Background

Heterocyclic compound are extensively distributed in nature and have versatile synthetic applicability and biological activity which implement the new approaches for the medicinal chemist to plan and organize towards the discovery of novel drugs [1]. Thiophene and its derivatives showed extensive significance in pharmaceutical field because of its varied biological and clinical applications [2].

Serious life-threatening infections have been increased because of the increase in the resistance of microorganism agents which is mainly caused by multi-drug resistant of Gram-positive and Gram-negative pathogenic bacteria [3]. So, there is an pressing need to designed an effective, potent and novel antimicrobial agents with better pharmacodynamic and pharmacokinetic properties [4]. Moreover, cancer also remains one of the primary causes of death in the world therefore effective treatments with novel medicines having improved tumor selectivity, efficacy, and safety remains desirable [5]. Ideal anticancer drugs eliminate the cancer cells without harming normal tissues [6]. Unfortunately, no currently available anticancer agents with desired therapeutic index that earned this criterion and clinical use of drugs which involves a weighing of benefits beside toxicity [7].

Recently, thiophene and its derivatives attracted researchers in expanding their potential in the field of antioxidant [8]. The main purpose of antioxidant is to neutralize free radicals in order to prevent various oxidative diseases like autoimmune, cardiovascular, and neurovascular diseases [9]. The setback of drug resistance and generation of free radicals has promoted the researchers to study and explore for new compounds to combat the problems of infection and the associated risks from generation of free radicals. Corrosion inhibition mechanism is a crucial problem which causes destruction of material (usually metal) by chemical and/or electrochemical reaction with its environment. Organic inhibitors generally containing heteroatom’s such as sulphur is found to have higher basicity and electron donating ability which act by adsorption on the metal surface and slab the active surface sites which leads in the reduction of corrosion rate [10, 11].

Thiophene derivatives show remarkable applications in different disciplines. In medicine, thiophene derivatives show antimicrobial [12], analgesic and anti-inflammatory [13], antihypertensive [14], and antitumor activity [15] while they are also used as inhibitors of corrosion of metals [16] or in the fabrication of light-emitting diodes in material science [17]. In light of afore-mentioned facts and in continuance of our research on the improvement of thiophene derivatives, we hereby report the synthesis and biological screening (i.e. antimicrobial, antioxidant, anticorrosion and anticancer) of thiophene derivatives with a variety of acceptor and donor groups.

Results and discussion

Chemistry

A series of ethyl-2-(substituted benzylideneamino)-4,5,6,7-tetrahydrobenzo[b]thiophene-3-carboxylate derivatives were synthesized (Scheme 1, S1–S18) by using Gewald synthesis. Initially, ethylcyanoacetate and cyclohexanone was reacted with sulphur at room temperature with continuous stirring in presence of diethylamine which resulted in the formation of intermediate-1. Later on intermediate-1 treated with substituted aromatic benzaldehyde and dioxane in presence of triethylamine yielded the final compounds (S1–S18). The structures of the entire newly synthesized compound were checked by IR data and 1H NMR which was in full favour with molecular structures assigned. Its physical characterizations were indicated in Table 1.

Scheme 1
scheme 1

Synthesis of ethyl-2-(substituted benzylideneamino)-4,5,6,7-tetra hydrobenzo[b]thiophene-3-carboxylate derivatives

Full size image
Table 1 The physicochemical properties of newly synthesized derivatives (S1–S18)
Full size table

Antimicrobial activity

In vitro antimicrobial activity of novel synthesized compounds were screened against the microorganisms such as Gram positive (Staphylococcus aureus, Bacillus subtilis), Gram negative (Escherichia coli, Salmonella typhi) and fungal strains (Aspergillus niger and Candida albicans) using tube dilution method. The results after examined were expressed as minimum inhibitory concentration (MIC i.e. lowest conc. required for the test substances to complete growth inhibition). The MIC values of standard and synthesized compounds were presented in Table 2. Antimicrobial evaluation results indicated that the entire synthesized compounds were found to have significant antimicrobial activity and different compound were found to be active against different microorganisms.

Table 2 Antimicrobial activity of newly synthesized compounds (S1–S18)
Full size table

Antimicrobial screening results showed that compound S1 was the most potent antibacterial agent against B. subtilis, S. aureus, E. coli and S. typhi having MIC value 0.81 µM/ml. Further having significant antibacterial activity, the synthesized compound S4 also displayed excellent antifungal activity against both A. niger and C. albicans and (MIC = 0.91 µM/ml). On the whole, antimicrobial activity results (Table 2) indicated that compounds S1 and S4 were found to be most potent antimicrobial agents.

