourless oil; [a]D ?40 (c 1.0, acetone); UV kmax (CH3OH): 312, 261, 213 nm; IR (KBr) cmax: 3363 (OH), 2923 (CH3), 2227 -1 ? (C:C) cm ; HRESIMS m/z: 255.0456 [M?Na] (calcd. for C13H12O2SNa, 255.0450); NMR data (CDCl3, 600 and 150 MHz); Whole plant of Xanthopappus subacaulis C. Winkl (family: Asteraceae) (Zhang et al. 2014) 14. 5-(But-3,4-dihydroxy-1-ynyl)-2-(E)-pent-3-ene-1-ynylthiophene

Colourless oil; [a]D ?40 (c 1.0, acetone); UV kmax (CH3OH): 313, 263, 2I5 nm; IR (KBr) cmax: 3344 (OH), 2924 (CH3), 2180 -1 ? (C:C) cm ; HRESIMS m/z: 255.0456 [M?Na] (calcd. for C13H12O2SNa, 255.0450); NMR data (CDCl3, 600 and 150 MHz); Whole plant of Xanthopappus subacaulis C. Winkl (family: Asteraceae) (Zhang et al. 2014) 15. 2-Acetyl-3-metoxy-5-(prop-1-ynyl) thiophen

-1 A white solid crystal; mp 71–73 °C; UV kmax 300 nm; IR mmax: 1545 (Ar), 1632 (CO), 2233 (C:C) cm ; CIMS m/z (rel. int.): 195 ? ? ? ? [M?H] (100); EIMS m/z (rel. int.): 194 [M] (93.3), 179 [M–CH3] (100), 165 (22.8), 151 [M–CH3CO] (30.5), 136 (20.9), 108 (26.6), 93 (20.5), 77 (21.9), 63 (61), 43 (67); NMR data (CDCl3, 400 and 100 MHz); Roots of Artemisia absinthium L. (family: Asteraceae) (Yamari et al. 2004)

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Table 1 continued 16. 5-Hydroxymethyl-2-(E)-hept-5-ene-1,3-diynylthiophene diol

Cream crystals; mp 72–75 °C; UV kmax (CH3OH): 217, 227 sh, 256, 261, 271, 304 sh, 321, 348 sh nm; IR (KBr) cmax: 3260, 2900, 210, 2150, 1613, 1440, 1362, 1349, 1285, 1182, 1130, 1030, 995, 942, 805 cm-1; EIMS m/z (rel. int.): 202 [M]? (100) 185 [M– ? ? ? ? H2O] (46), 74 [M-CO] (16), 171 [M–CH2OH] (20); HRTOFMS m/z: 203.0525 [M?H] (calcd. for C12H11OS, 203.0530); 1 13 NMR data (CDCl3, 90 MHz for H and 125 for C NMR); Roots of Leuzea carthamoides DC (syn. Rhaponticum carthamoides Willd. Iljin) (family: Asteraceae); (Szendrei et al. 1984; Tian et al. 2006) 17. (E)-2-[5-(Hept-5-en-1,3-diynyl)-thien-2-yl]-ethan-1,2-diol

Cream crystals; mp 96–98 °C; [a]D 0(c 0.083, acetone); UV kmax (CH3OH): 217, 255, 270, 304 sh, 321, 346 sh nm; IR (KBr) cmax: 3200 (br), 2870, 2160, 2100, 1610, 1435, 1285, 1200, 1160, 1090, 1055, 1040, 940, 875, 810 cm-1; EIMS m/z (rel. int.): 232 ? ? ? ? [M] (25), 201 [M-CH2OH] (l00), 171 [M–CH(OH)–CH2OH] ; HRTOFMS m/z: 255.0525 [M?Na] (calcd. for C13H12ONaS, 255.0455); NMR data (CDCl3, 600 and 150 MHz); Underground parts of Leuzea carthamoides DC (syn. Rhaponticum carthamoides Willd. Iljin) (family: Asteraceae) (Chobot et al. 2003; Szendrei et al. 1984; Tian et al. 2006) 18. 2-[Pent-1,3-diynyl]-5[4-hydroxybut-1-ynyl]-thiophene

-1 13 Yellowish oil; IR (KBr) cmax: 3490, 2230, 1640, 1100, 980 cm ; C NMR (CDCl3, 75 MHz): dC 135.62, 132.94, 128.12, 123.73, 1 95.46, 85.35, 75.28, 70.25, 67.98, 65.30, 61.96, 25.43, 4.91; EIMS m/z (rel. int.): 214 (30), 187 (100); H NMR data (CDCl3, 90 MHz); Roots of Echinops pappii Chiov (family: Asteraceae) (Abegaz 1991) 19. 2-[Cis-pent-3-en-l-ynyl]-5-[4-hydroxybut-l-ynyl]-thiophene 20. 2-[Trans-pent-3-en-l-ynyl]-5-[4-hydroxybut-l-ynyl]-thiophene

-1 13 Yellow waxy solid; IR (KBr) cmax: 3360, 3040, 2245, 2165, 1630, 1200, 1058, 960, 820 cm ; C NMR data (CDCl3, 75 MHz): dC 141.90, 140.60, 132.48, 132.55, 132.33, 132.06, 125.42, 125.10, 122.66, 111.63, 110.39, 92.80, 92.43, 92.31, 91.28, 83.80, 81.07, 77.79, 75.40, 61.17, 61.13, 24.02, 18.79, 16.19; HRMS m/z: 216.0612 (calcd. for C13H12OS; 216.0609); EIMS m/z (rel.

