Introduction

Cancer remains one of the most difficult life-threatening diseases to treat, although a lot of progress has been made in chemotherapy research in recent years. Therefore, the discovery of more efficacious and safer anticancer therapeutic agents is one of the fundamental goals in medicinal chemistry research. The pharmacoactive agents containing pyrazole scaffolds have received considerable attention owing to their anticancer activities (Abdou et al., 2004; Deverakonda et al., 2013; Maggio et al., 2014). Pyrazole motif makes up the core structure of numerous biologically active compounds (Elguero et al., 2002) including blockbuster drugs such as Celebrex (Penning et al., 1997) and Viagra (Terrett et al., 1996). Among the pyrazoles, particularly the indenopyrazoles have received extensive awareness in recent years due to their synthetic significance and broad range of biological activities (Lapenna and Giordano, 2009; Mohil et al., 2014; Usui et al., 2008; Yue et al., 2004). For instance, indeno[1,2-c]pyrazole-2-carboxamide/carbothioamide have recently been recognized as novel antitubercular analogues (Ahsan et al., 2011) and indeno[1,2-c]pyrazol-4-ones as potent and selective cyclin-dependent kinase (CDK) (Nugiel et al., 2002) and check point kinase (CHK1) inhibitors (Tao et al., 2005, 2007) showing admirable activity against various tumor cell linings. Indeno[1,2-c]pyrazoles are also known to act as potential CNS agents (Lemke et al., 1978), tyrosine kinase inhibitors (Yan et al., 2006), antidepressants (Flores and Loev, 1961; Loev and Mosher, 1961) and nonsteroidal anti-inflammatory drugs (Lemke et al., 1989). Consequently, various approaches have been developed for the synthesis of indenopyrazoles (Hamilton, 1976; Lemke and Sawhney, 1982; Minegishi et al., 2013; Mosher and Soeder, 1971; Usui et al., 2008). Similarly, thiazole derivatives also constitute a fascinating class of organic compounds as they have been endowed with ample number of pharmacological activities like anticancer (Gu and Jin, 2012; Kim et al., 2002; Misra et al., 2004), anticonvulsant (Arshad et al., 2014), analgesic (Thore et al., 2013), antimicrobial (Gaikwad et al., 2013), anti-inflammatory (Bhosale et al., 2012), antipyretic (Pignatello et al., 1991), antitubercular (Samadhiya et al., 2013), anti-HIV (Rawal et al., 2008), antioxidant (Shih et al., 2007), diuretic (Andreani et al., 1987), anti-allergic (Hargrave et al., 1983), antihypertensive (Patt et al., 1992), etc.

The precedent for broad bioactivity profiles for indenopyrazole and thiazole pharmacophores led us to perceive that fusion of these motifs might result in new bioactive molecules with interesting biological activities. For instance, thiazole-linked pyrazoles are endowed with anticancer (Altıntop et al., 2014; Chong and Duvadie, 2003; Dawood et al., 2013; Wang et al., 2013), anti-inflammatory (Bekhit et al., 2008; Farghaly et al., 2000), antimicrobial (Bekhit et al., 2008; Farghaly et al., 2000; Gaikwad et al., 2013; Mor et al., 2012a; Karale et al., 2015; Song et al., 2014) activities, etc. In addition, thiazole tethered indenopyrazoles have been recognized as cyclin-dependent kinase (CDK) inhibitors (Rostom, 2006; Yue et al., 2004). The above facts prompted us to direct our study toward the synthesis of new structural entities combining the indenopyrazole and thiazole scaffolds together in a molecular framework with a goal to explore their anticancer activity.

Further, Quantitative Structure Activity Relationship (QSAR) is a thriving technique that has gained considerable attention during the past decades because of good success in medicinal chemistry research, proteomics, metabolomics and bioinformatics (Tong et al., 2014). QSAR is a multivariate, mathematical relationship between a set of physicochemical properties and biological activity (Suresh, 2013) and has been successfully employed for drug design and prediction of drug activity (Sigroha et al., 2012).

In this perspective and in a quest for synthesizing biologically active nitrogen and sulfur heterocycles (Mor et al., 2012a, b, c), herein, we report a convenient synthesis, in vitro anticancer evaluation and QSAR studies of twenty-four new thiazole tethered indenopyrazoles (3a–3l & 4a–4l).

Materials and methods

Chemistry

All reagents were used as received from commercial suppliers without any additional purification. Melting points (mp, °C) of the synthesized compounds were determined in open capillaries on an Electrothermal Melting Point apparatus, LABCO Co, India, and are uncorrected. The FTIR spectra were recorded in KBr on IR affinity-1 FTIR (Shimadzu) spectrophotometer, and wave numbers (ν) are reported in cm−1. 1H NMR, 13C NMR, DEPT (distortionless enhancement by polarization transfer), and 2D-NMR [COSY (correlation spectroscopy), HSQC (heteronuclear single-quantum coherence) and HMBC (heteronuclear multiple bond correlation)] spectra were scanned on Bruker AVANCE II NMR spectrometer operating at 300/400 MHz using tetramethylsilane (TMS) as internal standard. Chemical shift (δ) values are given in parts per million (ppm), and coupling constants (J) are expressed in Hertz (Hz). Mass spectra were recorded on Waters Quadrupole Detector (TDQ) by electron spray ionization (ESI) technique in positive mode. Elemental analyses were performed on Thermo Scientific FLASH-2000 CHN analyser. Analytical results for C, H and N were found to be within ±0.4 % of the theoretical values. The purity of synthesized compounds was tested using precoated TLC plates (SIL G/UV554, ALUGRAM), and visualization was achieved via UV light.

General procedure for the synthesis of indeno[1,2-c]pyrazol-4(1H)-ones ( 3 )

The 2-acyl-(1H)-indene-1,3(2H)-diones (1) needed for the purpose were prepared by the Claisen condensation of diethylphthlate and appropriate ketone under the influence of sodium methoxide according to the procedure as described in literature (Dhawan et al., 1993; Shapiro et al., 1960). An equimolar mixture of 2-acyl-(1H)-indene-1,3(2H)-diones (1, 5 mmol) and thiosemicarbazide (5 mmol) in dry methanol (30 mL) was refluxed on water bath at 65 °C for 10–15 min. Thereafter, appropriate phenacyl bromide (2, 5 mmol), sodium acetate (0.41 g, 5 mmol) and glacial acetic acid (15 mL) were added to the above solution, and the reaction mixture was refluxed on a water bath at 80–85 °C for 6–7 h. Thereafter, the reaction mixture was concentrated and the solid thus obtained was separated by filtration and recrystallized from chloroform to furnish the target thiazole tethered indeno[1,2-c]pyrazol-4(1H)-ones (3a–3l) in high yields. The physical and spectral data of 3a–3l are described as follows:

3-Isobutyl-1-(4-phenylthiazol-2-yl)indeno[1,2-c]pyrazol-4(1H)-one ( 3a )

Yellowish green crystals; yield 81 %; mp 172–176 °C; IR (KBr): νmax 741, 1080, 1383, 1487, 1528, 1597 (C = N), 1703 (C = O), 2882, 2949 (aliphatic C–H stretch), 3078, 3111 (aromatic C–H stretch) cm−1; 1H NMR (300 MHz, CDCl3): δ = 1.02 (d, 6H, J = 6.6 Hz, –CH2CH(CH 3 )2), 2.25–2.34 (m, 1H, –CH2CH(CH3)2), 2.65 (d, 2H, J = 7.20 Hz, –CH 2 CH(CH3)2), 7.32–7.53 (m, 6H, H-6, H-7, H-5′, H-3″, H-4″, H-5″), 7.59 (d, 1H, J = 7.20 Hz, H-5), 7.93 (d, 2H, J = 8.40 Hz, H-2″, H-6″), 8.48 (d, 1H, J = 7.50 Hz, H-8); 13C NMR (100 MHz, CDCl3): δ = 22.46 (–CH2CH(CH 3 )2), 27.83 (–CH2 CH(CH3)2), 36.31 (–CH 2 CH(CH3)2), 110.75 (C-5′), 123.41 (C-3a), 123.84 (C-8), 124.22 (C-5), 126.10 (C-3″, C-5″), 128.57 (C-4″), 128.98 (C-2″, C-6″), 130.48 (C-6), 132.78 (C-1″), 133.42 (C-7), 134.05 (C-4a), 140.26 (C-8a), 152.42 (C-3), 153.22 (C-4′), 157.72 (C-8b), 160.03 (C-2′), 184.00 (C-4); ESI–MS m/z: 386.1 [M + 1]+; Anal. Calcd. for C23H19N3OS (385.12): C, 71.66; H, 4.97; N, 10.90. Found: C, 71.85; H, 5.17; N, 10.69.

1-(4-(4-Chlorophenyl)thiazol-2-yl)-3-isobutylindeno[1,2-c]pyrazol-4(1H)-one ( 3b )

Green crystals; yield 89 %; mp 184–189 °C; IR (KBr): νmax 742, 1092, 1387, 1481, 1528, 1597 (C = N), 1707 (C = O), 2872, 2957 (aliphatic C–H stretch), 3063, 3100 (aromatic C–H stretch) cm−1; 1H NMR (300 MHz, CDCl3): δ = 1.01 (d, 6H, J = 6.60 Hz, –CH2CH(CH 3 )2), 2.24–2.34 (m, 1H, –CH2CH(CH3)2), 2.64 (d, 2H, J = 7.50 Hz, –CH 2 CH(CH3)2), 7.31–7.49 (m, 5H, H-6, H-7, H-5′, H-3″, H-5″), 7.59 (d, 1H, J = 7.20 Hz, H-5), 7.84 (d, 2H, J = 8.40 Hz, H-2″, H-6″), 8.39 (d, 1H, J = 7.50 Hz, H-8); 13C NMR (100 MHz, CDCl3): δ = 22.47 (–CH2CH(CH 3 )2), 27.83 (–CH2 CH(CH3)2), 36.31 (–CH 2 CH(CH3)2), 111.01 (C-5′), 123.53 (C-3a), 123.63 (C-8), 124.30 (C-5), 127.32 (C-2″, C-6″), 129.17 (C-3″, C-5″), 130.54 (C-6), 132.52 (C-1″), 132.72 (C-4″), 133.31 (C-7), 134.42 (C-4a), 140.25 (C-8a), 152.06 (C-3), 152.54 (C-4′), 157.71 (C-8b), 160.29 (C-2′), 183.90 (C-4); ESI–MS m/z: 419.1 [M]+, 420.3 [M + 1]+; Anal. Calcd. for C23H18ClN3OS (419.09): C, 65.78; H, 4.32; N, 10.01. Found: C, 65.42; H, 4.58; N, 9.88.

