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. 2020 Sep 9;25(18):4124. doi: 10.3390/molecules25184124

Construction of Unusual Indole-Based Heterocycles from Tetrahydro-1H-pyridazino[3,4-b]indoles

Cecilia Ciccolini 1, Lucia De Crescentini 1, Fabio Mantellini 1, Giacomo Mari 1, Stefania Santeusanio 1, Gianfranco Favi 1,*
Editor: Pascale Moreau1
PMCID: PMC7571100  PMID: 32916997

Abstract

Herein, we report the successful syntheses of scarcely represented indole-based heterocycles which have a structural connection with biologically active natural-like molecules. The selective oxidation of indoline nucleus to indole, hydrolysis of ester and carbamoyl residues followed by decarboxylation with concomitant aromatization of the pyridazine ring starting from tetrahydro-1H-pyridazino[3,4-b]indole derivatives lead to fused indole-pyridazine compounds. On the other hand, non-fused indole-pyrazol-5-one scaffolds are easily prepared by subjecting the same C2,C3-fused indoline tetrahydropyridazines to treatment with trifluoroacetic acid (TFA). These methods feature mild conditions, easy operation, high yields in most cases avoiding the chromatographic purification, and broad substrate scope. Interestingly, the formation of indole linked pyrazol-5-one system serves as a good example of the application of the umpolung strategy in the synthesis of C3-alkylated indoles.

Keywords: indole-based heterocycles, C2-C3 indole oxidation, aromatization, ring-opening/ring-closing, umpolung

1. Introduction

Given the intriguing structures and the medicinal importance of polycyclic indole-based molecules [1,2,3,4,5,6,7], we envisioned that the amalgamation of the indole moiety [8,9,10,11,12,13] with the pyridazine ring [14,15,16,17,18,19] that potentially generates different isomeric scaffolds (a–c) would lead to entities endowed with either amplified or new biological activities (Figure 1). A large number of reports dedicated to the synthesis of these appealing frameworks and studies on their pharmacologic activities appeared in the literature in the past few decades [20,21,22,23,24,25,26,27,28,29]. Among the heterocyclic architectures a–c, the tricyclic fused indole-pyridazine system c which can be considered to be aza-analogous (or bioisoster) of β-carboline, the unique tricyclic pyrido[3,4-b]indole core amenable to an important family of bioactive natural products widely distributed in nature [30], has attracted our attention.

Figure 1.

Figure 1

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Different isomeric fused indole-pyridazine systems.

Although many of the above cited examples deal with 5H-pyridazino[4,3-b]indoles (a) [20,21] and 5H-pyridazino[4,5-b]indoles or 3H-pyridazino[4,5-b]indol-4(5H)-ones (b) [22,23,24,25,26], the chemistry of 9H-pyridazine[3,4-b]indoles (c) [27,28,29] is much less known.

Specifically, in 1964, Kobayashi and Furukawa presented the synthesis of various 3-phenyl-9H-pyridazine[3,4-b]indoles by heating 3-phenacyl-oxindoles and hydrazine hydrate in acetic acid solution [27]. In 1992, Shimoji and co-workers synthesized a series of methyl 9H-pyridazino[3,4-b]indole-3-carboxylates and related compounds using a Diels-Alder reaction of 3-(1H-indol-3-yl)-2-propenoates with dibenzyl dicarboxylate [28]. Several of these compounds were found to have high affinity for the benzodiazepine receptor. Recently, the design, synthesis, biological evaluation, and molecular modeling studies of several 3-aryl-9-acetyl-pyridazino[3,4-b]indoles were reported by Nepali et al. [29].

Following our sustained efforts toward the construction of novel azaheterocycles, especially around privileged structures [31], we have recently disclosed a zinc-catalyzed synthesis of polycylic fused indoline scaffolds through a substrate-guided reactivity switch [32]. With this as a background, we herein describe a successful procedure providing the fused indole-pyridazine scaffold of type c (Figure 1)through oxidation and hydrolysis processes from previously synthesized tetrahydro-1H-pyridazino[3,4-b]indole compounds (1). Surprised to observe a ring-opening/ring-closing pathway during the C2-C3 indole oxidation step, a facile and robust access to unusual non-fused derivative indole-pyrazol-5-ones 4 starting from the same cycloadducts (1) has been also achieved in excellent yields.

2. Results and Discussion

In a preliminary experiment, the conversion of tetrahydro-1H-pyridazino[3,4-b]indole 1b to fully aromatic 9H-pyridazino[3,4-b]indole 3b was attempted by a two-step process consisting of decarboxylation of the ester motif with removal of carbamoyl residue followed by oxidation of pyridazine ring under basic conditions. However, although removal of the ester with alcoholic KOH efficiently took place affording a mixture of CH/NH tautomers, unexpectedly pyridazine ring aromatization did not occur. With this failure in hand, we first turned our attention to the crucial C2-C3 indole oxidation step. A variety of oxidizing agents (e.g., DDQ [33], BQ, NBS/TBPB, MnO2 [34,35,36], Na2Cr2O7, I2, PCC [37], Pd/C [38], CAN [39] and CuCl2∙2H2O [40]) was investigated (Table 1). Common reagents such as BQ, Na2Cr2O7, PCC, Pd/C, I2, and NBS in combination with TBPB, although frequently used for oxidation of the indoline ring, proved unsuccessful. With stoichiometric DDQ in dioxane, only trace of the aromatic indole-pyridazine 2b was formed (Table 1, entry 1). The dehydrogenations reaction yield could be further improved by using toluene and CH2Cl2 as solvents (Table 1, entries 2 and 3). Manganese dioxide was reported in the literature to be suitable for the oxidation of indolines to indoles [34,35,36]. To our surprise, when compound 1b was subjected to treatment with MnO2 in benzene at 70 °C, the sole demethylated indole derivative 2b’ was isolated in 55% yield [41] (see Supplementary Materials). Reducing the amount of MnO2 displayed an incomplete reaction, decreasing the reaction yield (40% yield) of the obtained product (Table 1, entries 6 and 7).

Table 1.

Optimization conditions.

graphic file with name molecules-25-04124-i001.jpg

Entry Oxidant Solvent Temp.
(°C)		 Time
(h)		 2b(2b’)/4c Yield
(%)
1 DDQ (1 equiv) dioxane rt 2 2b trace
2 DDQ (1 equiv) toluene reflux 24 2b 21
3 DDQ (1.5 equiv) CH2Cl2 rt 15 2b 27
4 BQ (4 equiv) toluene reflux 12 – –
5 NBS/TBPB (0.4 equiv) CCl4 reflux 5 – – a
6 MnO2 (10 equiv) benzene 70 48 2b’ 40
7 MnO2 (25 equiv) benzene 70 24 2b’ 55
8 Na2Cr2O7 (1 equiv) CHCl3 reflux 12 – – a
9 I2 (2 equiv) CH3OH reflux 24 – – a
10 PCC (3.3 equiv) CH2Cl2 reflux 2 – –
11 Pd/C (1 equiv) AcOEt reflux 24 – – a
12 CAN (1 equiv) CH2Cl2 rt 24 4c 38
13 CuCl2∙2H2O (0.1 equiv) DMSO 100 8 4c 80
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a Starting 1b was recovered.