Antioxidant activity

Newly synthesized derivatives were investigated for their in vitro antioxidant activity by DPPH assay at absorbance 517. The percentage (%) inhibition was calculated from the Eq. 1 and IC50 value of newly synthesized compounds were obtained from the graph drawn between concentrations with % inhibition of test compound given in Figs. 1, 2 and 3. From the results obtained it was known that all the newly derived compounds were found to have good to moderate antioxidant activity. Among them, compound S4 and S6 exhibited excellent antioxidant activity with IC50 values 48.45 and 45.33 respectively when compared with the ascorbic acid as standard drug. The results were shown in Table 3.

Fig. 1
figure 1

Standard graph of ascorbic acid

Full size image
Fig. 2
figure 2

Graph of potent antioxidant compounds S4 and S6. Where Series 1 = Percentage inhibition of compound 4 and Series 2 = % inhibition of compound 6

Full size image
Fig. 3
figure 3

IC50 values of compounds S4 and S6 compared to ascorbic acid. Where 1, 2, 3 represent IC50 values of ascorbic acid, compounds S4 and S6 respectively

Full size image
Table 3 Antioxidant activity of newly synthesized derivatives (S1–S18)
Full size table

Anticorrosion activity

Corrosion inhibition of most of the active synthesized thiophene derivatives was evaluated by weight loss of mild steel in their presence and absence in 1.0 M HCl at 35 °C. The anticorrosion activity results of selected compound are indicated in Table 4.

Table 4 Corrosion inhibition efficiency of thiophene derivatives at 35 °C, for 12 h
Full size table

The graph of inhibition efficiency Vs concentration at 35 °C for 12 h presented in Fig. 4. The result of inhibition efficiency after 12 h at 35 °C indicated that compounds exhibited good corrosion inhibition potential. Compound S7 showed more anticorrosion efficiency (97.90%) and low corrosion rate (0.98). The anticorrosion screening results indicated that the corrosion rate decreases with increasing concentration of inhibitors.

Fig. 4
figure 4

Graph of inhibition efficiency vs conc. of inhibitors for 12 h

Full size image

Anticancer activity

The synthesized derivatives were screened for cytotoxicity by sulforhodamine-B (SRB) assay method on A-549 cell line and the results were compared with adriamycin as a standard drug. Among the screened compounds, compound S8 showed significant anticancer activity at dose 10−4 M against human lung cancer cell line (A-549) when compared with adriamycin as standard. Antitumor activity of the synthesized compound against human lung cancer cell line (A-549) was found to be improved because of the presence of electron withdrawing group (p-Br, Compound S8) on benzylidene portion. Anticancer activity of ethyl-2-(substituted benzylideneamino)-4,5,6,7-tetrahydrobenzo[b]thiophene-3-carboxylate derivatives were presented in Tables 5 and 6. The results obtained from graph drawn between tested compound and standard drug indicated in Fig. 5.

Table 5 Percentage control growth against human lung cancer cell line A-549
Full size table
Table 6 Anticancer activity of selected newly synthesized derivatives
Full size table
Fig. 5
figure 5

Graph plotted between tested compound and standard drug

Full size image

Structure activity relationship (SAR) studies

The subsequent structure activity relationship of newly synthesized ethyl-2-(substituted benzylideneamino)-4,5,6,7-tetrahydrobenzo[b]thiophene-3-carboxylate derivatives can be derived on the basis of the result obtained from antimicrobial, antioxidant, anticorrosion and anticancer testing as follows:

  1. 1.

    Antibacterial activity of the synthesized compound was enhanced due to the presence of electron withdrawing group (–Cl, Compound S1) on benzylidene portion against S. aureus, B. subtilis, E. coli and S. typhi. Because of the absence of hyper conjugation of methyl group favours the increase in activity of lead molecule. The structural variations such as chloro group at o and p position to the aromatic linkage favours the activity in positive manner.

  2. 2.

    Antifungal activity of the synthesized compound was enhanced because of the presence of electron releasing group (o-OCH3, Compound S4) in benzylidene portion against C. albicans and A. Niger.

  3. 3.

    Presence of electron releasing group (o-OCH3, Compound S4, and m-OCH3 and p-OH, Compound S6) in benzylidene portion showed the significant increase in antioxidant activity.

  4. 4.

    Anticorrosion activity was also found to be improved because of the presence of electron releasing group (m-OCH3 and p-OCH3, Compound S7).

  5. 5.

    Anticancer activity of the synthesized compound was found to be increased against human lung cancer cell line (A-549) by the addition of electron withdrawing group (p-Br, Compound S8).

  6. 6.

