Bioorganic & Medicinal Chemistry Letters 25 (2015) 259–261

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Ionic liquid catalyzed synthesis of 2-(indole-3-yl)-thiochroman-4ones and their novel antifungal activities Ya-Li Song a,b,⇑, Fan Wu a,b, Chao-Chao Zhang a,b, Guo-Chao Liang a,b, Guan Zhou a,b, Jiao-Jiao Yu a a b

Key Laboratory for Pharmaceutical Quality Control of Hebei Province, College of Pharmaceutical Sciences, Hebei University, Baoding, Hebei 071002, China Key Laboratory of Medical Chemistry and Molecular Diagnosis, Ministry of Education, Hebei University, Baoding, Hebei 071002, China

a r t i c l e

i n f o

Article history: Received 18 August 2014 Revised 5 November 2014 Accepted 20 November 2014 Available online 27 November 2014 Keywords: Indole Thiochromone Thiochroman-4-one Ionic liquid Antifungal activity

a b s t r a c t 2-(Indole-3-yl)-thiochroman-4-ones were synthesized via ionic liquid and tested for in vitro antifungal activity. The contribution of ionic liquid to Michael addition reaction is significant. Structures of all compounds are elucidated by 1H NMR, 13C NMR and HRMS. Most of these compounds showed better antifungal activity than fluconazole. The results suggest that 2-(indole-3-yl)-thiochroman-4-ones would be efficient antifungal agents. Ó 2014 Elsevier Ltd. All rights reserved.

Thiochroman-4-one compounds represent an attractive class of biologically active molecules.1–3 Hoettecke et al.4 had prepared 2alkenylthiochroman-4-ones and tested their antimicrobial activity. Furthermore, a large number of products exhibit considerable antimicrobial activity. In our previous work5, 1-(4-phenylthiazol-2-yl)1,4-dihydrothiochroman[4,3-c]pyrazole was synthesized in EtOH by thiochromanone-3-carbaldehyde, phenacyl bromide and thiosemicarbazide. These compounds showed good inhibition for Cryptococcus neoformans. The study of structure–activity relationship of thiochroman-4one compounds indicated that the position of the heterocyclic ring with various substituents was considerably important factors to affect the biological activities.6–8 We speculated that incorporation of bioactive heterocycle containing heteroatom would change the physicochemical properties and lead to stronger pharmacophore. The C-3 of thiochroman-4-one containing substituent or heterocyclic group can effectively enhance antifungal activity.9–11 However, that C-2 is attached substituent or heterocyclic group, particularly indole, has not been found. Indoles are important small molecules of nitrogen-containing heterocyclic with biological activity, found in many natural products, pharmaceuticals agents and polymer materials.12 In 2013, a series of bisindolylalkanes analogues such as 3,3-(thiochroman4,4-diyl)bis(1H-indole) were synthesized by our group.13 Most of them showed antitumor activities in vitro, and varying degree ⇑ Corresponding author. Tel.: +86 312 507 1108; fax: +86 312 507 1107. E-mail address: [email protected] (Y.-L. Song). http://dx.doi.org/10.1016/j.bmcl.2014.11.056 0960-894X/Ó 2014 Elsevier Ltd. All rights reserved.

topoisomerase II inhibitory activity. C-3 of indole has a certain nucleophilicity, which enables indole to undergo Michael addition reaction with a,b-unsaturated ketone acid catalyzed method to give 1,4-addition product. Zhan et al.14 reported that b-indolones were synthesized from a,b-unsaturated ketone and indole by acid or Lewis acid. However, the catalytic activity of inorganic acid is very poor, and the organic acid with indole will generate purple similar paste substances, which are difficult to handle and separate the pure product. Ionic liquids (ILs), usually composed of organic cations and inorganic/organic anions, are presented in molten salt liquid at room temperature, also known as room temperature ionic liquids.15–18 Gu et al.19 reported that various indole derivatives and a,b-unsaturated carbonyl compounds including some acid-labile substrates were successfully applied to this system with water as the sole solvent to afford the desired Michael adducts in high yields. He et al.20 had synthesized quaternary ammonium geminal Brønsted acid ionic liquids. The catalytic ionic liquids were applied in three-component Mannich reactions with an aldehyde, ketone, and amine at 25 °C in water. In this work, 2-indole-3-yl-thiochroman-4-ones were synthesized by an ionic liquid catalyzed method, and then their antifungal activity was tested. A convenient method for synthesis of 2-indole-3-yl-thiochroman-4-ones is shown in Scheme 1 and Table 1. As shown in Scheme 1, compounds 5a–5o could be synthesized by a four step method from thiophenol 1, b-chloropropionic acid and appropriate active indole derivatives.