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Table 1 continued

int.): 216 (24), 189 (100); NMR data (CDCl3, 300 and 75 MHz); Roots of Echinops pappii Chiov (family: Asteraceae) (Abegaz 1991) 21. PDDYT; 2-(Penta-1,3-diynyl)-5-(3,4-dihydroxybut-1-ynyl)-thiophene

? EIMS m/z: 230 [M] ; NMR data (CD3OD, 500 and 125 MHz); Roots of Echinops grijsii Hance (family: Asteraceae) (Jin et al. 2008; Shi et al. 2010) 22. 4-(5-(Penta-1,3-diynyl)thiophen-2-yl)but-3-ynyl acetate

NMR data (CDCl3, 90 MHz); Roots of Echinops hispidus Fresen (family: Asteraceae) (Abegaz et al. 1991) 23A. 2-Hydroxy-4-(5-(penta-1,3-diynyl)thiophen-2-yl)but-3-ynyl acetate

-1 IR (KBr) mmax: 2237, 1758, 1240 cm ; HRMS m/z (rel. int.): 272. 0507 (calcd. for C15H12O3S; 272.0510) (24), 254 (10), 212 (100), 199 (42), 170 (20); NMR data (CDCl3, 90 MHz); Roots of Echinops hispidus Fresen (family: Asteraceae) (Abegaz et al. 1991) 23B. 2-(Pant-1,3-diynyl)-5-(4-acetoxy-3-hydroxybuta-1-ynyl)-thiophene

? EIMS m/z: 290 [M] ; NMR data (CDCl3, 500 and 125 MHz); Stems and leaves of Pluchea indica (L.) Less. (family: Asteraceae) (Jin et al. 2008) 24. 1-Hydroxy-4-(5-(penta-1,3-diynyl)thiophen-2-yl)but-3-yn-2-yl acetate

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Table 1 continued

NMR data (CDCl3, 90 MHz); Roots of Echinops hispidus Fresen (family: Asteraceae) (Abegaz et al. 1991) 25. 4-(5-(Penta-1,3-diynyl)thiophen-2-yl)but-3-yne-1,2-diyl diacetate

NMR data (CDCl3, 90 MHz); Roots of Echinops hispidus Fresen (family: Asteraceae) (Abegaz et al. 1991) 26. 2-Chloro-4-(5-(penta-1,3-diynyl)thiophen-2-yl)but-3-yn-1-ol

NMR data (CDCl3, 90 MHz); Roots of Echinops hispidus Fresen (family: Asteraceae) (Abegaz et al. 1991) 27A. 4-[5-(Penta-1,3-diynyl)thien-2-yl]-2-chlorobut-3-ynyl acetate

? EIMS m/z (rel. int.): 292 (5), 290 [M] (17), 254 (28), 230 (100), 195 (59); NMR data (CDCl3, 400 and 100 MHz); Roots of Echinops transiliensis Golosh (family: Asteraceae) (Fokialakis et al. 2006) 27B. 2-(Pant-1,3-diynyl)-5-(4-acetoxy-3-chlorobuta-1-ynyl)-thiophene

? EIMS m/z: 272 [M] ; NMR data (CDCl3, 500 and 125 MHz); Stems and leaves of Pluchea indica (L.) Less. (family: Asteraceae) (Jin et al. 2008) 28. 5-(1,2-Diacetoxyethyl)-2-(E)-hept-5-ene-1,3-diynylthiophene diol

Yellow oil; UV kmax (CH3OH): 217, 256, 261, 270, 302 sh, 322, 344 sh nm; IR (KBr) cmax: 2960, 2200, 2140, 1755, 1422, 1370, 1226, 1049, 950, 870, 810 cm-1; EIMS m/z (rel. int.): 316 [M]? (41), (256) [M–AcOH]? (55), 214 [M–AcOH–CH,CO]? (l00),

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Table 1 continued

? ? ? 201 [M–Me–CH2CO] (82), 171 [M–CH(COOMe)–CH2COOMe] (9); HRTOFMS m/z: 239.0663 [M?Na] (calcd. for 1 13 C17H16O4NaS, 239.0667); NMR data (CDCl3, 90 MHz for H and 125 for C NMR); Roots of Leuzea carthamoides DC (syn. Rhaponticum carthamoides Willd. Iljin) (family: Asteraceae) (Szendrei et al. 1984; Tian et al. 2006) 29. 5-(1-Dihydroxy-2-acetoxyethyl)-2-(E)-hept-5-ene-1,3-diynylthiophene diol

Cream crystals; mp 82–84 °C; UV kmax (CH3OH): 217, 226 sh, 257, 270, 303 sh, 324, 346 sh nm; IR (KBr) cmax: 3310, 2940, 885, 2170, 2210, 1700, 1430, 1385, 1360, 1270, 1235, 1225, 1185, 145, 1080, 1035, 980, 944, 895, 802 cm-1;MSm/z (rel. int.): 274 ? ? ? ? ? [M] (15), 256 [M–H2O] (2), 214 [M–HOAc] (70), 201 [M–CH2COOMe] (l00), 185 [M–C7H5] (12), 171 [M–CH(OH)– ? CH2COOMe] (28); NMR data (CDCl3, 90 MHz); Roots of Leuzea carthamoides DC (syn. Rhaponticum carthamoides Willd. Iljin) (family: Asteraceae) (Szendrei et al. 1984)