    From these result we may conclude that for a compound to be most effective and potent against various target it is essential to have different structural requirements.

Experimental section

Materials and methods

Reagents and solvents were of both laboratory and analytical grade which were used in the study. Hi-media Laboratories were used to obtain the Media for antimicrobial activity and all the microbial type cell cultures (MTCC) were obtained from Institute of Microbial Technology and Gene bank (IMTECH), Chandigarh. Labtech melting point equipment was used to determined melting points by an open glass capillary method. Thin layer chromatography (TLC) was used for observing the reaction steps forward making use of commercial silica gel plates (Merck), Silica gel F254 on aluminium sheets. 1H NMR spectra were screened by Bruker Avance 400 NMR spectrometer in a suitable chloroform solvent and are expressed in parts per million (δ, ppm) downfield from tetramethyl silane (internal standard). 1H NMR data are given as multiplicity (s, singlet; d, doublet; t, triplet; m, multiplet) and number of protons. Infrared (IR) spectra were recorded on a Bruker FTIR spectrometer using KBr pellet method and expressed in cm−1. The mass spectra of derived compounds were done on Waters Micromass Q-ToF Micro instrument by mass spectrometer (Fig. 6).

Fig. 6
figure 6

Images of in vitro testing of samples for anti-cancer activity screening

Full size image

General procedure for the synthesis of (ethyl-2-amino-4,5,6,7tetrahydrobenzo[b] thiophene-3-carboxylate derivatives (S1–S18)

Step I: Synthesis of ethyl-2-amino-4,5,6,7-tetrahydrobenzo[b]thiophene-3-carboxylate

Sulphur (1.92 g, 0.06 mol) was added to a mixture of ethylcyanoacetate (5.32 ml, 0.05 mol) and cyclohexanone (5.2 ml, 0.05 mol) with continuous stirring at room temperature followed by the adding up of diethylamine (5.26 ml, 0.05 mol). The reaction mixture was then stirred at 40–50 °C for 2 h. Thin layer chromatography was used for the confirmation of reaction mixture. The resulting mixture was set aside for overnight at room temperature. The precipitate obtained was filtered, washed with water, dried and recrystallized from ethanol.

Step II: Synthesis of ethyl-2-(substituted benzylideneamino)-4,5,6,7-tetrahydrobenzo[b] thiophene-3-carboxylate

Intermediate 1 (0.05 mol) and different substituted aromatic benzaldehyde (0.05 mol) in 15 ml of dioxane and triethylamine (0.005 mol) were taken in RBF and refluxed for 2 h. The resulting mixture was cooled and after that it was poured into crushed ice to obtain precipitate. Then was filtered and washed with 1% potassium bicarbonate followed by water, dried and recrystallized it from ethanol. The structures of synthesized derivatives were established by IR, NMR and mass analysis.

Analytical data

Compound S1: Ethyl-2-(2,4-dichlorobenzylideneamino)-4,5,6,7-tetrahydrobenzo[b]thio-phene-3-carboxylate: M. p: 96–97 °C; yield: 84.71%; IR (KBr pellets, cm−1): 2977 (C–H str.), 1582 (C=C str.), 1657 (C=N str.), 1698 (C=O str., carbonyl), 1267 (C–O–C str.), 665.53 (C–S–C str., thiophene ring), 822 (C–Cl str., aromatic); 1H NMR (CDCl3, δppm): 7.28–7.50 (m, 3H, Ar–H), 8.12 (s, 1H, CH=N), 4.33 (q, 2H, CH2), 1.39 (t, 3H, CH3), 2.74 (t, 2H, CH2 cyclo), 1.67 (q, 2H, CH2 cyclo); 13C NMR (75 MHz, CDCl3) d 166.74, 163.75, 123.90, 122.84, 106.61, 51.20, 39.17, 32.08, 29.37, 27.50, 22.43; MS ES + (ToF): m/z 383 [M++1].

Compound S2: Ethyl-2-(3-bromobenzylideneamino)-4,5,6,7-tetrahydrobenzo[b]thiophene-3-carboxylate: M. p: 122–124 °C; yield: 75%; IR (KBr pellets, cm−1): 2970 (C–H str.), 1555 (C=C str.), 1598 (C=N str.), 1707 (C=O str., carbonyl), 1269 (C–O–C str.), 682 (C–S–C str., thiophene ring), 592 (C–Br str., aromatic); 1H NMR (CDCl3, δppm): 7.28–7.78 (m, 3H, Ar–H), 9.99 (s, 1H, CH=N), 4.35 (q, 2H, CH2), 1.40 (t, 3H, CH3), 2.71 (t, 2H, CH2 cyclo), 1.84 (q, 2H, CH2 cyclo); 13C NMR (75 MHz, CDCl3) d 165.80, 161.50, 127.46, 121.60, 106.16, 51.20, 30.20, 29.39; MS ES + (ToF): m/z 393 [M++1].