On the other hand, the use of oxidizing systems such as ceric ammonium nitrate (CAN) and catalytic copper (II) chloride dihydrate in DMSO did not afford the expected product 2b, providing instead the non-fused indole-pyrazole 4c (Table 1, entries 12 and 13).

With these results in hand, also tetracylic compounds incorporating 6 and 8 membered fused ring such as 1a and 1c were subjected to optimal oxidation conditions to obtain the relative compounds 2a/2a’ and 2c’ respectively (Scheme 1). Intrigued by the possibility of obtaining fully aromatic derivatives, the treatment of representative 6, 7, and 8 membered tetrahydro-1H-pyridazino[3,4-b]indole 2a, 2b’, 2c’ with alcoholic KOH was also carried out. To our delight, the formation of cyclo-fused pyridazino[3,4-b]indoles 3a, 3b’ and 3c’ (via hydrolysis of ester and carbamoyl residues followed by decarboxylation with concomitant aromatization of the pyridazine ring) was registered with success (43–68% yields) (Scheme 1).

Scheme 1.

Scheme 1

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Two-step synthesis of cyclo-fused pyridazino[3,4-b]indoles 3a, 3b’, 3c’.

Encouraged by the unexpected formation of non-fused indole-pyrazol-5-one 4c during the investigation for indoline oxidation to indole, we next focused our attention to exploring this transformation. We identified that the use of trifluoroacetic acid (TFA) as common Brønsted acid induced the ring-opening of the cycloadduct 1b to give non-fused N-polyheterocyclic compound 4c in excellent yield (95%). It was found that all the tetracyclic compounds embedding 6, 7 and 8 membered-ring well tolerated the acidic environment (Scheme 2). Changing the substituents (alkyl, benzyl) on the N-indole ring, the reaction proceeded satisfactorily (4f, 4g, and 4i). Also, the free NH-indole was proven to be a good candidate for this reaction furnishing the relative products 4h, 4j, and 4p in very excellent yields. As expected, conversion to NH-pyrazol-5-one 4e obtained by removal of the t-butoxycarbonyl (Boc) protective group was observed under such conditions. The wide functional groups tolerance of this procedure was validated from the introduction of electron-donor (4i, and 4j) or electron-attractor (4k, 4l, 4m, 4n) substituents on the aromatic component which led to the corresponding indole linked pyrazol-5-one systems in almost quantitative yields. Interesting to note that no purification of the obtained products by flash chromatography column was necessary because of their precipitation from the reaction medium. A plausible mechanism could involve TFA-induced ring-opening of the tetrahydropyridazine, followed by sequential intramolecular nucleophilic acyl substitution (ring-closing) to form the final product accompanied by elimination of an alcohol molecule. This pathway reflects the well-established tendency of polycyclic fused indoline structures without 3-substituents to rearomatization [42]. Also, the formation of NH tautomeric form of II previously described by our group [32] supports this.

Scheme 2.

Scheme 2

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Substrate scope for the synthesis of indole linked pyrazol-5-ones 4.

To illustrate the synthetic potential of this transformation, a one-pot reaction involving the in situ formation of [4+2] cycloadduct 1b from N-methyl indole A1 and cyclic azoalkene B1 [32] was also performed. The reaction proceeded very well, and desired product 4c was obtained in 93% overall yield (Scheme 3).

Scheme 3.

Scheme 3

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One-pot synthesis of 4c from N-methyl indole A1 and cyclic azoalkene B1.

To date, only one example of the synthesis of compound of type 4 was documented in the literature [43]. In their work, Shi and co-workers realized an umpolung of C3 indole reactivity, using 2-indolylmethanols (C) as an electrophile and pyrazol-5-ones (D) as a nucleophile to obtain, under the cooperative catalysis of Pd(0) and a chiral phosphoric acid, the strategic C-C bond formation.

Differently from Shi’s work, in our case the same C-C bond formation is realized exploiting the reversal of polarity of the azoalkene (B) system (“umpoled” of carbonyl compounds) [44,45,46,47]. Taking advantage of the intrinsic reactivity of the pertinent substrates, a new and complementary synthetic approach toward these unusual N-polyheterocyclic structures can be successfully achieved (Scheme 4). We believe that the present method is of operational interest in terms of forming otherwise challenging-to-prepare C−C bonds.

Scheme 4.

Scheme 4

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Different synthetic approaches to indole linked pyrazol-5-one derivatives 4.

3. Materials and Methods

3.1. General

All the commercially available reagents and solvents were used without further purification. Tetrahydro-1H-pyridazino[3,4-b]indoles 1 were prepared via a formal [4+2] cycloaddition reactions of 2,3-unsubstituted indoles with cyclic azoalkenes by known procedures [32]. Chromatographic purification of compounds was carried out on silica gel (60–200 μm). Thin-layer chromatography (TLC) analysis was performed on pre-loaded (0.25 mm) glass supported silica gel plates (Silica gel 60, F254, Merck; Darmstadt, Germany); compounds were visualized by exposure to UV light. Melting points (m.p.) were determined in open capillary tubes and are uncorrected.

All 1H-NMR and 13C-NMR spectra were recorded at 400 and 100 MHz, respectively at 25 °C on a Bruker Ultrashield 400 spectrometer (Bruker, Billerica, MA, USA). Proton and carbon spectra were referenced internally to residual solvent signals as follows: δ = 2.50 ppm for proton (middle peak) and δ = 39.50 ppm for carbon (middle peak) in DMSO-d6 and δ = 7.27 ppm for proton and δ = 77.00 ppm for carbon (middle peak) in CDCl3. The following abbreviations are used to describe peak patterns where appropriate: s = singlet, d = doublet, t = triplet q = quartet, m = multiplet and br = broad signal. All coupling constants (J) are given in Hz. FT-IR spectra were measured as Nujol mulls using a Nicolet Impact 400 (Thermo Scientific, Madison, WI, USA). Low-resolution mass spectra (LRMS) was performed on a Waters Micromass Q-ToF instrument (Waters, Milford, MA, USA) using an ESI source. Elemental analyses were within ± 0.4 of the theoretical values (C, H, N).