1977). Oleic acid has been proposed as a precursor in consistent with the desaturase pathway, the elimina- the biosynthesis of thiophenes via acetylene interme- tion hypothesis remains valid for polyketide-derived diates (Margl et al. 2001). Acetylenic natural products acetylenic natural products (Minto and Blacklock include all compounds containing a carbon–carbon 2008). triple bond or alkynyl functional group. Three fatty Sulfur, which is a heteroatom commonly intro- acids have been identiﬁed as the basic building blocks duced into polyacetylenes, is found in a wide range of of most acetylenic natural products, including ecologically signiﬁcant thiophenes and bithiophenes. crepenynic acid, stearolic acid and tariric acid (Minto The structures of these compounds vary considerably and Blacklock 2008). Oleic acid is converted to in terms of their number of thiophene rings (1–3) and trideca-3,5,7,9,11-pentayn-l-ene (PYE) via repeated the degree of unsaturation in their side chains (i.e., desaturation steps involving crepenynic acid and chain ene/yne) (Margl et al. 2001). Cysteine and H2S have shortening processes (Margl et al. 2001). PYE is then both been proposed as potential sources of sulfur converted to a variety of different thiophenes that (Bohlmann et al. 1973, 1988; Jente et al. 1988, 1981). subsequently accumulate in plant tissue (Fig. 2) The key step in the conversion of PYE to thiophenes is

(Jacobs et al. 1995). The biosynthesis of polyacetyle- the addition of H2S or its biochemical equivalent to a nes occurs in two stages, including (A) an oxidative conjugated triple bond, followed by a ring formation dehydrogenation (desaturation) mechanism, where the reaction, which is probably a two-step reaction existing alkene functionality undergoes a desaturation (Bohlmann et al. 1973). In addition to the formation reaction through an iron-catalyzed dehydrogenation of compounds containing two or three thiophene rings, with molecular oxygen. The electrons required by this the removal of a terminal methyl group and modiﬁ- reaction are provided by either NADH or NADPH. cation of a vinyl group are necessary to obtain the The second step (B) involves a decarboxylative enol various thiophenes that ultimately accumulate in plant elimination mechanism, which uses a divergent tissues (Fig. 4). approach for the formation of the second p-bond The addition of sulfur to a diyne unit leads to the (Fig. 3). The elimination of an activated enol car- formation of a thiophene ring via a stepwise process. boxylate intermediate is thermodynamically driven by The formal addition of H2S produces vinyl thiols that the formation of CO2, which could be accompanied by are intercepted in certain Asteraceae species to the hydrolysis of the pyrophosphate. According to the produce thioethers. Subsequent ring closure results original hypotheses, path A would operate with full- in the formation of thiophenes and the oxidative length acyl lipids, whereas path B would install formation of disulﬁde linkages that producing bithio- acetylenic groups during de novo fatty acid biosyn- phenes (Fig. 4). thesis. Although the current paradigm and all ex- The proportion of thiophenes found in the different periments dealing with fatty acid biosynthesis are parts of a plant can vary considerably based on the type 123 Phytochem Rev (2016) 15:197–220 207

Table 2 Naturally occurring thiophene: group-II: bithiophene 30. 5-Acetyl-2,20-bithiophene

mp 59–60 °C; UV kmax (CH3OH): 254, 365 nm; NMR data (CDCl3, 500 and 125 MHz); Roots of Echinops latifolius Tausch. (family: Asteraceae) (Wang et al. 2008) 31. 5-(4-Hydroxybut-1-ynyl)-2,20-bithiophene

UV kmax (CH3OH): 254, 365 nm; NMR data (CDCl3, 500 and 125 MHz); Roots of Echinops latifolius Tausch. (family: Asteraceae) (Wang et al. 2008) 32. BBT; 5-(But-3-en-1-ynyl)-2,20-bithiophene

UV kmax (CH3OH): 254, 365 nm; NMR data (CDCl3, 500 and 125 MHz); Roots of Echinops latifolius Tausch. (family: Asteraceae) (Margl et al. 2001; Wang et al. 2008) 33. 5-(3-Acetoxy -4-isovaleroyloxybut-1-ynyl-2,20-bithiophene

mp 94–95 °C; UV kmax (CH3OH): 254, 365 nm; NMR data (CDCl3, 500 and 125 MHz); Roots of Echinops latifolius Tausch. (family: Asteraceae) (Wang et al. 2008) 34. 5-(3-Hydroxmethyl-3-isovaleroyloxyprop-1-ynyl)-2,20-bithiophene

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Table 2 continued

Yellow oil; [a]D -9.0 (c 0.001, CHCl3); HRMS: m/z 334.0696 (calcd. for C17H18O3S2, 334.0671); ESIMS m/z (rel. int.): 357.0 [M?Na]? (9.0), 358.0 [M?H?Na]? (1.7), 359.0 [M?2H?Na]? (1.1), 360.2 [M?3H?Na]? (0.3), 254.8 [M?Na-102]? (100.0); NMR data (CDCl3, 300 and 75 MHz); Roots of Echinops latifolius Tausch. (family: Asteraceae) (Wang et al. 2006) 35. Grijisone A: 5-[(4-Isovaleroyloxy) buta-1-onyl]-2,20-bithiophene

Yellow powder (CDCl3); mp 62.3-62.7 °C; UV (MeOH) kmax (log e): 375 (4.3), 262 (3.2), 220 (3.6) nm; IR (KBr) mmax: 1729, -1 ? 1655, 839, 801, 717 cm ; HREIMS m/z: 336.0838 [M] (calcd. for C17H20O3S2, 336.0854); EIMS m/z (rel. int.): 336.1 (100), 337.1 (19.8), 338.0 (10.4), 166 (4.3); NMR data (CDCl3, 600 and 150 MHz); Roots of Echinops grijissi Hance (family: Asteraceae) (Zhang et al. 2008) 36. 5-(3,4-Diacetoxy-l-butynyl)-2,20-bithiophene