3-Isobutyl-1-(4-p-tolylthiazol-2-yl)indeno[1,2-c]pyrazol-4(1H)-one ( 3c )

Greenish yellow crystals; yield 87 %; mp 158–164 °C; IR (KBr): νmax 741, 1086, 1385, 1493, 1529, 1601 (C = N), 1713 (C = O), 2949 (aliphatic C–H stretch), 3111 (aromatic C–H stretch) cm−1; 1H NMR (400 MHz, CDCl3): δ = 1.01 (d, 6H, J = 6.64 Hz, –CH2CH(CH 3 )2), 2.23–2.34 (m, 1H, –CH2CH(CH3)2), 2.42 (s, 3H, CH 3 ), 2.63 (d, 2H, J = 7.32 Hz, –CH 2 CH(CH3)2), 7.23–7.47 (m, 5H, H-6, H-7, H-5′, H-3″, H-5″), 7.57 (d, 1H, J = 7.20 Hz, H-5), 7.79 (d, 2H, J = 7.96 Hz, H-2″, H-6″), 8.44 (d, 1H, J = 7.36 Hz, H-8); 13C NMR (100 MHz, CDCl3): δ = 21.35 (CH 3 ), 22.50 (–CH2CH(CH 3 )2), 27.88 (–CH2 CH(CH3)2), 36.36 (–CH 2 CH(CH3)2), 109.92 (C-5′), 123.42 (C-3a), 123.90 (C-8), 124.25 (C-5), 126.05 (C-3″, C-5″), 129.68 (C-2″, C-6″), 130.47 (C-6), 131.41 (C-1″), 132.86 (C-4a), 133.40 (C-7), 138.57 (C-4″), 140.34 (C-8a), 152.46 (C-3), 153.35 (C-4′), 157.75 (C-8b), 159.94 (C-2′), 184.08 (C-4); ESI–MS m/z: 400.43 [M + 1]+; Anal. Calcd. for C24H21N3OS (399.14): C, 72.15; H, 5.30; N, 10.52. Found: C, 72.36; H, 5.07; N, 10.78.

3-Isobutyl-1-(4-(4-methoxyphenyl)thiazol-2-yl)indeno[1,2-c]pyrazol-4(1H)-one ( 3d )

Green crystals; yield 88 %; mp 180–190 °C; IR (KBr): νmax 737, 1090, 1387, 1489, 1529, 1601 (C = N), 1707 (C = O), 2887, 2949 (aliphatic C–H stretch), 3019, 3096 (aromatic C–H stretch) cm−1; 1H NMR (400 MHz, CDCl3): δ = 1.03 (d, 6H, J = 6.60 Hz, –CH2CH(CH 3 )2), 2.28–2.35 (m, 1H, –CH2CH(CH3)2), 2.66 (d, 2H, J = 7.32 Hz, –CH 2 CH(CH3)2), 3.91 (s, 3H, OCH 3 ), 7.04 (2H, d, J = 8.72 Hz, H-3″, H-5″), 7.21 (s, 1H, H-5′), 7.34–7.51 (m, 2H, H-6, H-7), 7.60 (d, 1H, J = 7.16 Hz, H-5), 7.85 (d, 2H, J = 8.72 Hz, H-2″, H-6″), 8.46 (d, 1H, J = 7.40 Hz, H-8); 13C NMR (100 MHz, CDCl3): δ = 22.49 (–CH2CH(CH 3 )2), 27.87 (–CH2 CH(CH3)2), 36.35(–CH 2 CH(CH3)2), 55.42 (OCH 3 ), 108.91 (C-5′), 114.34 (C-3″, C-5″), 123.40 (C-3a), 123.84 (C-8), 124.26 (C-5), 127.00 (C-1″), 127.44 (C-2″, C-6″), 130.46 (C-6), 132.86 (C-4a), 133.38 (C-7), 140.34 (C-8a), 152.46 (C-3), 153.08 (C-4′), 157.70 (C-8b), 159.93 (2C, C-2′ & C-4″), 184.07 (C-4); ESI–MS m/z: 416.1 [M + 1]+, 417.2 [M + 2]+, 438.6 [M + Na]+; Anal. Calcd. for C24H21N3O2S (415.14): C, 69.37; H, 5.09; N, 10.11. Found: C, 69.52; H, 5.37; N, 10.42.

1-(4-Phenylthiazol-2-yl)-3-propylindeno[1,2-c]pyrazol-4(1H)-one ( 3e )

Yellow solid; yield 80 %; mp 120–124 °C; IR (KBr): νmax 739, 1088, 1474, 1535, 1605 (C = N), 1705 (C = O), 2853, 2959 (aliphatic C–H stretch), 3061, 3134 (aromatic C–H stretch) cm−1; 1H NMR (400 MHz, CDCl3): δ = 1.03 (t, 3H, –CH2CH2CH 3 ), 1.85–1.90 (m, 2H, –CH2CH 2 CH3), 2.73 (t, 2H, –CH 2 CH2CH3), 7.32–7.51 (m, 6H, H-6, H-7, H-5′, H-3″, H-4″, H-5″), 7.58 (d, 1H, J = 7.16 Hz, H-5), 7.91 (d, 2H, J = 8.40 Hz, H-2″, H-6″), 8.45 (d, 1H, J = 7.40 Hz, H-8); 13C NMR (100 MHz, CDCl3): δ = 13.90 (–CH2CH2 CH 3 ), 21.44 (–CH2 CH 2 CH3), 29.50 (–CH 2 CH2CH3), 110.70 (C-5′), 123.22 (C-3a), 123.89 (C-8), 124.29 (C-5), 126.15 (C-3″, C-5″), 128.61 (C-4″), 129.01 (C-2″, C-6″), 130.52 (C-6), 132.85 (C-1″), 133.43 (C-7), 134.09 (C-4a), 140.30 (C-8a), 153.23 (C-3), 153.28 (C-4′), 157.88 (C-8b), 160.07 (C-2′), 184.04 (C-4); ESI–MS m/z: 372.58 [M + 1]+; Anal. Calcd. for C22H17N3OS (371.11): C, 71.14; H, 4.61; N, 11.31. Found: C, 71.38; H, 4.92; N, 11.63.

1-(4-(4-Chlorophenyl)thiazol-2-yl)-3-propylindeno[1,2-c]pyrazol-4(1H)-one ( 3f )

Green crystals; yield 83 %; mp 185–188 °C; IR (KBr): νmax 746, 1094, 1474, 1541, 1603 (C = N), 1701 (C = O), 2849, 2957 (aliphatic C–H stretch), 3090 (aromatic C–H stretch) cm−1; 1H NMR (400 MHz, CDCl3): δ = 1.03 (t, 3H, –CH2CH2CH 3 ), 1.80–1.91 (m, 2H, –CH2CH 2 CH3), 2.72 (t, 2H, –CH 2 CH2CH3), 7.25–7.45 (5H, m, H-6, H-7, H-5′, H-3″, H-5″), 7.57 (d, 1H, J = 7.12 Hz, H-5), 7.81 (d, 2H, J = 8.36 Hz, H-2″, H-6″), 8.35 (d, 1H, J = 7.36 Hz, H-8); 13C NMR (100 MHz, CDCl3): δ = 13.86 (–CH2CH2 CH 3 ), 21.38 (–CH2 CH 2 CH3), 29.48 (–CH 2 CH2CH3), 111.00 (C-5′), 123.33 (C-3a), 123.65 (C-8), 124.32 (C-5), 127.35 (C-2″, C-6″), 129.19 (C-3″, C-5″), 130.56 (C-6), 132.57 (C-1″), 132.78 (C-4″), 133.32 (C-7), 134.47 (C-4a), 140.28 (C-8a), 152.12 (C-3), 153.29 (C-4′), 157.85 (C-8b), 160.32 (C-2′), 183.86 (C-4); ESI–MS m/z: 406.66 [M + 1]+, 407.68 [M + 2]+, 408.72 [M + 3]+; Anal. Calcd. for C22H16ClN3OS (405.07): C, 65.10; H, 3.97; N, 10.35. Found: C, 65.37; H, 4.23; N, 10.62.

3-Propyl-1-(4-p-tolylthiazol-2-yl)indeno[1,2-c]pyrazol-4(1H)-one ( 3g )

Yellow solid; yield 84 %; mp 175–177 °C; IR (KBr): νmax 739, 1092, 1474, 1541, 1605 (C = N), 1705 (C = O), 2872, 2963 (aliphatic C–H stretch), 3026, 3119 (aromatic C–H stretch) cm−1; 1H NMR (400 MHz, CDCl3): δ = 1.03 (t, 3H, –CH2CH2CH 3 ), 1.82–1.91 (m, 2H, –CH2CH 2 CH3), 2.42 (s, 3H, CH 3 ), 2.72 (t, 2H, –CH 2 CH2CH3), 7.25–7.47 (5H, m, H-6, H-7, H-5′, H-3″, H-5″), 7.57 (d, 1H, J = 7.16 Hz, H-5), 7.79 (d, 2H, J = 8.08 Hz, H-2″, H-6″), 8.44 (d, 1H, J = 7.40 Hz, H-8); 13C NMR (100 MHz, CDCl3): δ = 13.90 (–CH2CH2 CH 3 ), 21.35 (CH 3 ), 21.44 (–CH2 CH 2 CH3), 29.49 (–CH 2 CH2CH3), 109.90 (C-5′), 123.15 (C-3a), 123.92 (C-8), 124.24 (C-5), 126.04 (C-3″, C-5″), 129.67 (C-2″, C-6″), 130.48 (C-6), 131.38 (C-1″), 132.85 (C-4a), 133.39 (C-7), 138.56 (C-4″), 140.30 (C-8a), 153.17 (C-3), 153.34 (C-4′), 157.82 (C-8b), 159.89 (C-2′), 184.05 (C-4); ESI–MS m/z: 386.70 [M + 1]+, 408.32 [M + Na]+; Anal. Calcd. for C23H19N3OS (385.12): C, 71.66; H, 4.97; N, 10.90. Found: C, 71.89; H, 4.71; N, 10.68.