Compound S3: Ethyl-2-(3-nitrobenzylideneamino)-4,5,6,7-tetrahydrobenzo[b]thiophene-3-carboxylate: M. p: 87–89 °C; yield: 82.22%; IR (KBr pellets, cm−1): 2988 (C–H str.), 1599 (C=C str.), 1648 (C=N str.), 1686 (C=O str., carbonyl), 1272 (C–O–C str.), 697 (C–S–C str., thiophene ring), 1550, 1352 (N–O str., aromatic); 1H NMR (CDCl3, δppm): 8.04–8.93 (m, 3H, Ar–H), 10.12 (s, 1H, CH=N), 4.36 (q, 2H, CH2), 1.31 (t, 3H, CH3), 2.57 (t, 2H, CH2 cyclo), 1.67 (q, 2H, CH2 cyclo); 13C NMR (101 MHz, CDCl3) d 165.94, 160.53, 128.07, 122.37, 106.00, 50.97, 32.04, 29.75, 29.26, 29.90; MS ES + (ToF): m/z 359.41 [M++1].

Compound S4: Ethyl-2-(2-methoxybenzylideneamino)-4,5,6,7-tetrahydrobenzo[b]thiophene-3-carboxylate: M. p: 95–98 °C; yield: 72.67%; IR (KBr pellets, cm−1): 2934 (C–H str.), 1600 (C=C str.), 1657 (C=N str.), 1693 (C=O str., carbonyl), 1252 (C–O–C str.), 647 (C–S–C str., thiophene ring), 2842 (O–CH3 str., aromatic); 1H NMR (CDCl3, δppm): 6.84–7.33 (m, 3H, Ar–H), 3.76 (s, 3H, CH3 methoxy), 9.75 (s, 1H, CH=N), 4.36 (q, 2H, CH2), 1.35 (t, 3H, CH3), 2.68 (t, 2H, CH2 cyclo), 1.84 (q, 2H, CH2 cyclo); 13C NMR (101 MHz, CDCl3) d 166.84, 161.63, 127.77, 120.37, 107.00, 50.90, 41.04, 28.75, 29.20, 28.81; MS ES + (ToF): m/z 344.4 [M++1].

Compound S5: Ethyl-2-(4-(diethylamino)benzylideneamino)-4,5,6,7-tetrahydrobenzo[b] thiophene-3-carboxylate: M. p: 98–100 °C; yield: 78.86%; IR (KBr pellets, cm−1): 2985 (C–H str.), 1575 (C=C str.), 1649 (C=N str.), 1736 (C=O str., carbonyl), 1274 (C–O–C str.), 639 (C–S–C str., thiophene ring), 1333 (C–N str., aromatic); 1H NMR (CDCl3, δppm): 6.60–7.38 (m, 3H, Ar–H), 3.94 (q, 2H, N-CH2), 1.25 (t, 3H, N-CH3), 9.05 (s, 1H, CH=N), 4.29 (q, 2H, CH2), 1.34 (t, 3H, CH3), 2.53 (t, 2H, CH2 cyclo), 1.58 (q, 2H, CH2 cyclo); 13C NMR (101 MHz, CDCl3) d 165.50, 163.45, 129.90, 106.26, 106.82, 93.93, 74.51, 50.23, 32.24, 29.48, 22.45, 20.76, 15.11; MS ES + (ToF): m/z 385.53 [M++1].

Compound S6: Ethyl-2-(4-hydroxy-3-methoxybenzylideneamino)-4,5,6,7-tetrahydrobenzo [b]thiophene-3-carboxylate: M. p: 95–97 °C; yield: 82.43%; IR (KBr pellets, cm−1): 2985 (C–H str.), 1597 (C=C str.), 1645 (C=N str.), 1792 (C=O str., carbonyl), 1273 (C–O–C str.), 632 (C–S–C str., thiophene ring), 3403 (O–H str., aromatic); 1H NMR (CDCl3, δppm): 7.02–7.05 (m, 3H, Ar–H), 3.76 (s, 3H, CH3 methoxy), 5.07 (s, 1H, OH alcohol), 9.86 (s, 1H, CH=N), 4.26 (q, 2H, CH2), 1.34 (t, 3H, CH3), 2.53 (t, 2H, CH2 cyclo), 1.78 (q, 2H, CH2 cyclo); 13C NMR (75 MHz, CDCl3) d 165.90, 164.48, 131.27, 125.37, 116.11, 54.00, 39.06, 36.17, 30.82, 39.79, 20.83, 14.26; MS ES + (ToF): m/z 362 [M++1].