? ? Yellow oil; EIMS m/z (rel. int.): 334 [M] (25); 274 [M–AcOH] (l), 232 (49, 95 (4), 73 (5), 43 (100); C16H14O4S2; NMR data (CDCl3, 200 MHz); Roots of Tagetes patula L. (family: Asteraceae) (Menelaou et al. 1991) 37. 50-Methyl-[5-(4-acetoxy-1-butynyl)]-2,20-bithiophene

? ? Yellow oil; ESIMS m/z (rel. int.): 291 [M?H] (10), 301 (26), 245 (33), 229 (73), 313 [M?Na] (100); NMR data (CDCl3, 300 and 75 MHz); Aerial parts of Porophyllum ruderale (Jacq.) (family: Asteraceae) (Takahashi et al. 2013, 2011) 38. Methyl-5-[4-(3-methyl-1-oxobutoxy)-1-butynyl]-2,20-bithiophene

? Yellow needle-like crystals; UV kmax (Et2O): 347.2 nm; EIMS m/z (rel. int. %): 245.7 [M] (94.6), 228.8 (70), 216.8 (100); HREIMS m/z: 246.0155 (C13H10OS2); NMR (CDCl3, 300 and 75 MHz); Roots of Tagetes patula L. (family: Asteraceae) (Bano et al. 2002)

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Table 2 continued 39. Methyl-5-[4-(3-methyl-1-oxobutoxy)-1-butynyl]-2,20-bithiophene

Yellowish oil; UV kmax (Et2O): 339 nm (e 26,726); IR (CCl4) mmax: 2850 (C–H stretching), 2300 (C:C), 1717 (ester carbonyl), -1 ? ? 1600 (C=C) cm ; HREIMS m/z (rel. int.): 332.0903 M] (C18H20O2S2) (19), 230.0186 [M–C5H10O2] (100), 217 (9), 197 (5), 115 (7), 102 (4); NMR (CDCl3, 300 and 75 MHz); Roots of Tagetes patula L. (family: Asteraceae) (Bano et al. 2002) 40. Grijisyne A; 5-[2-[4-(5-Propyneylthiophen-2-yl)buta-1,3-diynyl]cyclobutaneyl]ethynyl]-2,20-bithiophene

Yellow powder; mp 134.2–135.0 °C; UV (MeOH) kmax (log e): 340.4 (4.2), 250.6 (3.5) nm; IR (KBr) mmax: 2192, 796, 836, -1 ? 674 cm ; HREIMS m/z: 412.0429 [M] (calcd. for C25H16S3, 412.0414); EIMS m/z: 413.0 (100), 414.0 (28.5), 415.1 (12.9), 166 (5.6); NMR data (CDCl3, 600 and 150 MHz); Roots of Echinops grijissi Hance (family: Asteraceae) (Zhang et al. 2008) 41. Cardopatine

Yellow plates; mp 123-125 °C; UV (MeOH) kmax (log e): 340.0 (4.82), 242.0 (4.33) nm; [a]D; IR (KBr) mmax: 840 (2-thienyl), 810 -1 (thiophen-2,5-diyl) cm ; EIMS: m/z (rel. int.): 432 (9) 216.0 (100), 171 (13), 95 (6); NMR data (CDCl3, 400 and 100 MHz); Stem and leaves of Echinops latifolius Tausch. (family: Asteraceae) (Selva et al. 1978; Zhang et al. 2007) 42. Isodopatine

Light yellow plates; mp 79–80 °C; UV (MeOH) kmax (log e): 340.0 (4.82), 242.0 (4.33) nm; IR (KBr) mmax: 840 (2-thienyl), 810 -1 (thiophen-2,5-diyl) cm ; EIMS: m/z (rel. int.): 432 (12), 216.0 (100), 171 (13), 95 (7); NMR data (CDCl3, 300 and 75 MHz); Stem and leaves of Echinops latifolius Tausch. (family: Asteraceae) (Selva et al. 1978; Zhang et al. 2007)

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Table 2 continued 43. Xanthopappin C; 1,2-Bis[5-(E)-hept-5-ene-1,3-diynylthiophen-2-yl]-2-hydroxypentane-1,4-dione