1-(4-(4-Methoxyphenyl)thiazol-2-yl)-3-propylindeno[1,2-c]pyrazol-4(1H)-one ( 3h )

Yellow solid; yield 81 %; mp 184–186 °C; IR (KBr): νmax 760, 1090, 1475, 1541, 1610 (C = N), 1703 (C = O), 2868, 2961 (aliphatic C–H stretch), 3030, 3115 (aromatic C–H stretch) cm−1; 1H NMR (400 MHz, CDCl3): δ = 1.03 (t, 3H, –CH2CH2CH 3 ), 1.83–1.92 (m, 2H, –CH2CH 2 CH3), 2.73 (t, 2H, –CH 2 CH2CH3), 3.88 (s, 3H, OCH 3 ), 7.01 (2H, d, J = 8.72 Hz, H-3″, H-5″), 7.18 (s, 1H, H-5′), 7.32–7.48 (m, 2H, H-6, H-7), 7.58 (d, 1H, J = 7.16 Hz, H-5), 7.84 (d, 2H, J = 8.68 Hz, H-2″, H-6″), 8.44 (d, 1H, J = 7.40 Hz, H-8); 13C NMR (100 MHz, CDCl3): δ = 13.90 (–CH2CH2 CH 3 ), 21.44 (–CH2 CH 2 CH3), 29.50 (–CH 2 CH2CH3), 55.42 (OCH 3 ), 108.90 (C-5′), 114.35 (C-3″, C-5″), 123.16 (C-3a), 123.86 (C-8), 124.27 (C-5), 127.01 (C-1″), 127.45 (C-2″, C-6″), 130.48 (C-6), 132.89 (C-4a), 133.39 (C-7), 140.34 (C-8a), 153.10 (C-3), 153.19 (C-4′), 157.80 (C-8b), 159.90 (C-2′), 159.94 (C-4″), 184.05 (C-4); ESI–MS m/z: 402.73 [M + 1]+; Anal. Calcd. for C23H19N3O2S (401.12): C, 68.81; H, 4.77; N, 10.47. Found: C, 68.58; H, 4.93; N, 10.12.

3-Isopropyl-1-(4-phenylthiazol-2-yl)indeno[1,2-c]pyrazol-4(1H)-one ( 3i )

Yellow solid; yield 78 %; mp 140–144 °C; IR (KBr): νmax 744, 878, 1094, 1475, 1543, 1605 (C = N), 1703 (C = O), 2880, 2968 (aliphatic C–H stretch), 3069, 3116 (aromatic C–H stretch) cm−1; 1H NMR (400 MHz, CDCl3): δ = 1.41 (d, 6H, J = 6.92 Hz, –CH(CH 3 )2), 3.06–3.13 (m, 1H, –CH(CH3)2), 7.32–7.51 (6H, m, H-6, H-7, H-3″, H-4″, H-5″, H-5′), 7.58 (d, 1H, J = 7.24 Hz, H-5), 7.91 (d, 2H, J = 8.00 Hz, H-2″, H-6″), 8.46 (d, 1H, J = 7.32 Hz, H-8); 13C NMR (100 MHz, CDCl3): δ = 21.28 (–CH(CH 3 )2), 28.14 (–CH(CH3)2), 110.68 (C-5′), 122.32 (C-3a), 123.84 (C-8), 124.26 (C-5), 126.14 (C-3″, C-5″), 128.59 (C-4″), 129.01 (C-2″, C-6″), 130.50 (C-6), 132.94 (C-1″), 133.38 (C-7), 134.12 (C-4a), 140.29 (C-8a), 153.25 (C-4′), 158.22 (C-8b), 159.10 (C-3), 160.14 (C-2′), 183.75 (C-4); ESI–MS m/z: 372.66 [M + 1]+, 373.64 [M + 2]+, 374.64 [M + 3]+; Anal. Calcd. for C22H17N3OS (371.11): C, 71.14; H, 4.61; N, 11.31. Found: C, 71.35; H, 4.97; N, 11.60.

1-(4-(4-Chlorophenyl)thiazol-2-yl)-3-isopropylindeno[1,2-c]pyrazol-4(1H)-one ( 3j )

Yellow solid; yield 75 %; mp 192–194 °C; IR (KBr): νmax 752, 878, 1094, 1472, 1545, 1603 (C = N), 1701 (C = O), 2880, 2970 (aliphatic C–H stretch), 3098 (aromatic C–H stretch) cm−1; 1H NMR (400 MHz, CDCl3): δ = 1.41 (d, 6H, J = 6.92 Hz, –CH(CH 3 )2), 3.08–3.11 (m, 1H, –CH(CH3)2), 7.30–7.47 (5H, m, H-6, H-7, H-5′, H-3″, H-5″), 7.59 (d, 1H, J = 7.16 Hz, H-5), 7.83 (d, 2H, J = 8.44 Hz, H-2″, H-6″), 8.37 (d, 1H, J = 7.36 Hz, H-8); 13C NMR (100 MHz, CDCl3): δ = 21.25 (–CH(CH 3 )2), 28.13 (–CH(CH3)2), 111.01 (C-5′), 122.41 (C-3a), 123.63 (C-8), 124.34 (C-5), 127.36 (C-2″, C-6″), 129.20 (C-3″, C-5″), 130.58 (C-6), 132.58 (C-1″), 132.87 (C-4″), 133.32 (C-7), 134.43 (C-4a), 140.26 (C-8a), 152.08 (C-4′), 158.21 (C-8b), 159.18 (C-3), 160.38 (C-2′), 183.65 (C-4); ESI–MS m/z: 406.66 [M + 1]+, 407.63 [M + 2]+, 408.65 [M + 3]+, 428.23 [M + Na]+; Anal. Calcd. for C22H16ClN3OS (405.07): C, 65.10; H, 3.97; N, 10.35. Found: C, 65.32; H, 4.25; N, 10.66.

3-Isopropyl-1-(4-p-tolylthiazol-2-yl)indeno[1,2-c]pyrazol-4(1H)-one ( 3k )

Green solid; yield 76 %; mp 160–164 °C; IR (KBr): νmax 741, 878, 1095, 1475, 1543, 1605 (C = N), 1711 (C = O), 2872, 2966 (aliphatic C–H stretch), 3067–3117 (aromatic C–H stretch) cm−1; 1H NMR (400 MHz, CDCl3): δ = 1.41 (d, 6H, J = 6.92 Hz, –CH(CH 3 )2), 2.42 (s, 3H, CH 3 ), 3.05–3.12 (m, 1H, –CH(CH3)2), 7.25–7.46 (5H, m, H-6, H-7, H-5′, H-3″, H-5″), 7.57 (d, 1H, J = 7.16 Hz, H-5), 7.79 (d, 2H, J = 8.08 Hz, H-2″, H-6″), 8.45 (d, 1H, J = 7.40 Hz, H-8); 13C NMR (100 MHz, CDCl3): δ = 21.28 (–CH(CH 3 )2), 21.34 (CH 3 ), 28.14 (–CH(CH3)2), 109.87 (C-5′), 122.27 (C-3a), 123.87 (C-8), 124.21 (C-5), 126.05 (C-3″, C-5″), 129.67 (C-2″, C-6″), 130.45 (C-6), 131.43 (C-1″), 132.98 (C-4a), 133.33 (C-7), 138.54 (C-4″), 140.32 (C-8a), 153.34 (C-4′), 158.17(C-8b), 159.04 (C-3), 160.00 (C-2′), 183.73 (C-4); ESI–MS m/z: 386.73 [M + 1]+; Anal. Calcd. for C23H19N3OS (385.12): C, 71.66; H, 4.97; N, 10.90. Found: C, 71.92; H, 4.68; N, 10.59.

3-Isopropyl-1-(4-(4-methoxyphenyl)thiazol-2-yl)indeno[1,2-c]pyrazol-4(1H)-one ( 3l )

Green crystals; yield 77 %; mp 188–190 °C; IR (KBr): νmax 746, 876, 1101, 1481, 1541, 1609 (C = N), 1705 (C = O), 2970 (aliphatic C–H stretch), 3087 (aromatic C–H stretch) cm−1; 1H NMR (400 MHz, CDCl3): δ = 1.41 (d, 6H, J = 6.96 Hz, –CH(CH 3 )2), 3.05–3.12 (m, 1H, –CH(CH3)2), 3.88 (s, 3H, OCH 3 ), 7.00 (d, 2H, J = 8.76 Hz, H-3″, H-5″), 7.16 (s, 1H, H-5′), 7.30–7.47 (m, 2H, H-6, H-7), 7.57 (d, 1H, J = 7.20 Hz, H-5), 7.83 (d, 2H, J = 8.76 Hz, H-2″, H-6″), 8.43 (d, 1H, J = 7.40 Hz, H-8); 13C NMR (100 MHz, CDCl3): δ = 21.27 (–CH(CH 3 )2), 28.14 (–CH(CH3)2), 55.42 (OCH 3 ), 108.85 (C-5′), 114.34 (C-3″, C-5″), 122.26 (C-3a), 123.81 (C-8), 124.22 (C-5), 127.03 (C-1″), 127.43 (C-2″, C-6″), 130.44 (C-6), 132.98 (C-4a), 133.31 (C-7), 140.33 (C-8a), 153.06 (C-4′), 158.13 (C-8b), 159.03 (C-3), 159.93 (C-4″), 159.98 (C-2′), 183.71 (C-4); ESI–MS m/z: 402.71 [M + 1]+, 403.78 [M + 2]+, 424.71 [M + Na]+; Anal. Calcd. for C23H19N3O2S (401.12): C, 68.81; H, 4.77; N, 10.47. Found: C, 68.63; H, 4.98; N, 10.21.