Compound S7: Ethyl-2-(3,4-dimethoxybenzylideneamino)-4,5,6,7-tetrahydrobenzo[b] thiophene-3-carboxylate: M. p: 97–99 °C; yield: 83.67%; IR (KBr pellets, cm−1): 2985 (C–H str.), 1574 (C=C str.), 1597 (C=N str.), 1648 (C=O str., carbonyl), 1274 (C–O–C str.), 639 (C–S–C str., thiophene ring), 2854 (O–CH3 str., aromatic); 1H NMR (CDCl3, δppm): 6.87–7.11 (m, 3H, Ar–H), 3.73 (s, 3H, CH3 methoxy) 9.02 (s, 1H, CH=N), 4.26 (q, 2H, CH2), 1.49 (t, 3H, CH3), 2.53 (t, 2H, CH2 cyclo), 1.67 (q, 2H, CH2 cyclo); 13C NMR (101 MHz, CDCl3) d 166.48, 163.45, 129.97, 107.25, 105.80, 94.93, 75.51, 55.23, 36.24, 29.48, 22.35, 19.76, 14.11; MS ES + (ToF): m/z 374 [M++1].

Compound S8: Ethyl-2-(4-bromobenzylideneamino)-4,5,6,7-tetrahydrobenzo[b]thiophene-3-carboxylate: M. p: 94–95 °C; Yield: 87.62%; IR (KBr pellets, cm−1): 2985 (C–H str.), 1596 (C=C str.), 1646 (C=N str.), 1696 (C=O str., carbonyl), 1274 (C–O–C str.), 639 (C–S–C str., thiophene ring), 607 (C–Br str., aromatic); 1H NMR (CDCl3, δppm): 7.64–7.86 (m, 3H, Ar–H), 8.34 (s, 1H, CH=N), 4.81 (q, 2H, CH2), 1.52 (t, 3H, CH3), 2.51 (t, 2H, CH2 cyclo), 1.88 (q, 2H, CH2 cyclo); 13C NMR (75 MHz, CDCl3) d 165.80, 161.48, 127.24, 121.35, 106.08, 51.00, 32.06, 31.16, 29.95, 29.68, 29.50, 29.88, 25.83, 15.26; MS ES + (ToF): m/z 393 [M++1].

Compound S9: Ethyl-2-(2-chlorobenzylideneamino)-4,5,6,7-tetrahydrobenzo[b]thiophene-3-carboxylate: M. p: 87–88 °C; yield: 85.63%; IR (KBr pellets, cm−1): 2977 (C–H str.), 1582 (C=C str.), 1657 (C=N str.), 1698 (C=O str., carbonyl), 1267 (C–O–C str.), 665 (C–S–C str., thiophene ring), 822 (C–Cl str., aromatic); 1H NMR (CDCl3, δppm): 7.17–7.74 (m, 3H, Ar–H), 10.09 (s, 1H, CH=N), 4.48 (q, 2H, CH2), 1.32 (t, 3H, CH3), 2.40 (t, 2H, CH2 cyclo), 1.69 (q, 2H, CH2 cyclo); 13C NMR (75 MHz, CDCl3) d 166.89, 161.47, 128.27, 122.37, 107.12, 51.01, 32.06, 31.17, 29.84, 29.80, 29.78, 29.68, 29.49, 29.08, 23.83, 14.26; MS ES + (ToF): m/z 349 [M++1].

Compound S10: Ethyl-2-(2-nitrobenzylideneamino)-4,5,6,7-tetrahydrobenzo[b]thiophene-3-carboxylate: M. p: 94–97 °C; yield: 79.80%; IR (KBr pellets, cm−1): 2983 (C–H str.), 1592 (C=C str.), 1649 (C=N str.), 1699 (C=O str., carbonyl), 1268 (C–O–C str.), 696 (C–S–C str., thiophene ring), 1533, 1344 (N–O str., aromatic); 1H NMR (CDCl3, δppm): 7.64–8.24 (m, 3H, Ar–H), 10.27 (s, 1H, CH=N), 4.60 (q, 2H, CH2), 1.38 (t, 3H, CH3), 2.51 (t, 2H, CH2 cyclo), 1.63 (q, 2H, CH2 cyclo);); 13C NMR (75 MHz, CDCl3) d 166.90, 164.48, 131.27, 125.37, 116.11, 54.00, 34.06, 36.17, 30.82, 39.80, 20.93, 15.26; MS ES + (ToF): m/z 359 [M++1].