? Brown oil; [a]D 0(c 0.085, acetone); UV kmax (CH3OH) (log e): 215 (4.72), 269 (4.83), 353 (4.52) nm; EIMS m/z: 456 [M] (4), ? ? ? 398 [M–CH3COCH3] (17), 370 [M–CH3COCH3–H2O] (9), 257 [M–HCOC4H2SC:CC:CCH=CHCH3] (25), 200 ? ? [HCOC4H2SC:CC:CCH=CHCH3] (18), 199 [HCOC4H2SC:CC:CCH=CHCH3–H] (100), 171 (6); HRTOFMS m/z: ? 479.0737 [M?Na] (calcd. for C27H20O3NaS2, 479.0751); NMR data (CDCl3, 500 and 125 MHz); The stems and roots of Xanthopappus subacaulis C. Winkl (family: Asteraceae) (Tian et al. 2006) of plant. For example, no thiophenes can be found in the resulting mixture of thiophenes mixture was then shoots of achenes, with bithienyls and traces of 5-(but- dissolved in EtOH and purified by preparative HPLC 0 3-en-1-ynyl)-2,2 -bithiophene (BBT) being identified over an octadecylsilane (C18) reversed-phase column, as the major chemicals in this case. a-Terthienyl, which using 72–85 % acetonitrile or 70–85 % MeOH as an can be found in the root of corresponding plants but not eluent (Downum et al. 1984). The compounds eluted in the shoots, and accumulates in flowers. Despite many from the column were detected using a UV spec- experiments, it remains to be shown whether thiophene trophotometer with a detection range of 320–350 nm metabolites originate exclusively in the roots, and that (Jin et al. 2008; Norton et al. 1985; Tosi et al. 1991). specific thiophenes are preferentially accumulated in Normal phase HPLC analyses were conducted using a the different parts of the plant, or whether enzymatic 95:5 (v/v) mixture of n-hexane and dioxane as the components of the thiophene pathway are expressed in a eluent (Szarka et al. 2006, 2007). HPLC was used to tissue-specific manner. It has been reported that methyl identify and quantify the different thiophene-contain- cleavage occurs prior to the formation of the second ing compounds (Camm et al. 1975; Croes et al. 1989). thiophene ring (Minto and Blacklock 2008). Thiophenes can generally be isolated by the extraction of plant materials with EtOH or MeOH, and the resulting thiophenes can then be further purified by Methods for separation of thiophenes partitioning the alcohol extract between n-hexane or pet ether (PE). The n-hexane or PE fraction can then be Thiophenes extraction and purification subjected to purification by column chromatography using n-hexane:EtOAc or PE:acetone as the eluent with To allow for the exclusive extraction and isolation of a gradient elution system. The isolated compounds can only thiophene-containing compounds, the plant ma- then be further purified by preparative HPLC. terials were extracted with a 1:1 (v/v) mixture of EtOH Another method for the isolation and purification of and H2O. The resulting thiophenes were then separated thiophenes is the direct extraction of plant materials by partitioning them between a 1:1 (v/v) mixture of n- with n-hexane or PE. The resulting extracts can be hexane and tert-butylmethylether (Jacobs et al. 1995). purified by column chromatography over silica gel The individual layers were collected and the organic eluting with an n-hexane:EtOAc or PE:acetone gradi- solvents were evaporated under a stream of N2 gas. The ent, followed by preparative HPLC.

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Table 3 Naturally occurring thiophene: group-III: terthiophene 44. a-Terthiophene

Colourless needles; mp 91–92 °C; IR (KBr) mmax: 3434, 2931, 2862, 1637, 1460, 1378, 1240, 1050, 1021, 969, 958, 837, -1 800 cm ;C12H8S3; NMR data (Acetone-d6, 500 and 125 MHz); Roots of Echinops grijissii Hance (family: Asteraceae) (Liu et al. 2002) 45. 5-Acetyl-a-terthiophene

-1 ? Yellow crystals; mp 135–1378 °C; C14H10S3O; IR (KBr) mmax: 2919, 2850, 1731, 1636 cm ; EIMS m/z: 290 [M] , 275 [M– ? ? CH3] , 247 [M–CH3CO] , 203, 138; NMR data (Acetone-d6, 500 and 125 MHz); Roots of Echinops grijisii Hance (family: Asteraceae) (Liu et al. 2002) 46. 5-Chloro-a-terthiophene

-1 ? Yellow crystals; mp 129–130 °C; C12H7S3Cl; IR (KBr) mmax: 2914, 1586, 1422, 833, 788, 686 cm ; EIMS m/z: 284 [M?2] , ? ? 282 [M] , 247 [M–Cl] , 237, 214, 203, 141, 127, 102, 93; NMR data (Acetone-d6, 500 and 125 MHz); Roots of Echinops grijisii Hance (family: Asteraceae) (Liu et al. 2002) 47. 5,500-Dichloro-a-terthiophene

-1 ? ? Yellow crystals; mp 134–135 °C; C12H6S3Cl2; IR (KBr) mmax: 2914, 1427, 849, 787 cm ; EIMS m/z: 320 [M?4] , 318 [M?2] , ? ? ? 316 [M] , 281 [M–Cl] , 246 [M–2Cl] , 237, 201, 158, 145, 119; NMR data (Acetone-d6, 500 and 125 MHz); Roots of Echinops grijisii Hance (family: Asteraceae) (Liu et al. 2002) 48. 5-Methyl-2,20:50,200-terthiophene

? Viscous yellow oil; ESIMS m/z (rel. int.): 185 (15), 229 (25), 263 [M?H] (36), 262 (100); NMR data (CDCl3, 300 and 75 MHz); Aerial parts of Porophyllum ruderale (family: Asteraceae) (Takahashi et al. 2013, 2011)

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Table 3 continued 49. Ecliptal; 5-Formyl-a-terthiophene

? mp 144–145 °C; EIMS m/z: 276 [M] ; NMR data (CDCl3, 300 and 75 MHz); Herbs of Eclipta alba Hassk (family: Asteraceae) (Das and Chakravarty 1991; Yuan et al. 2007)

Table 4 Naturally occurring thiophene: group-IV: quinquethiophene 50. 5-Methyl-2, 20,50,200,500,2000,5000,2000-quinquethiophene

-1 Brown needles; mp 215–216 °C; UV (MeOH) kmax: 334, 387 nm; IR (KBr) mmax: 2870, 1600 cm ; HRESIMS m/z: 427.6611 ? ? [M?H] (calcd. for C21H15S5, 427.6609); 428.6613 [M?2H] (calcd. for C21H16S5, 428.6609); NMR data (CDCl3, 500 and 125 MHz); Leaves of Tagetes minuta L. (family Asteraceae) (Al-Musayeib et al. 2014)