General procedure for the synthesis of indenopyrazoles ( 4 )

A solution of an appropriate indenopyrazole (3) (1 mmol), ethylene glycol (15 mL), hydrazine hydrate (1 mL) and KOH (0.5 mL) was refluxed at 197–200 °C for 5 h. Thereafter, the reaction mixture was poured in ice-cold water. The solid thus obtained was filtered, washed with water, dried and recrystallized from chloroform to afford indenopyrazoles (4a–4l) in moderate-to-good yields. The physical and spectral data of 4a–4l are given as follows:

2-(3-Isobutylindeno[1,2-c]pyrazol-1(4H)-yl)-4-phenylthiazole ( 4a )

Offwhite solid; yield 65 %; mp 116–120 °C; IR (KBr): νmax 731, 1020, 1080, 1472, 1508, 1537 (C = N), 2870, 2953 (aliphatic C–H stretch), 3055, 3127 (aromatic C–H stretch) cm−1; 1H NMR (400 MHz, CDCl3): δ = 0.98 (d, 6H, J = 6.80 Hz, –CH2CH(CH 3 )2), 2.02–2.08 (m, 1H, –CH2CH(CH3)2), 2.61 (d, 2H, J = 6.80 Hz, –CH 2 CH(CH3)2), 3.59 (s, 2H, H-4), 7.29–7.54 (m, 6H, H-6, H-7, H-5′, H-3″, H-4″, H-5″), 7.77 (d, 1H, J = 7.20 Hz, H-5), 8.00 (d, 2H, J = 7.20 Hz, H-2″, H-6″), 8.85 (d, 1H, J = 8.00 Hz, H-8); 13C NMR (100 MHz, CDCl3): δ = 22.13 (–CH2CH(CH 3 )2), 27.63 (C-4), 27.84 (–CH2 CH(CH3)2), 36.15 (–CH 2 CH(CH3)2), 110.83 (C-5′), 121.22 (C-8), 125.63 (C-7), 126.17 (C-2″, C-6″), 128.43 (C-4″), 128.76 (C-6), 128.92 (C-3a), 129.04 (C-3″, C-5″), 132.66 (C-1″), 134.78 (C-5), 138.51 (C-8a), 141.39 (C-4a), 148.59 (C-8b), 153.16 (C-4′), 154.37 (C-3), 159.65 (C-2′); ESI–MS m/z: 372.3 [M + 1]+; Anal. Calcd. for C23H21N3S (371.15): C, 74.36; H, 5.70; N, 11.31. Found: C, 74.59; H, 5.93; N, 11.03.

4-(4-Chlorophenyl)-2-(3-isobutylindeno[1,2-c]pyrazol-1(4H)-yl)thiazole ( 4b )

Offwhite solid; yield 64 %; mp 126–129 °C; IR (KBr): νmax 737, 1026, 1078, 1479, 1518, 1531 (C = N), 2889, 2932 (aliphatic C–H stretch), 3051, 3117 (aromatic C–H stretch) cm−1; 1H NMR (400 MHz, CDCl3): δ = 0.99 (d, 6H, J = 6.40 Hz, –CH2CH(CH 3 )2), 2.02–2.08 (m, 1H, –CH2CH(CH3)2), 2.61 (d, 2H, J = 6.80 Hz, –CH 2 CH(CH3)2), 3.59 (s, 2H, H-4), 7.24–7.55 (m, 5H, H-6, H-7, H-5′, H-3″, H-5″), 7.76 (d, 1H, J = 7.20 Hz, H-5), 7.92 (d, 2H, J = 8.40 Hz, H-2″, H-6″), 8.77 (d, 1H, J = 7.60 Hz, H-8); 13C NMR (100 MHz, CDCl3): δ = 22.20 (–CH2CH(CH 3 )2), 27.69 (C-4), 27.81 (–CH2 CH(CH3)2), 36.12 (–CH 2 CH(CH3)2), 111.09 (C-5′), 121.15 (C-8), 124.15 (C-7), 127.52 (C-2″, C-6″), 128.87 (C-3a), 129.25 (C-3″. C-5″), 129.87 (C-6), 132.53 (C-1″), 132.78 (C-4″), 134.60 (C-5), 138.42 (C-8a), 141.33 (C-4a), 148.66 (C-8b), 152.49 (C-4′), 154.37 (C-3), 159.63 (C-2′); ESI–MS m/z: 406.0 [M + 1]+, 407.2 [M + 2]+; Anal. Calcd. for C23H20ClN3S (405.11): C, 68.05; H, 4.97; N, 10.35. Found: C, 68.36; H, 4.71; N, 10.06.

2-(3-Isobutylindeno[1,2-c]pyrazol-1(4H)-yl)-4-p-tolylthiazole ( 4c )

Offwhite solid; yield 57 %; mp 115–118 °C; IR (KBr): νmax 742, 1020, 1067, 1460, 1529, 1530 (C = N), 2901, 2943 (aliphatic C–H stretch), 3049, 3121 (aromatic C–H stretch) cm−1; 1H NMR (400 MHz, CDCl3): δ = 0.99 (d, 6H, J = 6.40 Hz, –CH2CH(CH 3 )2), 2.02–2.09 (m, 1H, –CH2CH(CH3)2), 2.43 (s, 3H, CH 3 ), 2.61 (d, 2H, J = 7.20 Hz, –CH 2 CH(CH3)2), 3.59 (s, 2H, H-4), 7.19–7.53 (m, 5H, H-6, H-7, H-5′, H-3″, H-5″), 7.77 (d, 1H, J = 7.20 Hz, H-5), 7.88 (d, 2H, J = 8.40 Hz, H-2″, H-6″), 8.85 (d, 1H, J = 7.60 Hz, H-8); 13C NMR (100 MHz, CDCl3): δ = 21.32 (CH 3 ), 22.18 (–CH2CH(CH 3 )2), 27.67 (C-4), 27.74 (–CH2 CH(CH3)2), 36.20 (–CH 2 CH(CH3)2), 109.99 (C-5′), 121.17 (C-8), 125.35 (C-7), 126.09 (C-2″, C-6″), 128.89 (C-3a), 129.43 (C-6), 129.62 (C-3″, C-5″), 131.49 (C-1″), 134.75 (C-5), 138.34 (C-4″), 138.62 (C-8a), 141.49 (C-4a), 148.41 (C-8b), 153.51 (C-4′), 154.43 (C-3), 159.51 (C-2′); ESI–MS m/z: 386.2 [M + 1]+, 387.3 [M + 2]+, 408.2 [M + Na]+; Anal. Calcd. for C24H23N3S (385.16): C, 74.77; H, 6.01; N, 10.90; S, 8.32. Found: C, 74.46; H, 6.37; N, 10.68; S, 8.56.

2-(3-Isobutylindeno[1,2-c]pyrazol-1(4H)-yl)-4-(4-methoxyphenyl)thiazole ( 4d )

Offwhite solid; yield 55 %; mp 120–122 °C; IR (KBr): νmax 735, 1036, 1070, 1466, 1514, 1543 (C = N), 2851, 2955 (aliphatic C–H stretch), 3055, 3117 (aromatic C–H stretch) cm−1; 1H NMR (400 MHz, CDCl3): δ = 1.02 (d, 6H, J = 6.80 Hz, –CH2CH(CH 3 )2), 2.11–2.18 (m, 1H, –CH2CH(CH3)2), 2.65 (d, 2H, J = 7.60 Hz, –CH 2 CH(CH3)2), 3.62 (s, 2H, H-4), 3.89 (s, 3H, OCH 3 ), 7.03 (2H, d, J = 8.80 Hz, H-3″, H-5″), 7.11 (s, 1H, H-5′), 7.32–7.47 (m, 2H, H-6, H-7), 7.53 (d, 1H, J = 7.60 Hz, H-5), 7.92 (d, 2H, J = 8.80 Hz, H-2″, H-6″), 8.83 (d, 1H, J = 7.60 Hz, H-8); 13C NMR (100 MHz, CDCl3): δ = 22.16 (–CH2CH(CH 3 )2), 27.71 (C-4), 27.78 (–CH2 CH(CH3)2), 36.15(–CH 2 CH(CH3)2), 55.45 (OCH 3 ), 108.97 (C-5′), 114.25 (C-3″, C-5″), 121.13 (C-8), 124.38 (C-7), 127.08 (C-2″, C-6″), 127.53 (C-1″), 129.07 (C-3a), 129.72 (C-6), 134.64 (C-5), 138.51 (C-8a), 141.84 (C-4a), 148.56 (C-8b), 153.22 (C-4′), 154.37 (C-3), 159.95 (C-2′), 159.95 (C-4″); ESI–MS m/z: 402.0 [M + 1]+; Anal. Calcd. for C24H23N3OS (401.16): C, 71.79; H, 5.77; N, 10.47. Found: C, 71.98; H, 5.42; N, 10.75.

4-Phenyl-2-(3-propylindeno[1,2-c]pyrazol-1(4H)-yl)thiazole ( 4e )

Offwhite solid; yield 52 %; mp 96–98 °C; IR (KBr): νmax 731, 1072, 1470, 1510, 1539 (C = N), 2870, 2957 (aliphatic C–H stretch), 3051, 3128 (aromatic C–H stretch) cm−1; 1H NMR (400 MHz, CDCl3): δ = 0.98 (t, 3H, –CH2CH2CH 3 ), 1.72–1.81 (m, 2H, –CH2CH 2 CH3), 2.71 (t, 2H, –CH 2 CH2CH3), 3.59 (s, 2H, H-4), 7.24–7.35 (m, 4H, H-6, H-7, H-5′, H-4″), 7.48 (d, 2H, J = 7.40 Hz, H-3″, H-5″), 7.74 (d, 1H, J = 7.44 Hz, H-5), 7.99 (d, 2H, J = 7.40 Hz, H-2″, H-6″), 8.84 (d, 1H, J = 7.48 Hz, H-8); 13C NMR (100 MHz, CDCl3): δ = 13.91 (–CH2CH2 CH 3 ), 21.79 (–CH2 CH 2 CH3), 27.70 (C-4), 28.54 (–CH 2 CH2CH3), 110.81 (C-5′), 121.13 (C-8), 125.58 (C-7), 126.46 (C-2″, C-6″), 128.67 (C-4″), 128.74 (C-6), 128.80 (C-3″, C-5″), 129.01 (C-3a), 132.78 (C-1″), 134.81 (C-5), 138.48 (C-8a), 141.59 (C-4a), 148.62 (C-8b), 153.35 (C-4′), 154.57 (C-3), 159.62 (C-2′); ESI–MS m/z: 358.6 [M + 1]+; Anal. Calcd. for C22H19N3S (357.13): C, 73.92; H, 5.36; N, 11.75. Found: C, 73.65; H, 5.68; N, 11.37.