3.2. Two-Step Procedure for the Synthesis of 3

3.2.1. Procedure for the Oxidation of Indolines 1 to Indoles 2

To a stirred solution of compound 1 (0.5 mmol) in benzene (1 mL), MnO2 (25 eq) was added and the reaction was subsequently warmed up to 70 °C (oil bath). The reaction mixture was kept at this temperature until the reagent had been completely consumed as monitored by TLC (20 h). The crude mixture was then filtered and purified by column chromatography on silica gel using hexane-ethyl acetate as the eluent to afford product 2 (or 2’).

Ethyl 6-carbamoyl-7-methyl-2,3,4,6,7,11c-hexahydro-1H-indolo[2,3-c]cinnoline-11c-carboxylate (2a). Yield 26% (46.1 mg) as a whitish solid; m.p. 179–181 °C; 1H-NMR, 400 MHz, CDCl3): δ 1.18 (t, J = 7.2 Hz, 3 H), 1.49–1.89 (m, 4 H), 1.96–2.01 (m, 1 H), 2.47 (dt, J1 = 14.4 Hz, J2 = 5.2 Hz, 1 H), 2.69–2.75 (m, 1 H), 3.45–3.52 (m, 1 H), 3.72 (s, 3 H), 4.05–4.20 (m, 2 H), 5.14 (br, 1 H), 6.58 (br, 1 H), 7.09 (dt, J1 = 8.0 Hz, J2 = 1.2 Hz, 1 H), 7.17 (dt, J1 = 8.0 Hz, J2 = 1.2 Hz, 1 H), 7.29 (d, J = 8.0 Hz, 1 H), 7.66 (d, J = 8.0 Hz, 1 H) ppm; 13C-NMR (100 MHz, CDCl3): δ 14.1, 22.9, 26.0, 33.4, 33.6, 35.4, 50.8, 61.8, 96.5, 109.9, 119.8, 120.1, 121.3, 123.6, 131.3, 136.8, 154.4, 156.2, 170.5 ppm; IR (Nujol, cm−1): νmax 3491, 3363, 1734, 1703; MS (ESI): m/z 355 [M + H]+; anal. calcd. for C19H22N4O3 (354.40): C 64.39, H 6.26, N 15.81; found: C 64.25, H 6.32, N 15.94.

Ethyl 6-carbamoyl-2,3,4,6,7,11c-hexahydro-1H-indolo[2,3-c]cinnoline-11c-carboxylate (2a’). Yield 17% (28.9 mg) as a whitish solid; m.p. 159–161 °C; 1H-NMR, 400 MHz, CDCl3): δ 1.22 (t, J = 7.2 Hz, 3 H), 1.51–1.89 (m, 4 H), 1.95−2.02 (m, 1 H), 2.48 (dt, J1 = 13.2 Hz, J2 = 4.8 Hz, 1 H), 2.62 (d, J = 13.2 Hz, 1 H), 3.34 (d, J = 13.2 Hz, 1 H), 4.09–4.29 (m, 2 H), 5.26 (br, 1 H), 6.79 (br, 1 H), 7.07–7.14 (m, 2 H), 7.32 (d, J = 7.2 Hz, 1 H), 7.63 (d, J = 7.2 Hz, 1 H), 10.13 (s, 1 H) ppm; 13C-NMR (100 MHz, CDCl3): δ 14.3, 23.3, 26.8, 34.3, 36.4, 50.4, 61.8, 91.9, 111.1, 119.2, 120.2, 121.1, 123.8, 129.6, 132.7, 151.8, 155.1, 170.6 ppm; IR (Nujol, cm−1): νmax 3455, 3425, 3297, 1720, 1686; MS (ESI): m/z 341 [M + H]+; C18H20N4O3 (340.37): C 63.52, H 5.92, N 16.46; found: C 63.69, H 5.84, N 16.34.

Methyl 7-carbamoyl-8-methyl-1,2,3,4,5,7,8,12c-octahydrocyclohepta[5,6]pyridazino[3,4-b]indole-12c-carboxylate (2b). Yield 27% (47.8 mg) as a whitish oil; 1H-NMR (400 MHz, DMSO-d6, 25 °C): δ 1.00−1.10 (m, 1 H), 1.17–1.29 (m, 1 H), 1.40−1.49 (m, 1 H), 1.58–1.67 (m, 1 H), 1.75–1.82 (m, 1 H), 1.89−1.97 (m, 1 H), 2.17–2.24 (m, 1 H), 2.55−2.63 (m, 1 H), 2.77–2.83 (m, 1 H), 3.06–3.13 (m, 1 H), 3.53 (s, 3 H), 3.61 (s, 3 H), 6.99 (br, 2 H), 7.05 (dt, J1 = 8.0 Hz, J2 = 0.8 Hz, 1 H), 7.13 (dt, J1 = 8.0 Hz, J2 = 0.8 Hz, 1 H), 7.41 (d, J = 8.0 Hz, 1 H), 7.56 (d, J = 7.2 Hz, 1 H); 13C-NMR (100 MHz, DMSO-d6, 25 °C): δ 23.6, 28.9, 29.3, 32.3, 32.6, 33.4, 52.5, 52.6, 94.7, 109.9, 118.4, 119.9, 120.6, 122.7, 133.8, 136.3, 153.1, 158.9, 171.7; IR (Nujol, cm−1): νmax 3488, 3375, 1729, 1708; MS (ESI) m/z 355 [M + H]+; anal. calcd. for C19H22N4O3 (354.40): C 64.39, H 6.26, N 15.18; found: C 64.26, H 6.34, N 15.29.

Methyl 7-carbamoyl-1,2,3,4,5,7,8,12c-octahydrocyclohepta[5,6]pyridazino[3,4-b]indole-12c-carboxylate (2b’). Yield 55% (93.6 mg) as a white solid; m.p. 181–183 °C; 1H-NMR, 400 MHz, DMSO-d6): δ 1.10–1.23 (m, 1 H), 1.38–1.47 (m, 2 H), 1.65–1.74 (m, 2 H), 1.89–1.93 (m, 1 H), 2.19–2.35 (m, 2 H), 2.65–2.71 (m, 1 H), 2.84–2.90 (m, 1 H), 3.57 (s, 3 H), 6.93−7.01 (m, 2 H), 7.26 (s, 2 H), 7.44–7.50 (m, 2 H), 11.21 (s, 1 H) ppm; 13C-NMR (100 MHz, DMSO-d6): δ 22.9, 29.2, 29.6, 33.7, 33.8, 51.5, 52.4, 89.9, 112.2, 117.7, 119.5, 120.1, 123.1, 131.0, 133.4, 150.9, 154.1, 171.9 ppm; IR (Nujol, cm−1): νmax 3491, 3430, 3297, 1734, 1707; MS (ESI): m/z 341 [M + H]+; anal. calcd. for C18H20N4O3 (340.38): C 63.52, H 5.92, N 16.46; found: C 63.39, H 6.01, N 16.57.