TLC chromatography and detection of thiophenes novel compounds based on their 1D (1H, 13C and DEPT) and 2D (1H–1H COSY, HSQC/HMQC and The following solvent systems were used for TLC HMBC) NMR data (Tables 1, 2, 3, 4, 5). The analysis: PE:acetone (99:1), PE, PE:diethyl ether connectivities of the different atoms present in the (90:10), and n-hexane:dioxane:n-BuOH (75:25:1) thiophenes isolated in the current study were estab- (Margl et al. 2001). Thiophenes can be detected on a lished by NOE and ROESY experiments to determine TLC plate by their characteristic ﬂuorescence under the stereochemistries of the different thiophenes. long wave UV light or by their reaction with one of the following TLC stains: Mass spectroscopy (MS) 1. Vanillin spray reagent (0.5 g vanillin ? 9mL Mass spectroscopy has been used as an effective 95 % EtOH ? 0.5 mL conc. H SO ? 3 drops 2 4 method for the identiﬁcation and quantitative deter- glacial acetic acid (Picman et al. 1980). mination of thiophenes. The mass spectra of sulfur- 2. Isatin spray reagent (0.4 % isatin in conc. H SO ) 2 4 containing compounds generally contain a series of (Curtis and Phillips 1962). characteristic fragments, including [M]?,[M?H]? and [M?2H]? (corresponding to 4.5 % of the inten- Structural elucidation of the thiophenes sity of the M?Áion). Electrospray ionization (ESI) mass spectrometry generally gives [M?H]? and [M?Na]± Nuclear magnetic resonance spectroscopy ions for sulfur-containing compounds. It is noteworthy that sulfur can be lost from the M?Á ions of sulfur- Nuclear magnetic resonance spectroscopy (NMR) is containing compounds together with neighboring C one the most powerful techniques available for atoms as CHS fragments. These fragments would investigating the structural properties of different appear with m/z values of 45 (CHS?) and 44 (CS?Á), molecules. One of the main applications of NMR in and can be used as indicators for the presence of sulfur thiophene research is the structural elucidation of (Pretsch et al. 2009).

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Table 5 Naturally occurring thiophene: group-V: miscellaneous 51. Echinothiophenegenol; 5-Hydroxy-6-[(1E,3E)-6-hydroxy-1,3-hexadienyl]-2-hydroxymethyl-thieno[2,3-e]- isobenzofuran-8(6H)-one

-1 ? - Pale yellow powder; IR (KBr) mmax: 3436, 1697, 1467 cm ; ESIMS m/z: 332 [M?H] ; HRESIMS m/z: 331.0643 [M–H] (calcd. 331.0640); NMR data (DMSO-d6, 600 and 150 MHz); Roots of Echinops grijissii Hance (family: Asteraceae) (Zhanga et al. 2009) 52. Echinothiophene; 5-O-b-D-glucopyranosyl-6-[(1E,3E)-6-hydroxy-1,3-hexadienyl]-2-hydroxymethyl-thieno[2,3-e]- isobenzofuran-8(6H)-one

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Table 5 continued

-1 Pale-yellowish needles; mp 214–216 °C (dec); IR (KBr) mmax: 3421, 2924, 1750, 1645, 1094, 1056 cm ;UVkmax (MeOH) (log e): 254 (4.94), 319 (4.10) nm; HRFABMS m/z: 495.1339 [M?H]? (D0.0022 of the calcd.) and m/z: 517.1147 [M?Na]? (D0.0010 of the calcd.) (C23H26O10S); NMR data (DMSO-d6, 500 and 125 MHz); Roots of Echinops grijissii Hance (family: Asteraceae) (Koike et al. 1999)

Biological activity Rhizoctonia solani, Sclerotinia sclerotiorum, and Sclertium rolfsii. These results therefore indicated Despite the unique nature of their chemical structures that Tagetes minuta could be used as a potential relative to the many other different classes of naturally candidate for the production of natural fungicides. occurring compounds, thiophenes have not yet been Furthermore, the methanol extract of Tagetes patula well studied in terms of their potential pharmaco- exhibited a dose dependent anti-fungal activity to- logical activities. Several naturally occurring thio- wards several phytopathogenic fungi, including Botry- phenes and thiophene-rich extracts have exhibited a tis cinerea, Fusarium moniliforme, and Pythium variety of different biological effects, including ultimum. It is noteworthy that the methanol extract antimicrobial, cytotoxic, chemo-preventive, photo- of Tagetes patula exhibited much stronger antifungal toxic, insecticidal, herbicidal, and anti-leishmanial activity when it was used in light than it did in the dark. activities. The enhanced antifungal activity of the extract in the presence of light could be attributed to light-induced Antimicrobial activities changes in the fungal cell membranes involving the production of free radicals, which could result in the Some of the thiophenes isolated in the current study premature aging of the fungal mycelia (Mares et al. exhibited antibacterial, antifungal and antiviral ac- 2004). tivities towards a variety of different microorganisms. Compound 3, which was isolated from Xanthopap- Saha et al. (2013) reported that the isolation of a pus subcaulis, exhibited potent antibacterial activity thiophene-rich extract from Tagetes minuta exhibited against Bacillus subtilis with an MIC of 7.25 lg/mL. moderate antifungal activity towards several soil In contrast, the corresponding erythro isomer 4 borne and foliar plant pathogens, including exhibited broad spectrum antibacterial activity