4-(4-Chlorophenyl)-2-(3-propylindeno[1,2-c]pyrazol-1(4H)-yl)thiazole ( 4f )

Offwhite solid; yield 54 %; mp 116–118 °C; IR (KBr): νmax 727, 1082, 1472, 1512, 1539 (C = N), 2872, 2957 (aliphatic C–H stretch), 3051, 3128 (aromatic C–H stretch) cm−1; 1H NMR (400 MHz, CDCl3): δ = 0.96 (t, 3H, –CH2CH2CH 3 ), 1.70–1.80 (m, 2H, –CH2CH 2 CH3), 2.72 (t, 2H, –CH 2 CH2CH3), 3.58 (s, 2H, H-4), 7.24–7.33 (m, 3H, H-6, H-7, H-5′), 7.47 (d, 2H, J = 8.64 Hz, H-3″, H-5″), 7.73 (d, 1H, J = 7.40 Hz, H-5), 7.90 (d, 2H, J = 8.64 Hz, H-2″, H-6″), 8.77 (d, 1H, J = 7.60 Hz, H-8); 13C NMR (100 MHz, CDCl3): δ = 13.90 (–CH2CH2 CH 3 ), 21.80 (–CH2 CH 2 CH3), 27.71 (C-4), 28.56 (–CH 2 CH2CH3), 111.08 (C-5′), 121.12 (C-8), 124.04 (C-7), 126.79 (C-2″, C-6″), 128.89 (C-3a), 129.27 (C-3″, C-5″), 129.99 (C-6), 132.53 (C-1″), 132.81 (C-4″), 134.87 (C-5), 138.87 (C-8a), 141.23 (C-4a), 148.65 (C-8b), 153.51 (C-4′), 154.31 (C-3), 159.45 (C-2′); ESI–MS m/z: 392.1 [M + 1]+. Anal. Calcd. for C22H18ClN3S (391.09): C, 67.42; H, 4.63; N, 10.72. Found: C, 67.73; H, 4.34; N, 10.46.

2-(3-Propylindeno[1,2-c]pyrazol-1(4H)-yl)-4-p-tolylthiazole (4g)

Offwhite solid; yield 58 %; mp 117–120 °C; IR (KBr): νmax 727, 1082, 1474, 1508, 1543 (C = N), 2872, 2957 (aliphatic C–H stretch), 3053, 3128 (aromatic C–H stretch) cm−1; 1H NMR (400 MHz, CDCl3): δ = 0.95 (t, 3H, –CH2CH2CH 3 ), 1.69–1.79 (m, 2H, –CH2CH 2 CH3), 2.42 (s, 3H, CH 3 ), 2.70 (t, 2H, –CH 2 CH2CH3), 3.57 (s, 2H, H-4), 7.23–7.34 (m, 3H, H-6, H-7, H-5′), 7.47 (d, 2H, J = 8.24 Hz, H-3″, H-5″), 7.72 (d, 1H, J = 7.28 Hz, H-5), 8.04 (d, 2H, J = 8.24 Hz, H-2″, H-6″), 8.84 (d, 1H, J = 7.40 Hz, H-8); 13C NMR (100 MHz, CDCl3): δ = 13.90 (–CH2CH2 CH 3 ), 21.36 (CH 3 ), 21.80 (–CH2 CH 2 CH3), 27.70 (C-4), 28.55 (–CH 2 CH2CH3), 110.02 (C-5′), 121.12 (C-8), 124.61 (C-7), 125.75 (C-2″, C-6″), 129.05 (C-3a), 129.30 (C-3″, C-5″), 129.43 (C-6), 131.41 (C-1″), 134.92 (C-5), 138.57 (C-4″), 138.66 (C-8a), 141.35 (C-4a), 148.69 (C-8b), 153.41 (C-4′), 154.28 (C-3), 159.39 (C-2′); ESI–MS m/z: 372.2 [M + 1]+; Anal. Calcd. for C23H21N3S (371.15): C, 74.36; H, 5.70; N, 11.31. Found: C, 74.58; H, 5.43; N, 11.57.

4-(4-Methoxyphenyl)-2-(3-propylindeno[1,2-c]pyrazol-1(4H)-yl)thiazole ( 4h )

Offwhite solid; yield 63 %; mp 104–110 °C; IR (KBr): νmax 729, 1078, 1468, 1506, 1537 (C = N), 2875, 2955 (aliphatic C–H stretch), 3049, 3128 (aromatic C–H stretch) cm−1; 1H NMR (400 MHz, CDCl3): δ = 0.98 (t, 3H, –CH2CH2CH 3 ), 1.72–1.81 (m, 2H, –CH2CH 2 CH3), 2.71 (t, 2H, –CH 2 CH2CH3), 3.59 (s, 2H, H-4), 3.88 (s, 3H, OCH 3 ), 7.17 (d, 2H, J = 8.68 Hz, H-3″, H-5″), 7.24–7.49 (m, 5H, H-6, H-7, H-5′), 7.73 (d, 1H, J = 7.44 Hz, H-5), 7.93 (d, 2H, J = 8.24 Hz, H-2″, H-6″), 8.84 (d, 1H, J = 7.40 Hz, H-8); 13C NMR (100 MHz, CDCl3): δ = 13.91 (–CH2CH2 CH 3 ), 21.80 (–CH2 CH 2 CH3), 27.71 (C-4), 28.55 (–CH 2 CH2CH3), 55.42 (OCH 3 ), 109.46 (C-5′), 114.12 (C-3″, C-5″), 121.13 (C-8), 124.07 (C-7), 126.81 (C-2″, C-6″), 126.88 (C-1″), 129.07 (C-3a), 129.72 (C-6), 134.82 (C-5), 138.51 (C-8a), 141.76 (C-4a), 148.63 (C-8b), 153.37 (C-4′), 154.83 (C-3), 159.43 (C-2′), 159.54 (C-4″); ESI–MS m/z: 388.1 [M + 1]+; Anal. Calcd. for C23H21N3OS (387.14): C, 71.29; H, 5.46; N, 10.84. Found: C, 71.45; H, 5.68; N, 10.56.

2-(3-Isopropylindeno[1,2-c]pyrazol-1(4H)-yl)-4-phenylthiazole ( 4i )

Offwhite solid; yield 66 %; mp 97–101 °C; IR (KBr): νmax 731, 1074, 1466, 1504, 1543 (C = N), 2874, 2961 (aliphatic C–H stretch), 3057, 3211 (aromatic C–H stretch) cm−1; 1H NMR (400 MHz, CDCl3): δ = 1.35 (d, 6H, J = 6.96 Hz, –CH(CH 3 )2), 3.06–3.13 (m, 1H, –CH(CH3)2), 3.62 (s, 2H, H-4), 7.26–7.36 (m, 4H, H-6, H-7, H-5′, H-4″), 7.46 (d, 2H, J = 7.76 Hz, H-3″, H-5″,), 7.72 (d, 1H, J = 7.40 Hz, H-5), 8.14 (d, 2H, J = 7.76 Hz, H-2″, H-6″), 8.70 (d, 1H, J = 7.60 Hz, H-8); 13C NMR (100 MHz, CDCl3): δ = 21.98 (–CH(CH 3 ) 2 ), 26.28 (C-4), 29.09 (–CH(CH3)2), 110.76 (C-5′), 121.13 (C-8), 125.77 (C-7), 126.40 (C-2″, C-6″), 128.63 (C-4″), 128.76 (C-6), 128.82 (C-3a), 129.08 (C-3″, C-5″), 132.66 (C-1″), 134.78 (C-5), 138.42 (C-8a), 141.39 (C-4a), 148.74 (C-8b), 153.29 (C-4′), 155.24 (C-3), 159.27 (C-2′); ESI–MS m/z: 358.21 [M + 1]+; Anal. Calcd. for C22H19N3S (357.13): C, 73.92; H, 5.36; N, 11.75. Found: C, 73.62; H, 5.71; N, 11.39.

4-(4-Chlorophenyl)-2-(3-isopropylindeno[1,2-c]pyrazol-1(4H)-yl)thiazole ( 4j )

Offwhite solid; yield 54 %; mp 103–106 °C; IR (KBr): νmax 735, 1082, 1470, 1499, 1536 (C = N), 2910, 2963 (aliphatic C–H stretch), 3057, 3155 (aromatic C–H stretch) cm−1; 1H NMR (400 MHz, CDCl3): δ = 1.37 (d, 6H, J = 6.96 Hz, –CH(CH 3 )2), 3.07–3.14 (m, 1H, –CH(CH3)2), 3.64 (s, 2H, H-4), 7.24–7.34 (m, 3H, H-6, H-7, H-5′), 7.47 (d, 2H, J = 8.40 Hz, H-3″, H-5″), 7.74 (d, 1H, J = 7.36 Hz, H-5), 7.94 (d, 2H, J = 8.40 Hz, H-2″, H-6″), 8.74 (d, 1H, J = 7.60 Hz, H-8); 13C NMR (100 MHz, CDCl3): δ = 21.92 (–CH(CH 3 ) 2 ), 26.25 (C-4), 29.05 (–CH(CH3)2), 111.22 (C-5′), 121.16 (C-8), 124.49 (C-7), 127.34 (C-2″, C-6″), 128.81 (C-3a), 129.10 (C-3″, C-5″), 129.28 (C-6), 132.16 (C-1″), 132.78 (C-4″), 134.76 (C-5), 138.56 (C-8a), 141.19 (C-4a), 148.67 (C-8b), 152.67 (C-4′), 155.24 (C-3), 159.66 (C-2′); ESI–MS m/z: 392.4 [M + 1]+; Anal. Calcd. for C22H18ClN3S (391.09): C, 67.42; H, 4.63; N, 10.72. Found: C, 67.78; H, 4.32; N, 10.43.