Compound S11: Ethyl-2-(4-nitrobenzylideneamino)-4,5,6,7-tetrahydrobenzo[b]thiophene-3-carboxylate: M. p: 130–131 °C; yield: 72.58; IR (KBr pellets, cm−1): 2981 (C–H str.), 1577 (C=C str.), 1593 (C=N str.), 1713 (C=O str., carbonyl), 1269 (C–O–C str.), 690 (C–S–C str., thiophene ring), 1512, 1335 (N–O str., aromatic); 1H NMR (CDCl3, δppm): 7.98–8.24 (m, 3H, Ar–H), 10.15 (s, 1H, CH=N), 4.42 (q, 2H, CH2), 1.38 (t, 3H, CH3), 2.57 (t, 2H, CH2 cyclo), 1.64 (q, 2H, CH2 cyclo);); 13C NMR (75 MHz, CDCl3) d 165.83, 164.50, 131.37, 125.37, 117.11, 56.00, 39.05, 36.17, 30.82, 39.79, 21.83, 14.36; MS ES + (ToF): m/z 359 [M++1].

Compound S12:Ethyl-2-(benzylideneamino)-4,5,6,7-tetrahydrobenzo[b]thiophene-3-carboxylate: M. p: 90–93 °C; yield: 68.72; IR (KBr pellets, cm−1): 2985 (C–H str.), 1601 (C=C str.), 1651 (C=N str.), 1736 (C=O str., carbonyl), 1272 (C–O–C str.), 741 (C–S–C str., thiophene ring); 13C NMR (75 MHz, CDCl3) d 165.97, 163.48, 130.27, 122.37, 107.11, 51.20, 33.06, 32.17, 30.82, 29.79, 29.70, 29.50, 29.88, 23.83, 15.26; MS ES + (ToF): m/z 314 [M++1].

Compound S13: Ethyl-2-(4-(dimethylamino)benzylideneamino)-4,5,6,7-tetrahydrobenzo[b] thiophene-3-carboxylate: M. p: 134–135 °C; yield: 83.23%; IR (KBr pellets, cm−1): 2977 (C–H str.), 1582 (C=C str.), 1657 (C=N str.), 1698 (C=O str., carbonyl), 1267 (C–O–C str.), 665.53 (C–S–C str., thiophene ring), 822 (C–Cl str., aromatic); 1H NMR (CDCl3, δppm): 6.67–7.44 (m, 3H, Ar–H), 2.89 (s, 3H, N-CH3), 10.10 (s, 1H, CH=N), 4.74 (q, 2H, CH2), 1.38 (t, 3H, CH3), 2.64 (t, 2H, CH2 cyclo), 1.74 (q, 2H, CH2 cyclo); 13C NMR (75 MHz, CDCl3) d 166.89, 162.48, 137.27, 122.37, 106.00, 52.00, 33.06, 31.18, 30.82, 29.89, 29.78, 29.59, 29.88, 21.83, 14.36; MS ES + (ToF): m/z 358 [M++1].

Compound S14: Ethyl-2-(4-hydroxybenzylideneamino)-4,5,6,7-tetrahydrobenzo[b]thiophene-3-carboxylate: M. p: 110–111 °C; Yield: 73.65%; IR (KBr pellets, cm−1): 2977 (C–H str.), 1582 (C=C str.), 1657 (C=N str.), 1698 (C=O str., carbonyl), 1267 (C–O–C str.), 665.53 (C–S–C str., thiophene ring), 822 (C–Cl str., aromatic); 13C NMR (75 MHz, CDCl3) d 165.98, 161.40, 127.24, 121.35, 106.08, 50.00, 32.66, 33.16, 29.82, 29.68, 29.50, 29.08, 21.83, 15.26; MS ES + (ToF): m/z 331.5 [M++1].

Compound S15: Ethyl 2-(2-hydroxybenzylideneamino)-4,5,6,7-tetrahydrobenzo[b]thiophene-3-carboxylate: M. p: 108–109 °C; yield: 81.35%; IR (KBr pellets, cm−1): 2977 (C–H str.), 1582 (C=C str.), 1657 (C=N str.), 1698 (C=O str., carbonyl), 1267 (C–O–C str.), 665.53 (C–S–C str., thiophene ring), 822 (C–Cl str., aromatic); 13C NMR (75 MHz, CDCl3) d 165.00, 162.48, 128.24, 121.35, 106.08, 51.00, 32.06, 30.16, 29.82, 29.78, 29.50, 29.08, 23.83, 15.36; MS ES + (ToF): m/z 331.5 [M++1].