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Fig. 2 Biosynthesis of different thiophenes

towards Escherichia coli, B. cereus, Staphylococcus Trichophyton mentagrophytes (Ragasa et al. 1997). aureus, and Erwinia carotovora with MIC values of Compound 7 was isolated from Balsamorhiza sagit- 12.5, 15.5, 7.2, and 7.2 lg/mL, respectively. Several tata, which is a plant native to Northwestern America. other chlorinated derivatives (8–11), which were This plant has been reported as a folk medicine isolated from the same plant, exhibited only moderate because of its antibacterial and antifungal activities, activity towards E. coli, B. cereus, S. aureus, E. and compound 7 exhibited signiﬁcant activities again- carotovora, and B. subtilis (Zhang et al. 2014). st B. subtilis, S. aureus and S. aureus SA0017, which is Compound 5, which was isolated from the chloroform a methicillin-resistant strain of S. aureus. The an- extract of Chrysanthemum coronarium, exhibited tibacterial activity of compound 7 towards a variety of moderate antimicrobial activity towards B. subtilis, different bacteria was enhanced when the experiments Pseudomonus aeruginosa, Candida albicans and were conducted in the presence of UV-A light

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Fig. 3 The two distinct AB proposals for the biogenesis of acetylenic bonds

Fig. 4 Sulfur addition to polyacetylenes

(Matsuura et al. 1996). Furthermore, the antibacterial exhibited significant broad spectrum antifungal ac- activity of compound 7 was confirmed by Kundu and tivity towards a variety of different fungal strains, with Chatterjee (2013), who reported that this compound Trichophyton mentagrophytes var. mentagrophytes, exhibited MIC values in the range of 25–100 lg/mL Absidia corymbifera and Candida tropicalis being towards six different strains of S. aureus. The authors particularly sensitive to this compound (Chobot et al. of this study also conducted a series of mechanistic 2003). studies with compound 7, which revealed that this Thiophenes 18, 27, 31, 32, 36, 40 and 44 were compound exhibited bacteriostatic effects. Further- identified in the dichloromethane extract of Echinops more, compound 7 was determined to be a DNA ritro using an antifungal/biological activity guided polymerase inhibitor, as confirmed by agarose gel approach. Compounds 18, 31 and 44 exhibited the electrophoresis (Kundu and Chatterjee 2013). Com- most potent antifungal activities of the seven different pound 17 was isolated from Leuzea carthamoides, and compounds towards a variety of different plant 123 Phytochem Rev (2016) 15:197–220 217 pathogens, including Colletotrichum acutatum, Col- mitochondrial membrane were observed in promastig- letotrichum fragrariae and Colletotrichum gloeospo- otes treated with compound 37, as well variations in rioides at concentrations in the range of 3–30 lM. the morphological characteristics of the cells (Taka- Compound 44 appeared to be relatively selective hashi et al. 2013). towards Colletotrichum species, exhibiting a high Compound 44 also exhibited significant nematici- level of activity against C. gloeosporioides (IC50 \ dal activity when it was irradiated with near UV light. 1.6 lM), whilst showing only moderate levels of The nematicidal activity of this compound was activity towards C. acutatum and C. fragariae with attributed to the liberation of reactive oxygen species

IC50 values of 3.0 and 4.9 lM, respectively. Com- from the compound upon UV irradiation (Bakker et al. pound 18 appeared to demonstrate selective antifungal 1979). activity towards Phomopsis species, with moderate activities towards Phomopsis obscurans (IC50 = 2.9 - Phototoxic, insecticidal, and herbicidal effects lM) and Phomopsis viticola (IC50 \ 1.6 lM). Fur- thermore, compound 18 exhibited a high level of There is a growing interest in the discovery of activity towards Fusarium oxysporum with an IC50 of phototoxic phytochemicals, especially those charac- 9.5 lM. The high activity of compound 18 is terized by signiﬁcant increases in their activities particularly interesting because very few chemicals following exposure to light. These compounds are have been reported to inhibit the activity of F. mainly used as insecticides, herbicides and antimicro- oxysporum with IC50 values of \30 lM (Fokialakis bial agents. Thiophenes are a class of natural products et al. 2006). that have been extensively studied in terms of their Compound 49 showed promising inhibitory activity phototoxic effects. towards HIV-1 protease with an IC50 value of 58 lM, The herbicidal activity of a-terthienyl (44) was but did not show any activity towards HIV-1 integrase assessed in pot and ﬁeld trials by Lambert et al. (1991). (Tewtrakul et al. 2007). It is noteworthy that com- The results of this study revealed that compound 44 pound 44 exhibited a dose-dependent inhibitory acted as a contact herbicide in corn and broad leaf -1 activity towards HIV in the presence of UV-A light weeds with IC50 values in the range of 15–29 kg ha . (320–400 nm), but no activity in the presence of Compounds 2, 9, 16, 17, 28 and 43 showed photo- visible light or in the dark. However, compound 44 did activated insecticidal activity towards the fourth not exhibit any activity towards poliovirus or cox- instar-larvae of the Asian tiger mosquito with LC50 sackievirus (Hudson et al. 1993). values of 0.71, 0.53, 0.30, 4.2, 0.66, and 0.95 lg/mL,