Ethyl 8-carbamoyl-2,3,4,5,6,8,9,13c-octahydro-1H-cycloocta[5,6]pyridazino[3,4-b]indole-13c-carboxylate (2c’). Yield 45% (82.9 mg) as a white solid; m.p. 192–194 °C; 1H-NMR, 400 MHz, CDCl3): δ 1.15 (t, J = 7.2 Hz, 3 H), 1.18–1.24 (m, 1 H), 1.35–1.55 (m, 4 H), 1.61−1.81 (m, 2 H), 1.94–2.01 (m, 1 H), 2.33−2.48 (m, 2 H), 2.65–2.71 (m, 1 H), 2.87−2.93 (m, 1 H), 4.01–4.20 (m, 2 H), 5.07 (br, 1 H), 6.76 (br, 1 H), 7.06–7.14 (m, 2 H), 7.32 (d, J = 8.0 Hz, 1 H), 7.60 (d, J = 8.0 Hz, 1 H); 10.13 (s, 1 H) ppm; 13C-NMR (100 MHz, CDCl3): δ 14.2, 23.7, 24.2, 24.9, 28.3, 31.3, 32.9, 52.1, 61.7, 89.6, 111.2, 118.9, 120.3, 121.2, 124.3, 131.1, 132.8, 154.6, 155.0, 171.5 ppm; IR (Nujol, cm−1): νmax 3410, 3276, 3220, 1724, 1698; MS (ESI): m/z 369 [M + H]+; anal. calcd. for C20H24N4O3 (368.43): C 65.20, H 6.57, N 15.21; found: C 65.06, H 6.63, N 15.30.

3.2.2. Procedure for the Preparation of Fused Indole-Pyridazine 3 from 2

To a stirred solution of KOH (10 eq.) in alcohol (3 mL), the compound 2 (or 2’) (0.2 mmol) was added and the mixture was refluxed until the disappearance of the reagent (TLC check, 7 h). The crude mixture was then filtered and purified by column chromatography on silica gel to afford product 3(or 3’).

1,2,3,4,5,8-Hexahydrocyclohepta[5,6]pyridazino[3,4-b]indole (3a). Yield 43% (20.5 mg) as a yellowish solid; m.p. 167–169 °C; 1H-NMR (400 MHz, DMSO-d6): δ 1.92–1.96 (m, 3 H), 2.07–2.09 (m, 3 H), 2.13–2.20 (m, 2 H), 3.99 (s, 3 H), 7.31–7.36 (m, 1 H), 7.68−7.74 (m, 2 H), 8.20 (d, J = 8.0 Hz, 1 H) ppm; 13C-NMR (100 MHz, DMSO-d6): δ 21.6, 22.5, 25.7, 27.9, 29.8, 109.9, 116.4, 118.3, 120.4, 125.1, 129.4, 130.5, 141.5, 151.1, 152.2 ppm; IR (Nujol, cm−1): νmax no significant signals were detected; MS (ESI): m/z 238 [M + H]+; anal. calcd. for C15H15N3 (237.39): C 75.92, H 6.37, N 17.71; found: C 76.03, H 6.45, N 17.59.

1,2,3,4,5,8-Hexahydrocyclohepta[5,6]pyridazino[3,4-b]indole (3b’). Yield 52% (24.7 mg) as a yellowish solid; m.p. 180–182 °C; 1H-NMR (400 MHz, DMSO-d6): δ 1.66–1.71 (m, 2 H), 1.75–1.81 (m, 2 H), 1.88–1.94 (m, 2 H), 3.30–3.34 (m, 2 H), 3.39–3.43 (m, 2 H), 7.26 (t, J = 8.0 Hz, 1 H), 7.53 (d, J = 8.0 Hz, 1 H), 7.59 (t, J = 8.0 Hz, 1 H), 8.35 (d, J = 8.0 Hz, 1 H), 12.10 (s, 1 H) ppm; 13C-NMR (100 MHz, DMSO-d6): δ 26.8, 26.9, 30.2, 32.2, 34.9, 113.1, 115.3, 119.1, 121.2, 125.0, 130.7, 138.7, 142.4, 153.3, 156.4 ppm; IR (Nujol, cm−1): νmax no significant signals were detected; MS (ESI): m/z 238 [M + H]+; anal. calcd. for C15H15N3 (237.29): C 75.92, H 6.37, N 17.71; found: C 76.07, H 6.44, N 17.62.

2,3,4,5,6,9-Hexahydro-1H-cycloocta[5,6]pyridazino[3,4-b]indole (3c’). Yield 68% (34.2 mg) as a yellowish solid; m.p. 195–197 °C; 1H-NMR (400 MHz, DMSO-d6): δ 1.18–1.31 (m, 2 H), 1.38–1.44 (m, 2 H), 1.70–1.81 (m, 2 H), 1.83–1.95 (m, 2 H), 3.24–3.29 (m, 2 H), 3.31–3.41 (m, 2 H), 7.29 (t, J = 8.0 Hz, 1 H), 7.55 (d, J = 8.0 Hz, 1 H), 7.61 (t, J = 8.0 Hz, 1 H), 8.21 (d, J = 8.0 Hz, 1 H), 12.12 (s, 1 H) ppm; 13C-NMR (100 MHz, DMSO-d6): δ 25.5, 25.7, 26.5, 28.3, 31.4, 31.7, 111.8, 116.3, 118.5, 120.2, 124.6, 129.2, 133.5, 140.9, 153.8, 154.9 ppm; IR (Nujol, cm−1): νmax no significant signals were detected; MS (ESI): m/z 252 [M + H]+; anal. calcd. for C16H17N3 (251.32): C 76.46, H 6.82, N 16.72; found: C 76.32, H 6.89, N 16.60.

3.3. General Procedure for the Synthesis of Indole Linked Pyrazol-5-ones 4

To a stirred solution of compound 1 (0.2 mmol) in methylene chloride (2 mL), TFA (2.5 eq) was added at room temperature. The reaction was refluxed until TLC indicated the disappearance of the reagent (TLC check, 2 h). Removed the solvent under reduced pressure, the crude mixture was diluted with water and extracted with EtOAc (2 × 10 mL). The organic phase was dried with Na2SO4 and solvent was evaporated in vacuo. The compound 4 was collected by precipitation as a white solid, filtered, and washed with ethyl ether.