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Scheme 1. Synthesis of 2-indole-3-yl-thiochroman-4-ones derivatives. Reagents and conditions: (a) ClCH2CH2COOH/NaOH/H2O/reflux/3 h/92–97%; (b) H2SO4/12 h/81–87%; (c) PCl5 (3 equiv)/C6H6/reflux/2 h/70–85%; (d) indole (1.5 equiv)/[bmim]HSO4 (0.2 equiv)/EtOH/reflux/1.5–3 h/55–85%.

Table 1 Synthesis of 2-indole-3-yl-thiochroman-4-ones 5a–5o

a

No.

R1

R2

R3

R4

R5

Timea (h)

Yielda (%)

Mp (°C)

5a 5b 5c 5d 5e 5f 5g 5h 5i 5j 5k 5l 5m 5n 5o

F CH3 Cl Cl Cl H H H H H H CH3 CH3 CH3 CH3

H H H H H H H H H H H F H H H

H H H H H F F F CH3 CH3 Cl H CH3 CH3 CH3

H H H H CH3 H H H H CH3 CH3 H H H H

H H H Cl H H Cl OH H H H H H Cl OH

1.5 2 1.5 1.5 1.5 1.5 1.5 3 2 2 2 1.5 2 1.5 3

81 77 76 80 85 82 81 68 75 80 84 79 76 77 55

139–142 169–170 153–156 182–184 138–140 122–125 177–180 95–97 147–149 96–98 119–121 124–126 117–119 169–172 90–92

All compounds was catalyzed by ionic liquid—[bmim]HSO4.

Thiochromone 4 was prepared according to the reported method.21 Substituted thiophenol 1 (0.2 mol), b-chloropropionic acid (0.24 mol) and sodium hydroxide (0.48 mol) were reflux in water for 3 h. The solution was adjusted by hydrochloric acid to pH = 1 and found a large number of white precipitations 2, which were filtrated and purified. Drying compound 2 (20 g) was dehy-

drated and cyclized by concentrated sulfuric acid for 12 h. After adding a lot of ice, a large number substituted thiochroman-4one 3 was precipitated. Phosphorus pentachloride (30 mmol) and 3 (10 mmol) were reflux in benzene for 2 h. Thiochromone 4 was precipitated, and the pure product was recrystallized from methanol. In the final, Micheal addition reaction of thiochromone 4 and indole would be carried out by a suitable acid. In order to study the optimum conditions of Michael addition reaction, taking 6-fluorothiochromone for instance, catalyst and solvent were respectively investigated. As shown in Table 2, inorganic acid such as hydrochloric acid is not a good catalyst for this reaction, and the yield is only 9%. Organic acid such as p-toluenesulfonic acid or trifluoroacetic acid can increase the yield to reach 50%. In addition, ionic liquids can simultaneously act as an organic acid and solvent, so 1-butyl-3-methylimdazolium hydrosulfate ([bmim]HSO4)22,23 was prepared and selected as acidic catalyst. In order to save the use of the catalyst, the reaction was catalyzed by [bmim]HSO4 in methanol, ethanol or acetonitrile, respectively. When the solvent is ethanol, the yield is highest (81%) and the time is shortest (1.5 h). Compounds 5a–5o were synthesized by Michael addition reaction of 4 (1 mmol) with appropriate indole or substituted indole. When 4 with 1.5 equiv amounts of appropriate indole and 20 mol % [bmim]HSO4 in ethanol was reflux for 1.5 h to 3 h, compounds 5a–5o were found. As follow extracted with ethyl acetate, the product was isolated and purified by silica gel column chromatography. Most of these addition product had overall high yields of 55–85%. Furthermore, the ionic liquid could be repeated to use in the reaction by the extraction and drying to constant weight, so the recovery and reuse of the ionic liquid was investigated. After completion of the reaction, the system was concentrated under reduced pressure to remove the ethanol and then added in water. The solution was extracted by ethyl acetate. Ionic liquid remained in the lower aqueous layer, which immediately was concentrated

Table 2 Effect of the different catalysts and solventsa

a b c

Catalyst

Amount of catalyst (mol %)

Solvent

Temp (°C)

Time (h)

Isolated yield (%)

HCl pTSAb TFA c [bmim]HSO4 [bmim]HSO4 [bmim]HSO4

20 20 20 20 20 20

Ethanol Ethanol Ethanol Methanol Ethanol Acetonitrile

Reflux Reflux Reflux Reflux Reflux Reflux

12 4 3 2 1.5 2

9 50 51 70 81 74

6-Fluorothiochromone (1 mmol) and indole (1.5 mmol) were as reactants in 10 mL solvent. pTSA = p-toluenesulfonic acid. TFA = trifluoroacetic acid.