2-(3-Isopropylindeno[1,2-c]pyrazol-1(4H)-yl)-4-p-tolylthiazole ( 4k )

Offwhite solid; yield 56 %; mp 96–102 °C; IR (KBr): νmax 735, 1071, 1477, 1506, 1545 (C = N), 2874, 2961 (aliphatic C–H stretch), 3059, 3159 (aromatic C–H stretch) cm−1; 1H NMR (400 MHz, CDCl3): δ = 1.37 (d, 6H, J = 6.96 Hz, –CH(CH 3 )2), 2.42 (s, 3H, CH 3 ), 3.07–3.14 (m, 1H, –CH(CH3)2), 3.64 (s, 2H, H-4), 7.17–7.48 (m, 5H, H-6, H-7, H-5′, H-3″, H-5″), 7.74 (d, 1H, J = 7.40 Hz, H-5), 7.87 (d, 2H, J = 8.04 Hz, H-2″, H-6″), 8.86 (d, 1H, J = 7.60 Hz, H-8); 13C NMR (100 MHz, CDCl3): δ = 21.32 (CH 3 ), 21.92 (–CH(CH 3 )2), 26.24 (C-4), 29.04 (–CH(CH3)2), 108.11 (C-5′), 121.09 (C-8), 125.52 (C-7), 125.76 (C-2″, C-6″), 128.96 (C-3), 129.55 (C-6), 129.63 (C-3″, C-5″), 132.00 (C-1″), 134.75 (C-5), 138.04 (C-4″), 138.65 (C-8a), 141.24 (C-4a), 148.66 (C-8b), 152.75 (C-4′), 155.28 (C-3), 159.64 (C-2′); ESI–MS m/z: 372.1 [M + H]+; Anal. Calcd. for C23H21N3S (371.15): C, 74.36; H, 5.70; N, 11.31. Found: C, 74.65; H, 5.39; N, 11.62.

2-(3-Isopropylindeno[1,2-c]pyrazol-1(4H)-yl)-4-(4-methoxyphenyl)thiazole ( 4l )

Offwhite solid; yield 51 %; mp 110–114 °C; IR (KBr): νmax 731, 1072, 1474, 1506, 1541 (C = N), 2876, 2961 (aliphatic C–H stretch), 3057, 3157 (aromatic C–H stretch) cm−1; 1H NMR (400 MHz, CDCl3): δ = 1.38 (d, 6H, J = 6.92 Hz, –CH(CH 3 )2), 3.07–3.16 (m, 1H, –CH(CH3)2), 3.64 (s, 2H, H-4), 3.88 (s, 3H, OCH 3 ), 7.04–7.48 (m, 5H, H-6, H-7, H-5′, H-3″, H-5″), 7.74 (d, 1H, J = 7.40 Hz, H-5), 8.04 (d, 2H, J = 8.24 Hz, H-2″, H-6″), 8.84 (d, 1H, J = 7.64 Hz, H-8); 13C NMR (100 MHz, CDCl3): δ = 21.95 (–CH(CH 3 )2), 26.23 (C-4), 29.06 (–CH(CH3)2), 55.45 (OCH 3 ), 109.91 (C-5′), 114.10 (C-3″, C-5″), 121.13 (C-8), 125.52 (C-7), 127.02 (C-2″, C-6″), 127.39 (C-1″), 128.64 (C-3a), 129.36 (C-6), 134.27 (C-5), 138.45 (C-8a), 141.35 (C-4a), 148.62 (C-8b), 153.52 (C-4′), 155.28 (C-3), 159.88 (C-2′), 159.97 (C-4″); ESI–MS m/z: 388.3 [M + 1]+; Anal. Calcd. for C23H21N3OS (387.14): C, 71.29; H, 5.46; N, 10.84. Found: C, 71.53; H, 5.75; N, 10.48.

In vitro Anticancer Evaluation

Materials

Dulbecco’s modified Eagle’s medium (DMEM) and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) were obtained from Sigma Chemical Co. (St. Louis, MO, USA).

Cell culture and treatment

A498 (Human renal carcinoma), HT29 (Human colorectal adenocarcinoma), MCF-7 (Human breast adenocarcinoma), HepG2 (Human hepatocellular carcinoma) and NRK (Normal rat kidney epithelial) cell lines were obtained from National Centre for Cancer Sciences (NCCS), Pune, India, and grown as a monolayer in DMEM supplemented with 10 % FBS (Fetal Bovine Serum), 100 µg/mL streptomycin and 100 units/mL penicillin. Cells were incubated at 37 °C in an atmosphere of 5 % CO2. For 96-well plates, cells were seeded at approximately 1.5 × 104 cells per well.

MTT assay

The cell viability was assessed by the MTT colorimetric assay (Denizot and Lang, 1986) which is based on the reduction of MTT by the mitochondrial succinate dehydrogenase of intact cells to a purple formazan product. Briefly, cells were incubated in 96-well micro titer plates for 24 h at 37 °C in a 5 % CO2 incubator. Following the addition of the test compounds, the plates were incubated for an additional 48 h. Control wells contained medium alone. Three replicate wells were used at each point in the experiments. After 24 h incubation, MTT solution (5 mg/mL in phosphate-buffered saline) was added and incubated for another 4 h. The resulting MTT/formazan product was dissolved by 0.1 mL of isopropanol, and the plates were gently shaken to solubilize the formed formazan. The amount of formazan was determined by measuring the absorbance (OD) at 570 nm using a Bio-Rad 550 enzyme-linked immunosorbent assay (ELISA) microplate reader.

Cell survival was calculated as the percentage MTT inhibition as follows:

$$\% \,{\text{growth}}\,{\text{inhibition}} = 100 - \left( {{\text{mean}}\,{\text{OD}}\,{\text{of}}\,{\text{individual}}\,{\text{test}}\,{\text{group}}/{\text{mean}}\,{\text{OD}}\,{\text{of}}\,{\text{each}}\,{\text{control}}\,{\text{group}}} \right) \times 100.$$

The values of IC50 (concentration of test compound that is needed to reduce the cell survival fraction to 50 %) were calculated and used as a measure of cellular sensitivity to a given treatment.

QSAR studies

Dataset

The data given in the Table 1 was used to carry out this study. The entire dataset of 22 compounds was divided into training set (18 compounds) and test set (04 compounds), and training set was employed to develop QSAR models.

Table 1 In vitro anticancer activity of thiazole tethered indenopyrazoles (3a–3l & 4a–4l)
Full size table

Structure generation

The structures of the molecules were sketched and optimized using Marvin Sketch. The molecules were prepared on the same conformation of basic skeleton indenopyrazole.

Calculation of descriptors

In order to get a QSAR model, compounds were characterized by the molecular descriptors at all times. The molecular descriptors (863 descriptors including one-, two- and three-dimensional descriptors) were calculated with PaDEL Descriptor 2.12 program (Yap, 2011). The different descriptors can portray a molecule from dissimilar aspects, but a little of them may signify the similar meanings with the same or similar values. The invariable or near-constant descriptors and descriptors with value zero for even one molecule were deleted to lessen repetition and errors. If the pairwise correlation of two descriptors was more than 0.75, the one having higher pair correlation with other descriptors was left out from the original matrix of variables to reduce redundant information. The remaining descriptors underwent the consequent variable selection process.

QSAR modeling and validation

Multiple linear regression (MLR) was used for the development of QSAR models, and the stepwise multiple linear regression variable subset selection was applied for variable selection with SPSS software package [SPSS, 1996]. Different parameters were employed to validate the models. The correlation coefficient R was utilized as an assessment of the goodness-of-fit. Further fitting criteria used were \(R_{\text{adj}}^{2}\), R 2–\(R_{\text{adj}}^{2}\), RMSE (Root mean squared error), MAE (Mean average error), s (standard error of estimate) and F (Fischer’s value). In order to authenticate model’s robustness and predictive power, the cross-validation coefficient \(Q_{\text{LOO}}^{2}\) (leave-one-out) was employed where a model is developed by n − 1 compounds and the nth compound is predicted. Each compound is iteratively excluded from the set used for model building and predicted sequentially. An indication of the model performance is accomplished from the cross-validation coefficient, which may be defined as

$$Q_{\text{LOO}}^{2} = \left( {1{-}{\text{PRESS}}/{\text{TSS}}} \right)$$

where TSS is the total sum of squares. PRESS (predictive error sum of squares) is the sum of the squared difference between the observed and the predicted values when the compound is held out from the procedure of fitting. The model with high \(Q_{\text{LOO}}^{2}\) value is thought to have high predictive ability. This is the only method that uses all the information on hand and is very pertinent mainly in small datasets as is our case.

However, many studies have proved that \(Q_{\text{LOO}}^{2}\) is an imperfect measure of a model predictive ability (Baumann and Stiefl, 2004), and although it is necessary at the time of model development, it is still insufficient for a reliable estimate of model predictive power for totally novel compounds. Leave-many-out cross-validation (LMO-CV) is a stronger CV technique where more than one compound is excluded at a time for the validation. LMO is used to counteract the slight over optimism of \(Q_{\text{LOO}}^{2}\). Moreover, it has been established on different datasets that the strongest CV, that can furnish a more realistic indication of the true internal predictivity in small datasets (20–30 compounds), is LMO 30 % (Gramatica 2007). Therefore, we have used LMO with 30 % compounds left at a time.