3a-(1-methyl-1H-indol-3-yl)-3-oxo-3,3a,4,5,6,7-hexahydro-2H-indazole-2-carboxamide (4a). Yield 92% (57.2 mg) as a white solid; m.p. 192–194 °C; 1H-NMR (400 MHz, DMSO-d6): δ 1.39–1.52 (m, 1 H), 1.58–1.78 (m, 3 H), 1.92–1.99 (m, 1 H), 2.21 (dt, J1 = 13.2 Hz, J2 = 5.6 Hz, 1 H), 2.59 (d, J = 12.8 Hz, 1 H), 2.82 (d, J = 12.8 Hz, 1 H), 3.80 (s, 3 H), 7.02 (t, J = 8.0 Hz, 1 H), 7.18 (t, J = 8.0 Hz, 1 H), 7.24 (d, J = 8.0 Hz, 1 H), 7.26 (br, 1 H), 7.45 (d, J = 8.0 Hz, 1 H), 7.53 (br, 1 H), 7.57 (s, 1 H) ppm; 13C-NMR (100 MHz, DMSO-d6): δ 20.9, 27.0, 28.0, 32.6, 33.5, 54.3, 105.7, 110.3, 117.8, 119.5, 121.7, 125.1, 129.1, 136.9, 149.7, 166.5, 176,8 ppm; IR (Nujol, cm−1): νmax 3399, 3251, 1726, 1702; MS (ESI): m/z 311 [M + H]+; anal. calcd. for C17H18N4O2 (310.35): C 65.79, H 5.85, N 18.05; found: C 65.64, H 5.92, N 17.92.

3a-(1-methyl-1H-indol-3-yl)-3-oxo-N-phenyl-3,3a,4,5,6,7-hexahydro-2H-indazole-2-carboxamide (4b). Yield 90% (69.6 mg) as a white solid; m.p. 220–222 °C; 1H-NMR (400 MHz, DMSO-d6): δ 1.44–1.57 (m, 1 H), 1.59–1.75 (m, 2 H), 1.86 (dt, J1 = 13.2 Hz, J2 = 5.6 Hz, 1 H), 1.96–2.01 (m, 1 H), 2.26 (dt, J1 = 13.2 Hz, J2 = 5.6 Hz, 1 H), 2.68 (d, J = 12.8 Hz, 1 H), 2.85 (d, J = 12.8 Hz, 1 H), 3.81 (s, 3 H), 7.04 (dt, J1 = 8.0 Hz, J2 = 0.8 Hz, 1 H), 7.09 (dt, J1 = 8.0 Hz, J2 = 0.8 Hz, 1 H), 7.18 (dt, J1 = 8.0 Hz, J2 = 0.8 Hz, 1 H), 7.27–7.38 (m, 3 H), 7.46 (d, J = 8.0 Hz, 1 H), 7.57 (d, J = 8.0 Hz, 2 H), 7.62 (s, 1 H), 9.82 (s, 1 H) ppm; 13C-NMR (100 MHz, DMSO-d6): δ 20.9, 27.1, 28.0, 32.6, 33.5, 54.5, 105.6, 110.4, 117.9, 119.7, 120.1, 121.7, 123.9, 125.2, 128.9, 129.2, 137.0, 137.4, 147.0, 167.2, 176.6 ppm; IR (Nujol, cm−1): νmax 3236, 1744, 1709; MS (ESI): m/z 387 [M + H]+; anal. calcd. for C23H22N4O2 (386.45): C 71.48, H 5.74, N 14.50; found: C 71.35, H 5.83, N 14.62.

8a-(1-Methyl-1H-indol-3-yl)-1-oxo-4,5,6,7,8,8a-hexahydrocyclohepta[c]pyrazole-2(1H)-carboxamide (4c). Yield 95% (61.6 mg) as a white solid; m.p. 131–133 °C; 1H-NMR (400 MHz, DMSO-d6): δ 1.28–1.35 (m, 1 H), 1.48–1.69 (m, 4 H), 1.76–1.85 (m, 1 H), 2.15–2.23 (m, 1 H), 2.37–2.46 (m, 2 H), 2.69–2.75 (m, 1 H), 3.78 (s, 3 H), 7.02 (dt, J1 = 8.0 Hz, J2 = 0.8 Hz, 1 H), 7.17 (dt, J1 = 8.0 Hz, J2 = 0.8 Hz, 1 H), 7.24 (br, 1 H), 7.31 (d, J = 8.0 Hz, 1 H), 7.44 (d, J = 8.0 Hz, 1 H), 7.46 (s, 1 H), 7.53 (br, 1 H) ppm; 13C-NMR (100 MHz, DMSO-d6): δ 24.7, 25.9, 28.5, 29.3, 32.6, 32.8, 58.5, 106.9, 110.3, 118.4, 119.5, 121.7, 124.8, 128.5, 136.9, 149.4, 166.7, 176.6 ppm; IR (Nujol, cm−1): νmax 3425, 3267, 1729, 1683; MS (ESI): m/z 325 (M + H)+; anal. calcd. for C18H20N4O2 (324.38): C 66.65, H 6.21, N 17.27; found: C 66.51, H 6.28, N 17.39; found: C 66.64, H 6.17, N 17.27.

3a-(1-methyl-1H-indol-3-yl)-3-oxo-N-phenyl-3a,4,5,6,7,8-hexahydrocyclohepta[c]pyrazole-2(3H)-carboxamide (4d). Yield 90% (72.1 mg) as a white solid; m.p. 166-168 °C; 1H-NMR (400 MHz, DMSO-d6): δ 1.35–1.45 (m, 1 H), 1.50–1.71 (m, 4 H), 1.78–1.87 (m, 1 H), 2.23–2.31 (m, 1 H), 2.43–2.55 (m, 2 H), 2.77–2.85 (m, 1 H), 3.79 (s, 3 H), 7.05 (t, J = 7.2 Hz, 1 H), 7.09 (t, J = 7.2 Hz, 1 H), 7.18 (t, J = 7.6 Hz, 1 H), 7.33 (t, J = 8.0 Hz, 2 H), 7.41 (d, J = 8.0 Hz, 1 H), 7.45 (d, J = 8.0 Hz, 1 H), 7.50 (s, 1 H), 7.58 (d, J = 8.0 Hz, 2 H), 9.78 (s, 1 H) ppm; 13C-NMR (100 MHz, DMSO-d6): δ 24.7, 25.7, 28.4, 29.4, 32.5, 32.8, 58.7, 106.7, 110.3, 118.6, 119.6, 120.1, 121.7, 123.9, 124.9, 128.6, 128.8, 136.9, 137.3, 146.7, 167.4, 176.4 ppm; IR (Nujol, cm−1): νmax 3251, 1739, 1708; MS (ESI): m/z 401 (M + H)+; anal. calcd. for C24H24N4O2 (400.47): C 71.98, H 6.04, N 13.99; found: C 71.90, H 6.15, N 14.15.