Compound S16: Ethyl-2-(3-hydroxybenzylideneamino)-4,5,6,7-tetrahydrobenzo[b]thiophene -3-carboxylate: M. p: 93–94 °C; yield: 69.87%; IR (KBr pellets, cm−1): 2977 (C–H str.), 1582 (C=C str.), 1657 (C=N str.), 1698 (C=O str., carbonyl), 1267 (C–O–C str.), 665.53 (C–S–C str., thiophene ring), 822 (C–Cl str., aromatic); 13C NMR (75 MHz, CDCl3) d 165.89, 160.47, 127.27, 121.47, 106.12, 51.01, 32.06, 31.17, 29.84, 29.80, 29.88, 29.60, 29.49, 29.07, 22.84, 14.28; MS ES + (ToF): m/z 331.5 [M++1].

Compound S17: Ethyl2-(4-methoxybenzylideneamino)-4,5,6,7-tetrahydrobenzo[b]thiophene-3-carboxylate: M. p: 93–95 °C; yield: 81.81%; IR (KBr pellets, cm−1): 2977 (C–H str.), 1582 (C=C str.), 1657 (C=N str.), 1698 (C=O str., carbonyl), 1267 (C–O–C str.), 665.53 (C–S–C str., thiophene ring), 822 (C–Cl str., aromatic); 13C NMR (75 MHz, CDCl3) d 166.89, 161.48, 127.20, 121.37, 106.11, 51.00, 32.06, 31.17, 29.80, 29.79, 29.68, 29.48, 29.08, 22.80, 14.29; MS ES + (ToF): m/z 344.4 [M++1].

Compound S18: Ethyl-2-(2,4-dimethoxybenzylideneamino)-4,5,6,7-tetrahydrobenzo[b] thiophene-3-carboxylate: M. p: 97–99 °C; yield: 76.69%; IR (KBr pellets, cm−1): 2977 (C–H str.), 1582 (C=C str.), 1657 (C=N str.), 1698 (C=O str., carbonyl), 1267 (C–O–C str.), 665.53 (C–S–C str., thiophene ring), 822 (C–Cl str., aromatic); 13C NMR (75 MHz, CDCl3) d 165.70, 161.57, 127.27, 121.37, 107.12, 52.01, 33.06, 31.18, 29.83, 29.81, 29.78, 29.58, 29.49, 29.09, 23.83, 14.27; MS ES + (ToF): m/z 374 [M++1].

Evaluation of antimicrobial activity

Newly derived derivatives were screened for in vitro antimicrobial activity against Gram positive bacteria: Bacillus subtilis (MTCC 441), Staphylococcus aureus (MTCC 3160), Gram negative bacteria Salmonella typhi (MTCC 3216), Escherichia coli (MTCC 443) and fungal strains Candida albicans (MTCC 227) and Aspergillus niger (MTCC 281) by tube dilution method [18] and results were compared with fluconazole (antifungal) and cefadroxil (antibacterial) as standard drugs. The stock solutions were prepared in DMSO having 100 µg/ml concentrations for standard and test drugs. Fresh pure cultures were used to prepare the bacterial and fungal inoculums. In the test-tubes containing serial dilutions (50, 25, 12.5, 6.25 and 3.12 µg/ml) of test and standard compounds in nutrient broth and Sabouraud dextrose broth, 100 µl of inoculum was added. After that it was incubated at 37 ± 1 °C for 24 h (bacteria), at 25 ± 1 °C for 7 days (A. niger) and at 37 ± 1 °C for 48 h (C. albicans). Antimicrobial screening results were recorded in terms of lowest concentration of test substances which inhibited the growth of microorganisms i.e. MIC.

Evaluation of antioxidant activity by DPPH (1,1-diphenyl-2-picrylhydrazyl) method

Antioxidant activity was evaluated spectrophotometrically by using free radical scavenging method i.e. DPPH assay. The DPPH is a stable free radical and it reacts with hydrogen donors showing the reduction to its corresponding hydrazine with the maximum absorption at 517 nm. Colour of DPPH changes from violet to yellow, indicating the reaction of DPPH with an antioxidant agent as it can donate hydrogen to get reduced with a considerable decrease in absorption at 517 nm. DPPH solution (3 µg/ml) was prepared in methanol. The solution of methanol and DPPH (1:1) was used for blank reference. Four dilutions of different concentrations (25 µg/ml, 50 µg/ml, 75 µg/ml, 100 µg/ml) of each synthesized compound and standard (ascorbic acid) were prepared in the methanol and 1 ml of each concentration was added to 1 ml of DPPH solution. The solution mixture was placed in dark place for 30 min at room temperature after vigorous shaking and their absorbance was measured by UV at 517 nm [19]. Percentage (%) inhibition of free radical DPPH was calculated as follows:

$$\% {\text{ Inhibition }} = \frac{{{\text{A}}_{\text{Blank}} {-}{\text{ A}}_{\text{Sample}} }}{{{\text{A}}_{\text{Blank}} }} \times 100$$
(1)

where, ABlank = absorbance of the blank reaction, ASample  = absorbance of the test compounds.