Compounds 44, 48, and 49 exhibited photo-induced respectively. In the absence of light, the LC50 values of inhibitory activity towards the growth of S. aureus.It compounds 2, 16, 17, 28 and 43 were [10 lg/mL, is noteworthy that the unsubstituted 2,20:50,200-terthio- while that of compound 9 was 5.1 lg/mL. These phene (44) was the only one of these three compounds results demonstrated that the irradiation of compounds to exhibit inhibitory activity towards E. coli with an 2, 16, 17, 28 and 43 with light led to 14.1-, 15.2-, 10.5-, MIC value of 0.62 lg/mL. Furthermore, none of these 33.3- and 2.4-fold increases in their activity, respec- three compounds exhibited inhibitory activity towards tively, as well as a 9.6-fold increase in the activity of 9. P. aeruginosa (Ciofalo et al. 1996). The photo-activated insecticidal effects of these compounds were attributed to light dependent toxicity Antiparasitic activity mechanisms involving the photo-oxidation of insect targets resulting in membrane damage, enzyme inac- Compounds 37 and 48 exhibited antileishmanial tivation, cell death and other biological loss of activity towards the promastigote and axenic forms function mechanisms (Tian et al. 2006). of Leishmania amazonensis with IC50 values of 7.7 Compound 17 was isolated from the roots of Leuzea and 21.3 lg/mL, and 19.0 and 28.7 lg/mL, respec- carthamoides and exhibited potent phototoxic effects tively (Takahashi et al. 2011). Both of these com- in histidine photo-oxidation, Artemia salina and pounds were shown to be highly selective towards Tubifex assays compared with the known phototoxic intracellular amastigotes with minimal toxicity to- agent xanthotoxin. The higher activity of 17 towards wards human cells. Furthermore, changes in the A. salina could be attributed to the release of singlet 123 218 Phytochem Rev (2016) 15:197–220 oxygen from 17 following its irradiation with light and chemopreventive effects. Compound 5 was rather than the release of a superoxide anion, as is the investigated in terms of its antimutagenic effects case with xanthotoxin. A. salina is much more using a micronucleus test. At a dose of 8 mg/kg bwt, sensitive to singlet oxygen than it is to superoxide compound 5 reduced the number of micronucleated anion radicals, which explains the higher activity of 17 polychromatic erythrocytes by 66.5 % (Ragasa et al. towards A. salina compared with xanthotoxin (Chobot 1997). Compound 21 was reported to possess potent et al. 2003). NAD(P)H: quinine oxidoreductase 1 (NQO1) induc- ing activity in murine Hepa1c1c7 cells. The maximum Cytotoxic effect induction of this compound was 3.3-fold greater than that of 40-bromoﬂavone (positive control) at a con- Several thiophenes were screened to determine their centration of 40 lM. As a phase 2 detoxifying enzyme cytotoxic effects against a wide range of human cancer inducer, the mechanism of action of compound 21 was cell lines. The marine sponge-derived thiophene 1 investigated to determine whether it was monofunc- exhibited weak cytotoxicity towards Vero cells tional (i.e., progressing through the Keap1-Nrf2

(African green monkey kidney cells) with an IC50 pathway) or bifunctional (i.e., progressing through value of 31 lM (Pedpradab and Suwanborirux 2011). the aryl hydrocarbon receptor-xenobiotic response Compounds 35 and 40, which were isolated from the element pathway). The study concluded that com- roots of Echinops grijisii, were evaluated in terms of pound 21 was acting in a mono-functional manner their cytotoxic activity towards a variety of different though the activation of the Keap1-Nrf2 pathway (Shi cancer cell lines, including HL-60, K562 and MCF-7 et al. 2010). cells. Compound 35 exhibited moderate activities against HL60 and K562 cells, with IC50 values of 21.1 and 25.2 lg/mL, respectively. Compound 40 also Conclusions exhibited moderate levels of activity against HL60,

K562, and MCF-7 cells, with IC50 values of 19.6, 18.9 Thiophenes are a class of heterocyclic aromatic and 28.7 lg/mL (Zhang et al. 2008). Compounds 25, compounds that fulfill all the requirements for being 32, 33, 36, 40, and 43 were also isolated from Echinops lead compounds in a number of different therapeutic grijisii and screened for their cytotoxic activity against areas. Compounds belonging to this class possess a HepG2, K562, HL60, and MCF-7 cells. Compound 36 variety of different chemical compositions, and have exhibit a high level of activity towards HL60 and K562 been reported to exhibit a wide range of biological cells with IC50 values of 12 lg/mL), while 33 showed activities. In this review, we have described the potent activity towards K562 cells (IC50 = 7 lg/mL). biosynthetic pathways, spectral data, sources and Jin et al. (2008) reported that most thiophenes are biological activities of 52 different thiophenes. cytotoxic after UV irradiation. The UV light-mediated cytotoxicity of thiophenes has been attributed to them Conflict of interest The authors declare that they have no being highly conjugated and becoming increasingly conflicts of interest. unstable under UV irradiation conditions. The irradiation of these compounds with UV light would therefore result in the liberation of free radicals that References would attack the cells. However, the main interest of the authors of this particular study was the structure Abegaz BM (1991) Polyacetylenic thiophenes and terpenoids activity relationships of compounds that exhibited from the roots of Echinops pappii. Phytochemistry cytotoxic activity in the absence of light. The authors 30:879–881 Abegaz BM, Tadesse M, Majinda R (1991) Distribution of reported that the introduction of an acyl substituent as a sesquiterpene lactones and polyacetylenic thiophenes in side chain was essential to the cytotoxic activity of Echinops. Biochem Syst Ecol 19:323–328 these compounds, especially in the non-radiated Al-Musayeib NM, Mohamed GA, Ibrahim SR et al (2014) New bithiophenes. thiophene and flavonoid from Tagetes minuta leaves growing in Saudi Arabia. Molecules 19:2819–2828 Thiophenes have also been reported to exhibit a Bakker J, Gommers FJ, Nieuwenhuis I et al (1979) Photoacti- variety of other activities, including antimutagenic vation of the nematicidal compound alpha-terthienyl from 123 Phytochem Rev (2016) 15:197–220 219

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