3a-(1-methyl-1H-indol-3-yl)-3a,4,5,6,7,8-hexahydrocyclohepta[c]pyrazol-3(2H)-one (4e). Yield 97% (54.6 mg) as a white solid; m.p. 250–252 °C; 1H-NMR (400 MHz, DMSO-d6): δ 1.20–1.31 (m, 1 H), 1.41–1.63 (m, 4 H), 1.77–1.85 (m, 1 H), 1.98–2.05 (m, 1 H), 2.25–2.34 (m, 2 H), 2.56–2.63 (m, 1 H), 3.77 (s, 3 H), 6.98 (dt, J1 = 8.0 Hz, J2 = 0.8 Hz, 1 H), 7.14 (dt, J1 = 8.0 Hz, J2 = 0.8 Hz, 1 H), 7.32 (d, J = 8.0 Hz, 1 H), 7.37 (s, 1 H), 7.41 (d, J = 8.0 Hz, 1 H), 11.14 (s, 1 H) ppm; 13C-NMR (100 MHz, DMSO-d6): δ 24.8, 26.3, 28.7, 29.4, 32.4, 32.9, 55.4, 108.3, 109.9, 18.7, 119.0, 121.3, 125.2, 128.1, 136.8, 165.9, 178.9 ppm; IR (Nujol, cm−1): νmax 3165, 1714; MS (ESI): m/z 282 [M + H]+; anal. calcd. for C17H19N3O (281.35): C 72.57, H 6.81, N 14.94; found: C 72.43, H 6.90, N 15.02.

3a-(1-ethyl-1H-indol-3-yl)-3-oxo-3a,4,5,6,7,8-hexahydrocyclohepta[c]pyrazole-2(3H)-carboxamide (4f). Yield 93% (62.9 mg) as a white solid; m.p. 191-193 °C; 1H-NMR (400 MHz, DMSO-d6): δ 1.27–1.38 (m, 1 H), 1.36 (t, J = 7.2 Hz, 3 H), 1.45–1.68 (m, 4 H), 1.77–1.85 (m, 1 H), 2.16–2.24 (m, 1 H), 2.40–2.46 (m, 2 H), 2.68–2.75 (m, 1 H), 4.21 (q, J = 7.2 Hz, 2 H), 7.01 (dt, J1 = 7.2 Hz, J2 = 0.8 Hz, 1 H), 7.16 (dt, J1 = 7.2 Hz, J2 = 0.8 Hz, 1 H), 7.25 (br, 1 H), 7.31 (d, J = 8.0 Hz, 1 H), 7.49 (d, J = 8.0 Hz, 1 H), 7.51 (s, 1 H), 7.53 (br, 1 H) ppm; 13C-NMR (100 MHz, DMSO-d6): δ 15.3, 24.7, 25.9, 28.5, 29.3, 32.8, 40.4, 58.5, 107.1, 110.3, 118.5, 119.5, 121.6, 124.9, 126.9, 135.9, 149.4, 166.7, 176.7 ppm; IR (Nujol, cm−1): νmax 3384, 3175, 1734, 1683; MS (ESI): m/z 339 [M + H]+; anal. calcd. for C19H22N4O2 (338.40): C 67.44, H 6.55, N 16.56.; found: C 67.29, H 6.62, N 16.43.

3a-(1-benzyl-1H-indol-3-yl)-3-oxo-3a,4,5,6,7,8-hexahydrocyclohepta[c]pyrazole-2(3H)-carboxamide (4g). Yield quant. (80.0 mg) as a white solid; m.p. 181-183 °C; 1H-NMR (400 MHz, DMSO-d6): δ 1.30–1.41 (m, 1 H), 1.44–1.67 (m, 4 H), 1.77–1.83 (m, 1 H), 2.14–2.21 (m, 1 H), 2.43–2.49 (m, 2 H), 2.71–2.77 (m, 1 H), 5.44 (s, 2 H), 7.02 (t, J = 8.0 Hz, 1 H), 7.12 (t, J = 8.0 Hz, 1 H), 7.19–7.33 (m, 6 H), 7.35 (d, J = 8.0 Hz, 1 H), 7.45 (d, J = 8.0 Hz, 1 H), 7.59 (br, 1 H), 7.70 (s, 1 H) ppm; 13C-NMR (100 MHz, DMSO-d6): δ 24.8, 25.7, 28.4, 29.4, 32.9, 49.2, 58.5, 107.5, 110.8, 118.6, 119.7, 121.8, 125.2, 126.9, 127.4, 128.2, 128.5, 136.3, 137.9, 149.4, 166.7, 176.6 ppm; IR (Nujol, cm−1): νmax 3409, 3257, 1759, 1724; MS (ESI): m/z 401 [M + H]+; anal. calcd. for C24H24N4O2 (400.47): C 71.98, H 6.04, N 13.99; found: C 72.12, H 5.95, N 13.85.

3a-(1H-indol-3-yl)-3-oxo-3a,4,5,6,7,8-hexahydrocyclohepta[c]pyrazole-2(3H)-carboxamide (4h). Yield quant. (62.1 mg) as a white solid; m.p. 201–203 °C; 1H-NMR (400 MHz, DMSO-d6): δ 1.17–1.81 (m, 6 H), 2.14–2.22 (m, 1 H), 2.30–2.45 (m, 2 H), 2.68–2.82 (m, 1 H), 6.98–7.57 (m, 7 H), 11.35 (s, 1 H) ppm; 13C-NMR (100 MHz, DMSO-d6): δ 24.7, 26.0, 28.5, 29.3, 32.7, 58.6, 107.8, 112.0, 118.2, 119.4, 121.6, 124.4, 124.5, 136.6, 149.5, 166.8, 176.8 ppm; IR (Nujol, cm−1): νmax 3404, 3287, 3257, 1734, 1703; MS (ESI): m/z 311 [M + H]+; anal. calcd. for C17H18N4O2 (310.35): C 65.79, H 5.85, N 18.05; found: C 65.94, H 5.79, N 17.91.