IC50 value was calculated from the graph plotted between % inhibition and synthesized derivatives. The antioxidant screening results indicated that few synthesized compound exhibited significant antioxidant activity while other showed good to moderate antioxidant activity. In them, compound S4 and S6 exhibited excellent antioxidant activity and found to have IC50 value and (%) inhibition comparable to ascorbic acid.

Evaluation of anticorrosion activity by gravimetric method (weight loss)

The corrosion medium was made by dilution of 1.0 M HCl of analytical grade 37% with double distilled water. The test solutions were made up by dilution of 0.5 g thiophene derivatives with 250 ml of 1.0 M HCl solution to make 200 ppm solution (stock solution). The desired concentrations (50, 100, 200, 300 ppm) of thiophene derivatives were prepared from stock solution of 200 ppm.

The gravimetric method is commonly used corrosion monitoring method. The weight loss study has been carried out in 1.0 M HCl solution. Firstly, all mild steel samples were cut into 1 × 3 cm then scrapped with emery paper of different grade (100–1000) and rinsed with distilled water, acetone and after all dried between filter paper and weighed. The weight loss study was carried out in 25 ml of 1.0 M HCl solution filled in 50 ml beaker in presence and absence of various concentration of corrosion inhibitor for 12 h at 35 °C. After the immersion of sample in test solution, specimens were put outside and washed with double distilled water and acetone, dried and weighed again. This process of weight loss study was carried out in triplicate and the average weight loss was calculated. The % corrosion inhibition efficiency and surface coverage were calculated by following equations.

$$\eta w = \frac{Wo - W}{Wo} \times 100$$
(2)
$$\theta = \frac{Wo - W}{Wo}$$
(3)

where, Wo = weight loss value of mild steel in absence of test compound, W = weight loss value of mild steel in presence of test compound.

The equation used for calculation of corrosion rate of mild steel was given by:

$${\text{CR}}\,\left( {{\text{mmy}} - 1} \right) = \frac{{87.6 \times {\text{W}}}}{\text{AtD}}$$
(4)

where, W = weight loss of mild steel (mg), A = area of sample (cm2), t = exposure time (h) and D = density of mild steel (g cm−3).

Evaluation of in vitro anticancer activity

Sulforhodamine-B (SRB) assay method was used for screening in vitro cytotoxicity of the synthesized thiophene derivative on A-549 (human lung cancer cell line) [20]. Its mechanism is focused on the capability to bind electrostatically, the protein dye sulforhodamine B and pH dependent on protein basic amino acid residues of trichloroacetic acid-fixed cells. Anticancer screening results were shown as GI50 (concentration of drug causing 50% inhibition of cell growth) and the results were matched with the standard anticancer drug i.e. adriamycin.

The entire synthesized compounds which have to be screened initially at dose (10−7 to 10−4) were submitted at anti-cancer screening facility (ACDSF) at ACTREC Mumbai (Tata Memorial Centre).

Conclusion

Summarizing, we may conclude that novel thiophene derivatives were synthesized with different donor or acceptor groups on aromatic rings. Compound S1 showed the highest activity against various Gram positive and Gram negative bacterial strains. The di-halogen substituted derivatives were found to have significant antibacterial activities, particularly the compounds having two substituted chloro groups at the same ring. Compound S4 emerged as the most potent antifungal agent because of the presence of o-OCH3 group. Compound S4 and S6 exhibited excellent antioxidant activity and compound S7 showed more anticorrosion efficiency with low corrosion rate due to the presence of electron releasing groups on benzylidene portion which found to have higher basicity and electron donating ability which act by adsorption on the metal surface and block the active surface sites, thus reducing the corrosion rate. Compound S8 possessed significant cytotoxicity GI50 = 8E−04 molar against human lung cancer cell line (A-549). The results of anticancer screening indicated that the synthesized compounds having electron withdrawing groups (p-Br) on benzylidene portion found to possessed significant activity. So, these thiophene derivatives unquestionably grasp a greater assure in discovering as lead compound for the progress of novel therapeutic agents.