Y.-L. Song et al. / Bioorg. Med. Chem. Lett. 25 (2015) 259–261

under reduced pressure to remove water. Residual ionic liquid was placed into vacuo dryness case at 60 °C to constant weight. Recovery of ionic liquid would continue to catalyze 6-fluorothiochromone and indole in ethanol. The reaction was repeated six times in accordance with the above procedure, and yield and time of this reaction were listed in Table 3. Before repeated three times, the yield basically kept constant, but the catalytic effect significantly began to decline from the fourth to sixth time. Furthermore, the objective product was rapidly reduced. The extraction and transfer of [bmim]HSO4 was a little loss, we speculate that, which was the main cause of deterioration of the catalyst effect. These synthesized compounds were screened for antifungal activity against Candida albicas (C. a), Cryptococcus neoformans (C. n), Mucor racemosa (M. r), Microsporum gypseum (M. g) and Epidermophyton floccosum (E. f) by serial plate dilution method.24 The stock solution from each molecule was prepared in dimethyl sulfoxide. The compounds using RPMI 1640 media buffered with 3-[Nmorpholino]-propane sulfonic acid (MOPS) were determined in 96-well tissue culture plates. Then, individual fungal inoculums were added to each well of microtiter plate containing the compounds. The final concentration of all the compounds and drug was 128–0.25 lg/mL. The Petri dishes were prepared in triplicate and maintained at 37 °C for 2–7 days. The minimum inhibitory concentration (MIC) was noted by seeing the lowest concentration of the drug at which there was no visible growth. Amphotericin B and fluconazole were used as positive control.25 As shown in Table 4, most of the compounds have more excellent inhibition than fluconazole for the five fungi. Wherein, the best MIC values of compound 5 g (inhibition of C. n and E. f), 5d (inhibition of C. n) and 5 h (inhibition of E. f) can be achieved 4 lg/mL. These compounds for inhibition of shallow fungi are stronger than deep. Comparison of different substituent compounds with their antifungal activity, structure-activity relationships of these compounds can be proposed. When the phenyl ring of thiochroman-4-one has the electron-withdrawing group Table 3 Recovery and reuse of the ionic liquida 1

2

3

4

5

6

Yield (%) Time (h)

81 1.5

80 1.5

80 1.5

71 2

52 3

22 5

a 6-fluorothiochromone (1 mmol) and indole (1.5 mmol) were as reactants with 10 mL ethanol in the flask of ionic liquid.

Table 4 Antifungal activities in vitro for compounds 5a–5o

5a 5b 5c 5d 5e 5f 5g 5h 5i 5j 5k 5l 5m 5n 5o Fa Bb a b

MIC values/(lg/mL) C. a

C. n

M. r

M. g

E. f

128 128 64 64 64 32 32 16 64 128 32 >128 64 32 64 32 2

64 32 32 4 128 16 4 8 64 32 64 64 32 16 16 8 4

32 128 64 8 32 32 8 16 64 64 32 128 128 32 32 32 4

32 >128 64 16 128 8 8 16 64 32 32 64 64 16 16 64 2

32 >128 32 8 128 8 4 4 128 64 32 64 128 16 16 32 2

F = Fluconazole. B = Amphotericin B.

such as fluoro or chloro, the activity is relatively better than containing electron-donating group such as methyl. When C-8 of thiochroman-4-one is attached the substituent, they have stronger antifungal activity. Comparison substituent with activity can be found that 5a (6-F), 5b (6-CH3) and 5c (6-Cl) have poor activity than 5f (8-F), 5i (8-CH3) and 5k (8-Cl). Furthermore, when indole contains substituents in the phenyl ring, the activity has greatly improved. Chloro is slightly more positive antifungal activity than hydroxyl in the indol. However, the methyl instead of hydrogen on the nitrogen, to a certain extent, can decrease antifungal activity. In conclusion, an efficient synthesis and evaluation of antifungal activity of thiochroman-4-one derivatives was found. 2-Indole-3yl-thiochroman-4-ones were synthesized from thiochromone and indole, and catalyzed by [bmim]HSO4. The catalyst could be recycled 3 times for the next reaction. The preliminary bioassay showed that most compounds have more efficient antifungal activity than fluconazole. Among them, MIC values of 5d, 5f, 5g and 5h for particular fungi can reach 8 or 4 lg/mL. Acknowledgments We gratefully acknowledge Hebei University Natural Science Foundation (2010-194); The National Major Scientific and Technological Special Project for ‘Significant New Drugs Development’ (2012ZX09103-101-057); The Key Project supported by the Research Award Fund for Scientific and Technological in Higher Education institutions of Hebei Province (ZD2010234), China, the Open Project Program of Key Laboratory of Pharmaceutical Quality Control of Hebei Province, for financial support. Supplementary data Supplementary data (synthetic experimental procedures and compound characterization) associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.bmcl.2014.11.056. References and notes

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