3a-(5-methyl-1-propyl-1H-indol-3-yl)-3-oxo-3a,4,5,6,7,8-hexahydrocyclohepta[c]pyrazole-2(3H)-carboxamide (4i). Yield quant. (73.3 mg) as a white solid; m.p. 195–197 °C; 1H-NMR (400 MHz, DMSO-d6): δ 0.80 (t, J = 7.2 Hz, 3 H), 1.30–1.41 (m, 1 H), 1.44–1.63 (m, 4 H), 1.72 (sex, J = 7.2 Hz, 2 H), 1.79–1.86 (m, 1 H), 2.10–2.18 (m, 1 H), 2.33 (s, 3 H), 2.38–2.47 (m, 2 H), 2.67–2.78 (m, 1 H), 4.10 (t, J = 7.2 Hz, 2 H), 6.97 (d, J = 8.4 Hz, 1 H), 7.10 (s, 1 H), 7.28 (br, 1 H), 7.37 (d, J = 8.4 Hz, 1 H), 7.43 (s, 1 H), 7.57 (br, 1 H) ppm; 13C-NMR (100 MHz, DMSO-d6): δ 11.1, 21.3, 22.9, 24.8, 25.7, 28.4, 29.4, 32.9, 47.1, 58.5, 106.2, 110.2, 118.1, 123.1, 125.2, 127.7, 127.9, 134.7, 149.5, 166.7, 176.7 ppm; IR (Nujol, cm−1): νmax 3369, 3175, 1734, 1688; MS (ESI): m/z 367 [M + H]+; anal. calcd. for C21H26N4O2 (366.45): C 68.83, H 7.15, N 15.29; found: C 68.99, H 7.06, N 15.16.

3a-(5-methyl-1H-indol-3-yl)-3-oxo-3a,4,5,6,7,8-hexahydrocyclohepta[c]pyrazole-2(3H)-carboxamide (4j). Yield quant. (64.9 mg) as a white solid; m.p. 209–211 °C; 1H-NMR (400 MHz, DMSO-d6): δ 1.23–1.37 (m, 1 H), 1.40–1.69 (m, 4 H), 1.73–1.84 (m, 1 H), 2.15–2.23 (m, 1 H), 2.32 (s, 3 H), 2.39–2.45 (m, 2 H), 2.69–2.75 (m, 1 H), 6.93 (d, J = 8.4 Hz, 1 H), 7.09 (s, 1 H), 7.28 (d, J = 8.4 Hz, 1 H), 7.31 (br, 1 H), 7.37 (d, J = 2.4 Hz, 1 H), 7.56 (br, 1 H), 11.21 (s, 1 H) ppm; 13C-NMR (100 MHz, DMSO-d6): δ 21.4, 24.7, 25.9, 28.5, 29.3, 32.8, 58.6, 107.2, 111.7, 117.8, 123.1, 124.3, 124.7, 127.8, 134.9, 149.5, 166.8, 176.8 ppm; IR (Nujol, cm−1): νmax 3399, 3328, 3272, 1729, 1714; MS (ESI): m/z 325 [M + H]+; anal. calcd. for C18H20N4O2 (324.37): C 66.65, H 6.21, N 17.27; found: C 66.52, H 6.13, N 17.39.

3a-(6-chloro-1-methyl-1H-indol-3-yl)-3-oxo-3a,4,5,6,7,8-hexahydrocyclohepta[c]pyrazole-2(3H)-carboxamide (4k). Yield 96% (68.9 mg) as a white solid; m.p. 229–231 °C; 1H-NMR (400 MHz, DMSO-d6): δ 1.29–1.35 (m, 1 H), 1.44–1.67 (m, 4 H), 1.72–1.82 (m, 1 H), 2.15–2.23 (m, 1 H), 2.34– 2.48 (m, 2 H), 269–2.76 (m, 1 H), 3.78 (s, 3 H), 7.07 (dd, J1 = 8.8 Hz, J2 = 2.0 Hz, 1 H), 7.23 (br, 1 H), 7.33 (d, J = 8.8 Hz, 1 H), 7.51 (s, 1 H), 7.57 (br, 1 H), 7.60 (d, J = 2.0 Hz, 1 H) ppm; 13C-NMR (100 MHz, DMSO-d6): δ 24.7, 25.9, 28.4, 29.3, 32.7, 32.8, 58.4, 107.4, 110.3, 119.8, 119.9, 123.5, 126.7, 129.6, 137.4, 149.3, 166.5, 176.3 ppm; IR (Nujol, cm−1): νmax 3420, 3262, 1754, 1724; MS (ESI): m/z 359 [M + H]+; anal. calcd. for C18H19ClN4O2 (358.82): C 60.25, H 5.34, N 15.61; found: C 60.41, H 5.27, N 15.50.

3a-(5-cyano-1-methyl-1H-indol-3-yl)-3-oxo-3a,4,5,6,7,8-hexahydrocyclohepta[c]pyrazole-2(3H)-carboxamide (4l). Yield 99% (69.2 mg) as a white solid; m.p. 176-178 °C; 1H-NMR (400 MHz, DMSO-d6): δ 1.23–1.38 (m, 1 H), 1.40–1.79 (m, 5 H), 2.26–2.55 (m, 3 H), 2.76–2.82 (m, 1 H), 3.84 (s, 3 H), 7.25 (br, 1 H), 7.54 (d, J = 8.4 Hz, 1 H), 7.57 (br, 1 H), 7.65 (s, 1 H), 7.66 (d, J = 8.4 Hz, 1 H), 7.88 (s, 1 H) ppm; 13C-NMR (100 MHz, DMSO-d6): δ 24.6, 26.0, 28.5, 29.4, 32.7, 32.8, 58.4, 101.7, 108.5, 111.9, 120.3, 124.2, 124.3, 124.4, 131.1, 138.6, 149.3, 166.2, 176.0 ppm; IR (Nujol, cm−1): νmax 3399, 3302, 1739, 1724; MS (ESI): m/z 350 [M + H]+; anal. calcd. for C19H19N5O2 (349.38): C 65.32, H 5.48, N 20.04; found: C 65.47, H 5.37, N 19.96.

Methyl 3-(2-carbamoyl-3-oxo-2,3,3a,4,5,6,7,8-octahydrocyclohepta[c]pyrazol-3a-yl)-1-methyl-1H-indole-5-carboxylate (4m). Yield 98% (74.9 mg) as a white solid; m.p. 201–203 °C; 1H-NMR (400 MHz, DMSO-d6): δ 1.25–1.38 (m, 1 H), 1.45–1.83 (m, 5 H), 2.22–2.43 (m, 2 H), 2.49–2.56 (m, 2 H), 2.73–2.85 (m, 1 H), 3.82 (s, 3 H), 3.84 (s, 3 H), 7.25 (br, 1 H), 7.55 (d, J = 8.8 Hz, 1 H), 7.59 (s, 1 H), 7.79 (dd, J1 = 8.8 Hz, J2 = 1.2 Hz, 1 H), 8.17 (d, J = 1.2 Hz, 1 H) ppm; 13C-NMR (100 MHz, DMSO-d6): δ 24.7, 26.0, 28.0, 29.4, 32.8, 33.0, 51.8, 58.5, 108.8, 110.4, 120.9, 121.6, 122.5, 124.4, 130.3, 139.3, 149.3, 166.3, 166.9, 176.3 ppm; IR (Nujol, cm−1): νmax 3440, 3343, 1754, 1709; MS (ESI): m/z 383 [M + H]+; anal. calcd. for C20H22N4O4 (382.41): C 62.82, H 5.80, N 14.65; found: C 62.98, H 5.74, N 14.53.