Further, to ensure the robustness and the statistical importance of this QSAR study, Y-randomization test was also exercised to validate the developed models. In this test, the Y vector is shuffled randomly and the calculation procedure is repeated various times. The resulting models following several repetitions are supposed to have less significant correlation coefficient values than the ones of the original model. If all models attained by the Y-randomization test have comparatively high values for R2 statistics, then this is because of a chance correlation and suggests that the present modeling technique cannot lead to a satisfactory model using the existing dataset [Long et al., 2013].

In light of Tropsha’s criteria, the predictive power of a QSAR model should be evaluated on an external dataset that has not been taken into account during the process of model building. Therefore, the dataset was split into training (18 compounds) and test set (04 compounds) in order to confirm the true predictive power of the developed models (Tropsha, 2010). \(Q_{\text{ext}}^{2}\) was used as a criterion for this estimation which was calculated by formula described in literature (Shi et al., 2001).

Results and discussion

Chemistry

The synthetic route for the preparation of thiazole tethered indenopyrazoles (3 & 4) is outlined in Scheme 1. The starting 1,3-diketones, i.e, 2-acyl-(1H)-indene-1,3(2H)-diones (1) were prepared by the Claisen condensation of appropriate ketones and diethylphthlate under the influence of sodium methoxide as per literature procedure (Dhawan et al., 1993; Shapiro et al., 1960). The synthesis of twelve new indeno[1,2-c]pyrazol-4-ones (3) was carried out by a convenient one-pot three-component reaction of 1,3-diketones (1), thiosemicarbazide and α-bromoketones (2). Initially, a mixture of appropriate 2-acyl-(1H)-indene-1,3(2H)-dione (1) and thiosemicarbazide was refluxed in methanol for 10 min. Thereafter, the reaction mixture was charged with an appropriate phenacyl bromide (2) and glacial acetic acid, and subsequently refluxed for 6–7 h to furnish the corresponding thiazole tethered indeno[1,2-c]pyrazol-4-ones (3a–3l) in high yields. Further, the indeno[1,2-c]pyrazol-4-ones (3a–3l) were subjected to Wolff-Kishner reduction (Gupta et al., 1979) to afford the corresponding indenopyrazoles (4a–4l) in moderate-to-good yields.

Scheme 1
scheme 1

Synthetic route of thiazole tethered indenopyrazoles (3a–3l & 4a–4l)

Full size image

The structures of all the newly synthesized thiazole tethered indenopyrazoles (3 & 4) were well established by satisfactory IR, NMR (1H and 13C) and mass spectroscopic data. The IR spectra of indenopyrazoles (3a–3l), in each case, displayed a strong absorption band in the region at 1701–1713 cm−1 due to >C=O stretching. The salient feature of 1H NMR spectra of indenopyrazoles (3) is the downfield shifting of C8–H (δ 8.35–8.48) as compared to other aromatic protons due to anisotropic-diamagnetic effect of lone pair of electrons present on nitrogen and/or sulfur of thiazole moiety (Dhawan et al., 1994). The other aromatic and aliphatic protons appeared in the expected regions. The 13C NMR spectra of indenopyrazoles (3), in each case, displayed a signal in the most downfield region at δ 183.65–184.08 which can safely be assigned to the carbonyl carbon (C4) (Hughes et al., 1977; Patra and Mishra, 1991). The signals appearing in the regions at δ 152.06–159.18, δ 122.26–123.53 and δ 157.70–158.22 were assigned to the carbon atoms C3, C3a and C8b, respectively, of the pyrazole ring (Singh et al., 1989). The chemical shifts exhibited in the regions at δ 159.89–160.38 (C2′), δ 152.08–153.35 (C4′) and δ 108.85–111.01 (C5′) corroborated the thiazole character of thiazolyl moiety (Thakar et al., 2010). The signals due to the remaining aliphatic and aromatic carbons were obtained in the expected regions. In addition, the compound 3d was also characterized by 1D DEPT and 2D NMR techniques such as 1H-1H COSY, HSQC and HMBC.

The IR spectra of indenopyrazoles (4a–4l), in each case, exhibited a strong absorption band in the region at 1530–1545 cm−1 which was presumably attributed due to CN stretching (Pandya and Khan, 2008). Further, the reduction of carbonyl group of indeno[1,2-c]pyrazol-4-ones (3) by Wolff-Kishner reduction was confirmed by the disappearance of the absorption band due to > C=O stretching in the IR spectra of indenopyrazoles (4) as exhibited by indeno[1,2-c]pyrazol-4-ones (3). Also, the appearance of a singlet in 1H NMR spectra of indenopyrazoles (4a–4l), in each case, in the region at δ 3.57–3.64 integrating for two protons due to C4 protons and the downfield shifting of C8–H (δ 8.70–8.86) most likely due to increased anisotropic-diamagnetic effect of thiazole moiety, further confirms that the hybridization state of C4 changes from sp2 to sp3 by Wolff-Kishner reduction of 3. The other aliphatic and aromatic protons appeared in the expected regions. The conversion of indeno[1,2-c]pyrazol-4-ones (3) into indenopyrazoles (4) was further supported by disappearance of the signal due to carbonyl carbon (>C=O) and appearance of signal due to CH2 of indenopyrazole moiety in the region at δ 26.23–27.71 in the 13C NMR spectra of 4. The remaining aliphatic and aromatic carbons were observed in the expected regions. Further, the results obtained from mass spectral analysis and analytical data of 3 and 4 were found in accordance with their molecular formulae.

In vitro anticancer evaluation

The newly synthesized thiazole tethered indenopyrazole derivatives (3a–3l & 4a–4l) were evaluated for their in vitro cytotoxicity against A498 (Human renal carcinoma), HT29 (Human colorectal adenocarcinoma), MCF-7 (Human breast adenocarcinoma), HepG2 (Human hepatocellular carcinoma) and NRK (Normal rat kidney epithelial) cell lines by performing the standard MTT assay (Denizot and Lang, 1986). Cisplatin was used as reference drug in the present investigation because it is one of the most potent chemotherapeutic drugs used worldwide for over 40 years and easily available. Cisplatin is not specific against any particular organ and has been successfully employed for treatment of various human cancers including bladder, lung, head and neck, ovarian, and testicular cancers. It is effective against many types of cancers including germ cell tumors, carcinomas, sarcomas, lymphomas, and cancers of soft tissue, muscles, bones, and blood vessels. In the present study, cytotoxicity of the synthesized compounds was tested against cell lines of different organs, so a drug specific to a particular organ could not be used as reference molecule.

IC50 values of the indenopyrazoles (3a–3l & 4a–4l) against different human cancer cell lines under study are presented in Table 1. It is inferred from the data presented in Table 1 that the indenopyrazoles (4a–4l) were found more active than indeno[1,2-c]pyrazol-4-ones (3a–3l) against all the tested human cancer cell lines. Among all the tested indenopyrazoles, the compound 4d (IC50, 44.31 µM) was found to be most effective against A498 cancer cell line and the derivatives 4a (IC50, 34.77 µM), 4d (IC50, 35.07 µM) and 4h (IC50, 35.10 µM) exhibited better activity against HT29 cancer cell line. The compound 4a (IC50, 32.09 µM) showed highest activity against MCF-7 cancer cell line, whereas 4b (IC50, 34.70 µM) was found more active against HepG2 cancer cell line as compared to other tested derivatives. Further, the results illustrated that compound 3l was inactive against all the tested human cancer cell lines. It is worth mentioning here that all the tested compounds were found to be less toxic against normal cell line (NRK).

The tested compounds displayed different activities against different cell lines because the selected cell lines belong to different organs of human body. The difference in activities of control cisplatin and tested compounds may be due to their different mechanisms of action. Cisplatin acts by cross-linking DNA in different ways (Kartalou and Essigmann, 2001; Rabik and Dolan, 2007) thereby interfering with cell division by mitosis, which in turn leads to cell cycle arrest and apoptosis. On the other hand, indenopyrazoles are well-known CDK (Nugiel et al., 2002; Usui et al., 2008; Yue et al., 2004) and CHK inhibitors (Tao et al., 2005, 2007).

Quantitative structure–activity relationship (QSAR) studies

In order to understand the observed activity trend on structural basis, QSAR studies were performed for anticancer activities of the synthesized indenopyrazoles (3a–3l & 4a–4l). Anticancer activity data determined as IC50 values was first transformed into pIC50 values and used as dependent variables in QSAR analysis and is depicted in Table 2. Different 2D and 3D descriptors were calculated for all the active compounds and used as independent variables in this study. The values of descriptors used in successful model development are presented in Table 2. In the present QSAR model development study, a dataset containing twenty-three compounds was used (except compound 3l) and it was split into training set and a test set excluding outliers. The first step in getting a model with statistical significance is to inspect whether any colinearity exists between the descriptors used. This is accomplished by obtaining correlation matrix (Agrawal et al., 2002). Therefore, correlation matrix displaying correlation between the activity and descriptors as well as intercorrelation among descriptors is displayed in Table 3.

Table 2 pIC50 and calculated descriptors of the compounds under study
Full size table
Table 3 Correlation matrix
Full size table

Multiple linear regression analysis was used for creation of QSAR equations. Statistically significant QSAR models were developed, and best split QSAR model developed for A498 cells is expressed by Eq. (1).

QSAR model for anticancer activity against A498 cells

$${\text{pIC}}_{50} {\text{A}}498\,{\text{cells}} = {-}0.1182( \pm 0.0143)\,{\text{MDEC - }}33 + 2.6596( \pm 0.4024)$$
(1)

(n = 18; R = 0.900; \(R_{\text{adj}}^{2}\) = 0.799; R 2–\(R_{\text{adj}}^{2}\) = 0.012; s = 0.112; F = 68.590; RMSE = 0.106; \(Q_{\text{LOO}}^{2}\) = 0.763; \(Q_{\text{LMO}}^{2}\) = 0.500, R 2Yscr = 0.058; RMSEcv = 0.202; \(Q_{\text{ext}}^{2}\) = 0.7134).