3a-(1-methyl-5-nitro-1H-indol-3-yl)-3-oxo-3a,4,5,6,7,8-hexahydrocyclohepta[c]pyrazole-2(3H)-carboxamide (4n). Yield 98% (72.4 mg) as a yellow solid; m.p. 232–234 °C; 1H-NMR (400 MHz, DMSO-d6): δ 1.25–1.75 (m, 6 H), 2.27–2.44 (m, 2 H), 2.48–2.58 (m, 1 H), 2.72–2.88 (m, 1 H), 3.87 (s, 3 H), 7.25 (br, 1 H), 7.61 (br, 1 H), 7.68 (d, J = 8.4 Hz, 1 H), 7.75 (s, 1 H), 8.07 (d, J = 8.4 Hz, 1 H), 8.40 (s, 1 H) ppm; 13C-NMR (100 MHz, DMSO-d6): δ 24.6, 26.1, 28.5, 29.4, 33.0, 33.2, 58.4, 110.1, 111.2, 115.9, 116.9, 123.8, 132.3, 139.9, 140.9, 149.2, 166.1, 175.9 ppm; IR (Nujol, cm−1): νmax 3399, 3308, 1739, 1719; MS (ESI): m/z 370 [M + H]+; anal. calcd. for C18H19N5O4 (369.37): C 58.53, H 5.18, N 18.96; found: 58.68, H 5.11, N 18.84.

3a-(1-methyl-1H-indol-3-yl)-3-oxo-3,3a,4,5,6,7,8,9-octahydro-2H-cycloocta[c]pyrazole-2-carboxamide (4o). Yield quant. (67.7 mg) as a white solid; m.p. 207–209 °C; 1H-NMR (400 MHz, DMSO-d6): δ 0.89–1.04 (m, 1 H), 1.36–1.79 (m, 7 H), 2.12–2.26 (m, 1 H), 2.42–2.54 (m, 3 H), 3.79 (s, 3 H), 7.01 (t, J = 7.6 Hz, 1 H), 7.07 (d, J = 7.6 Hz, 1 H), 7.17 (t, J = 7.6 Hz, 1 H), 7.37 (br, 1 H), 7.44 (d, J = 7.6 Hz, 1 H), 7.57 (s, 1 H), 7.68 (br, 1 H) ppm; 13C-NMR (100 MHz, DMSO-d6): δ 22.4, 25.1, 25.5, 28.0, 28.9, 31.3, 32.6, 57.9, 107.5, 110.3, 117.3, 119.6, 121.7, 124.7, 128.8, 136.9, 149.3, 167.1, 176.9 ppm; IR (Nujol, cm−1): νmax 3404, 3272, 1739, 1698; MS (ESI): m/z 339 [M + H]+; anal. calcd. for C19H22N4O2 (338.40): C 67.44, H 6.55, N 16.56; found: C 67.29, H 6.48, N 16.67.

3a-(1H-indol-3-yl)-3-oxo-3,3a,4,5,6,7,8,9-octahydro-2H-cycloocta[c]pyrazole-2-carboxamide (4p). Yield 90% (58.4 mg) as a white solid; m.p. 215–217 °C; 1H-NMR (400 MHz, DMSO-d6): δ 0.92–1.03 (m, 1 H), 1.38–1.80 (m, 7 H), 2.10–2.19 (m, 1 H), 2.41–2.51 (m, 2 H), 2.56–2.60 (m, 1 H), 6.96 (dt, J1 = 7.6 Hz, J2 = 1.2 Hz, 1 H), 7.02 (d, J = 7.6 Hz, 1 H), 7.09 (dt, J1 = 7.6 Hz, J2 = 1.2 Hz, 1 H), 7.36 (br, 1 H), 7.39 (d, J = 7.6 Hz, 1 H), 7.56 (d, J = 2.4 Hz, 1 H), 7.65 (br, 1 H), 11.37 (s, 1 H) ppm; 13C-NMR (100 MHz, DMSO-d6): δ 22.4, 25.1, 25.5, 28.0, 28.9, 31.2, 58.0, 108.3, 112.1, 117.1, 119.5, 121.6, 124.3, 124.7, 136.5, 149.4, 167.3, 177.0 ppm; IR (Nujol, cm−1): νmax 3379, 3297, 3241, 1744, 1714; MS (ESI): m/z 325 [M + H]+; anal. calcd. for C18H20N4O2 (324.37): C 66.65, H 6.21, N 17.27; found: C 66.49, H 6.30, N 17.38.

3.4. One-Pot Procedure for the Synthesis of 4c

A mixture of N-methyl indole A1 (1.4 mmol), cyclic azoalkene B1 (1.0 mmol) and zinc dichloride (0.1 mmol) was stirred in dry dichloromethane (2 mL) at room temperature. After the disappearance of azoalkene B1 (TLC check), TFA (2.5 eq) was directly added to the reaction medium. The reaction was refluxed until TLC indicated the disappearance of the intermediate (TLC check, 2 h). Removed the solvent under reduced pressure, the crude mixture was diluted with water and extracted with EtOAc (2 × 10 mL). The organic phase was dried with Na2SO4 and solvent was evaporated in vacuo. The crude mixture was purified by column chromatography on silica gel to afford product 4c in 93% yield.

4. Conclusions

In conclusion, successful routes providing indole-based heterocycles such as fused indole-pyridazines and non-fused indole-pyrazol-5-ones have been accomplished. Mild and practical reaction conditions, wide substrate scope in conjunction with functional group tolerance make these protocols particularly attractive. Moreover, crude products are usually obtained in high purity and high yield by simple precipitation from the reaction medium. We expect that these methodologies and the chemistry described here would be a new addition to the indole chemistry and would find wide usage in both organic and medicinal chemistry.

Acknowledgments

The authors gratefully thank Anna Maria Gioacchini who competently performed the mass spectra and the Department of Biomolecular Science of the University of Urbino for the economic support.

Supplementary Materials

The following are available online: copies of 1H-NMR and 13C-MNR spectra of all newly synthesized compounds; copies of HMQC and HMBC of compound 2b’.

Click here for additional data file. (5.1MB, pdf)

Author Contributions

C.C. Conceptualization, Investigation, and Methodology; L.D.C. Validation; F.M. Funding Acquisition; G.M. Data Curation; S.S. Supervision; G.F. Project Administration and Writing—Review and Editing. All authors have read and agreed to the published version of the manuscript.

Funding

The authors declare no competing financial interest.

Conflicts of Interest

There are no conflict to declare.

Footnotes

Sample Availability: Samples of the compounds are not available from the authors.

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