Here and after that, n = number of compounds, R = correlation coefficient, \(R_{\text{adj}}^{2}\) = adjusted coefficient of determination, s = standard error, F = Fischer’s value, \(Q_{\text{LOO}}^{2}\) = cross-validated coefficient (leave one out), \(Q_{\text{LMO}}^{2}\) = cross-validated coefficient (leave many out), RMSE = Root Mean Squared Error, RMSEcv = Root Mean Squared Error (cross-validation), R 2Yscr = R 2 scrambled, \(Q_{\text{ext}}^{2}\) = external cross-validated coefficient.

Statistical data of the model developed reveals that it is a good QSAR model and have high-quality internal predictive ability with values of \(Q_{\text{LOO}}^{2}\) = 0.543 and \(Q_{\text{LMO}}^{2}\) = 0.500. The value of \(Q_{\text{LOO}}^{2}\) more than 0.5 is an essential condition for a valid QSAR model (Golbraikh and Tropsha, 2002). Low value of the averaged R 2 scrambled (0.058) was sign of a well-established original model and proved that the model was not obtained by chance. Low values of RMSE prove that the model is free from errors. The high value of \(Q_{\text{ext}}^{2}\) (0.7134) verified that the developed model has good external predictive power. The activities predicted by this equation are displayed in Table 4 in conjunction with observed activities and both are matched in the graph shown in Fig. 1. The plot between residuals and observed activities (Fig. 2) demonstrates that the values of residuals are spread on both positive and negative sides of the zero which further verifies that the built model is free from systematic errors. The negative sign of coefficient of MDEC-33 indicate that the activity is correlated with this descriptor in inverse manner. MDEC-33 is molecular distance edge (MDE) 2D parameter describing molecular distance edge between all tertiary carbons (Liu et al., 1998).

Table 4 Observed (Obs) and calculated (Cal) activities along with residuals (Res) obtained by the developed QSAR models
Full size table
Fig. 1
figure 1

Plot of observed pIC50 against calculated pIC50 for the QSAR model of A498 cell lines

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Fig. 2
figure 2

Plot of observed pIC50 against residuals for the QSAR model of A498 cell lines

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MDEC-23 and h max described the anticancer activity of the compounds (3a–3l & 4a–4l) against pHT29 cells, and the best split QSAR model developed is depicted in Eq. (2).

QSAR model for anticancer activity against pHT29 cells

$${\text{pIC}}_{50} {\text{HT}}29\,{\text{cells}} = 15.497( \pm 5.58)\,{\text{h}}\hbox{max} + 0.0598( \pm 0.0453)\,{\text{MDEC - }}23{-}9.7005( \pm 3.1918)$$
(2)

(n = 18; R = 0.893; \(R_{\text{adj}}^{2}\) = 0.770; R 2–\(R_{\text{adj}}^{2}\) = 0.027; s = 0.135; F = 29.407; \(Q_{\text{LOO}}^{2}\) = 0.721; \(Q_{\text{LMO}}^{2}\) = 0.5875; R 2Yscr = 0.1203; RMSE = 0.123; RMSEcv = 0.144; \(Q_{\text{ext}}^{2}\) = 0.920)

Compound 3a was found to be outliers in this QSAR model. Positive sign of h max and MDEC-23 established that these descriptors are directly correlated with the activity. The standardized coefficient for h max and MDEC-23 were 0.7243 and 0.3441, respectively. This shows that h max is more contributing toward the activity than MDEC-23. The parameter h max is the maximum hydrogen Estate value for all the atoms in the molecule. It illustrates the hydrogen accessibility of the molecule (Hall and Kier, 1995). MDEC-23 is molecular distance edge (MDE) 2D parameter explaining molecular distance edge between all secondary and tertiary carbons (Liu et al., 1998).

The anticancer activity against MCF-7 cells was described by the subsequent split QSAR Eq. (3).

QSAR model for anticancer activity against MCF-7 cells

$${\text{pIC}}_{50} {\text{MCF - }}7\,{\text{cells}} = 4.5459( \pm 1.309)\,\hbox{max} {\text{sCH}}3{-}9.0565( \pm 2.8741)$$
(3)

(n = 18; R = 0.879; \(R_{\text{adj}}^{2}\) = 0.758; R 2–\(R_{\text{adj}}^{2}\) = 0.014; s = 0.165; F = 54.199; \(Q_{\text{LOO}}^{2}\) = 0.712; \(Q_{\text{LMO}}^{2}\) = 0.625; R 2Yscr = 0.059; RMSE = 0.155; RMSEcv = 0.175; \(Q_{\text{ext}}^{2}\) = 0.566).

Compound 4l was found to be outlier; hence, this was left out in model development process. It is a monoparametric model in which descriptor maxsCH3 is correlated with the observed activity in a positive way as the sign of coefficient of this factor is positive. It is an electrotopological state atom type descriptor that explains maximum hydrogen Estate value for all the methyl groups present in the molecule (Hall and Kier, 1995).

Best QSAR model developed for describing anticancer activity against HepG2 cells was biparametric model obtained for 22 compounds (Eq. (4)). Again, compound 4l was found to be outlier and that is why not used in development of QSAR model.

QSAR model for anticancer activity against HepG2 cells

$${\text{pIC}}_{50} {\text{HepG}}2\,{\text{cells}} = 37.5356( \pm \,8.9206)\,{\text{ETA}}\_{\text{EtaP}}\_{\text{L}}{-}2.9826( \pm 1.9572)\,{\text{Weta}}2.{\text{eneg}}{-}7.438( \pm 2.463)$$
(4)

(n = 18; R = 0.925; \(R_{\text{adj}}^{2}\) = 0.837; R 2–\(R_{\text{adj}}^{2}\) = 0.019; s = 0.129; F = 44.550; \(Q_{\text{LOO}}^{2}\) = 0.783; \(Q_{\text{LMO}}^{2}\) = 0.751; R 2Yscr = 0.121; RMSE = 0.118; RMSEcv = 0.145; \(Q_{\text{ext}}^{2}\) = 0.852).

The coefficient of ETA_EtaP_L is positive illustrating that the activity is related with this descriptor in positive way, whereas Weta2.eneg is correlated in negative manner to the activity, as the coefficient of this parameter is negative in the equation. The standardized coefficients for ETA_EtaP_L and Weta2.eneg are 0.8795 and −0.3185, respectively, establishing more involvement of ETA_EtaP_L in activity determination in contrast to Weta2.eneg. ETA_EtaP_L is an extended topochemical atom descriptor explaining local index extended topochemical atom _local relative to molecular size (Roy and Das, 2011; Roy and Ghosh, 2004). Weta2.eneg is a directional WHIM, weighted by Mulliken atomic electronegativities (Todeschini and Gramatica, 1998).

All the aforementioned models are statistically considerable models as all the statistical parameters are in applicable range. The value of correlation coefficient is more than 0.85 for all the models. In addition, the values of \(R_{\text{adj}}^{2}\) are very high and the difference between R 2–\(R_{\text{adj}}^{2}\) is very less for all models proving that \(R_{\text{adj}}^{2}\) is very close to coefficient of determination of the models. The value of standard error is very small for every model. All these specifics confirm that the developed models have very good fitting capability. The internal predictive ability of all these models is substantiated by high value of leave-one-out cross-validation coefficient (\(Q_{\text{LOO}}^{2}\)) as well as leave-many-out cross-validation coefficient (\(Q_{\text{LMO}}^{2}\)) which are more than 0.5 and low values of RMSE (Golbraikh and Tropsha, 2002). These facts prove the robustness of the developed models. The difference between RMSE and RMSEcv is very less establishing the fact that described models have sufficient generalizability (Gramatica, 2007).

The developed QSAR models have excellent external predictive power as shown by high values of \(Q_{\text{ext}}^{2}\). The calculated activities, residuals together with observed activities for all above explained QSAR models are presented in Table 4. The plots between observed activity and calculated activity as well as residuals are exhibited in Figs. 3, 4, 5, 6, 7 and 8.

Fig. 3
figure 3

Plot of observed pIC50 against calculated pIC50 for the QSAR model of HT29 cell lines

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Fig. 4
figure 4

Plot of observed pIC50 against residuals for the QSAR model of HT29 cell lines

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Fig. 5
figure 5

Plot of observed pIC50 against calculated pIC50 for the QSAR model of MCF-7 cell lines

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Fig. 6
figure 6

Plot of observed pIC50 against residuals for the QSAR model of MCF-7 cell lines

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Fig. 7
figure 7

Plot of observed pIC50 against calculated pIC50 for the QSAR model of HepG2 cell lines

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Fig. 8
figure 8

Plot of observed pIC50 against residuals for the QSAR model of HepG2 cell lines

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Compound 3a was outlier for QSAR models 1 and 2, while compound 4l was outlier for models 3 and 4. These were response outliers for which the reference value of response is invalid as QSAR models developed including these molecules displayed high residual values (Furusjo et al., 2006).

Further, it can be noted that the model no. 2 and 4 which are biparametric models are free from problem of colinearity as the correlation coefficient between the used descriptors is less than 0.4 (Agrawal et al., 2002).

Conclusion

In conclusion, thiazole tethered indeno[1,2-c]pyrazol-4-ones (3) have been conveniently prepared by three-component one-pot synthesis. Indenopyrazoles (4) were synthesized by Wolff-Kishner reduction of 3. All the newly synthesized thiazole tethered indenopyrazoles (3 & 4) are characterized by using different spectral techniques. These indenopyrazoles were evaluated for their in vitro cytotoxicity against A498, HT29, MCF-7, HepG2 and NRK cell lines. Among all the tested indenopyrazoles, 4a, 4d and 4h exhibited better activity against HT29 cancer cell line. Interestingly, all the tested compounds were found to be less toxic toward normal cell line. Further, robust as well as generalizable QSAR models with good fitting ability and internal predictive power were developed for all the cancer cell lines. Molecular distance edge parameters MDEC-33 and MDEC-23 were important for anticancer activity against A498 and HT29 cancer cell lines, respectively, while electrotopological state atom type descriptor maxsCH3 was significant in MCF-7 cell line. Extended topochemical atom descriptor ETA_EtaP_L and directional WHIM parameter Weta2.eneg explained activity trend of HepG2 cell line.