# Alkylation and 1,3-Dipolar Cycloaddition of 6-Styryl-4,5-dihydro-2H-pyridazin-3-one: Synthesis of Novel N-Substituted Pyridazinones and Triazolo[4,3-b]pyridazinones

Authors: Souad Mojahidi; Hanan Sekkak; El Mostapha Rakib; Maria G. P. M. S. Neves; Maria A. F. Faustino; José A. S. Cavaleiro; Hafid Zouihri
Journal: Journal of Chemistry
 (2013)

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## Abstract

Some newN-substituted pyridazinones and triazolo[4,3-b]pyridazinones were synthesized, respectively, by simple alkylation and 1,3-dipolar cycloaddition of pyridazin-3-one with nitrile imines. The regioselectivity of the reactions was ascertained by 1H, 13C NMR spectroscopy and X-ray diffraction of the synthesized compounds.


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## Body

## 1. Introduction

Pyridazinone derivatives have been reported to exhibit a wide range of pharmacological activities such as antihypertensive [1, 2], anti-HIV [3], antibacterial, [4] aldose reductase inhibitors [5], hepatoprotective agents [6], and COX-2 inhibitors [7]. It has also been reported that pyridazinone derivatives have remarkable anticancer activity [8, 9]. Recently, our research group has reported the synthesis and the antiproliferative activities of new pyridazinone derivatives. Some of these compounds exhibited significant cytotoxicity against human and murine cell lines (A2780, A549, P388, and P815) [10, 11]. As a part of our program we focused on pyridazinones with biological activity, and in connection with our interest in the chemistry of annelated pyridazinones [10–12], in this paper we report the synthesis of a new series of N-substituted pyridazinones and triazolo[4,3-b]pyridazinones, which were obtained, respectively, by alkylation and 1,3-dipolar cycloaddition of 6-styryl-4,5-dihydro-2H-pyridazin-3-one.


## 2. Experimental Section

Melting points were determined using a Büchi-Tottoli apparatus and are uncorrected.1H and 13C NMR spectra were recorded in CDCl3 or DMSO-d6 and solution (unless otherwise specified) with TMS as an internal reference using a Bruker AC 300 (1H) or 75 MHz (13C) instruments. Chemical shifts are given in δ parts per million (ppm). Multiplicities of 13C NMR resources were assigned by distortionless enhancement by polarization transfer (DEPT) experiments. IR spectra were recorded on a Perkin-Elmer 577 spectrometer (Perkin-Elmer, USA) using KBr disks; only noteworthy IR absorptions are listed (cm−1). High resolution mass spectra were recorded on an Agilent ESI-TOF mass spectrometer. Column chromatography was carried out on SiO2 (silica gel 60 Merck 0.063–0.200 mm). Thin-layer chromatography (TLC) was carried out on SiO2 (silica gel 60, F 254 Merck 0.063–0.200 mm), and the spots were located with UV light (254 nm). Commercial reagents were used without further purification unless stated. Compounds 4a, b were prepared according to the literature methods  [13, 14].


### 2.1. Synthesis of Pyridazin-3-ones4a, b

A mixture of the appropriate aldehyde (4 mmol), levulinic acid (4 mmol, 0.46 g), morpholine (3 drops), and glacial acetic acid (9 drops) was heated in toluene at 60°C for 12 h. The solvent was evaporated; the reaction mixture was cooled and washed with acetic acid : water (1 : 4). In each case the formed precipitate was filtered and dried to give the corresponding compound3. A solution of each compound 3a, b(4.3 mmol) in 20 mL of glacial acetic acid containing hydrazine hydrate (0.5 g, 10 mmol) was heated at reflux for 30 h. The acetic acid was evaporated under vacuum and the residue was taken up with cold water. The precipitate was filtered, washed with cold water, dried, and purified by column chromatography (EtOAc/hexane 4/6).


### 2.2. 6-Styryl-4,5-dihydro-2H-pyridazin-3-one (4a)

Yield: 65%; mp: 160–162°C; IR (KBr, cm−1): 3420–3350 (NH), 1680 (CO); 1H NMR (DMSO-d6): δ 2.42 (t, 2H, CH2, J=7.8 Hz), 2.78 (t, 2H, CH2, J=7.8 Hz), 6.88 (d, 1H, J=16.6 Hz), 7.05 (d, 1H, J=16.6 Hz), 7.27–7.41 (m, 3H, ArH), 7.57–7.60 (m, 2H, ArH), 10.89 (s, 1H, NH); 13C NMR (DMSO-d6):  20.6 (CH2), 26.3 (CH2), 126.8 (CH-vinyl), 127.4 (2CH), 128.9 (CH), 129.3 (2CH), 133.9 (CH-vinyl), 136.5 (C), 151.2 (C), 167.7 (CO).


### 2.3. 6-[2-(4-Methoxyphenyl)vinyl]-4,5-dihydro-2H-pyridazin-3-one (4b)

Yield: 62%; mp: 164–166°C; IR (KBr, cm−1): 3450–3350 (NH), 1685 (C=O); 1H NMR (DMSO-d6): δ 2.38 (t, 2H, CH2, J=7.6 Hz), 2.75 (t, 2H, CH2, J=7.6 Hz), 3.78 (s, 3H, CH3O), 6,74 (d, 1H, J=16.8 Hz), 6.92 (d, 2H, J=8.7 Hz), 6.98 (d, 1H, J=16.8 Hz), 7.50 (d, 2H, J=8.7 Hz), 10.80 (s, 1H, NH); 13C NMR (DMSO-d6):  20.6 (CH2), 26.4 (CH2), 55.6 (CH3O), 114.7 (2CH), 124.6 (CH-vinyl), 128.8 (2CH), 129.2 (C), 133.6 (CH-vinyl), 151.4 (C), 160.1 (C), 167.7 (CO).


### 2.4. Synthesis of N-Substituted Pyridazinones5a–c

To a solution of compound4a (1.22 g, 6.13 mmol) in dry THF (30 mL) was added potassium carbonate (2.50 g, 18.30 mmol). The selected alkyl halide (7.40 mmol) was added dropwise. Upon disappearance of the starting material as indicated by TLC, the solvent was evaporated under vacuum. The crude material was dissolved with CH2Cl2 (50 mL), washed with water and brine, dried over MgSO4 and the solvent was evaporated at reduced pressure. The resulting residue was purified by column chromatography (EtOAc/hexane 3/7).


### 2.5. 2-Methyl-6-styryl-4,5-dihydro-2H-pyridazin-3-one (5a)

Yield: 85%; mp: 136–138°C; IR (KBr, cm−1): 1666 (CO); 1H NMR (CDCl3): δ 2.53 (t, 2H, CH2, J=7.4 Hz), 2.79 (t, 2H, CH2, J=7.4 Hz), 3.45 (s, 3H, NCH3), 6.84 (d, 1H, J=16.5 Hz), 6.93 (d, 1H, J=16.5 Hz), 7.24–7.48 (m, 5H); 13C NMR (CDCl3): δ 21.2 (CH2), 26.8 (CH2), 36.6 (NCH3), 126.8 (CH-vinyl), 127.4 (2CH), 128.7 (CH), 128.9 (2CH), 134.5 (CH-vinyl), 135.8 (C), 151.7 (C), 165.8 (CO); HRMS (ESI-TOF) m/z: calculated for C13H15N2O [M + H]+: 215.11844 found: 215.11816.


### 2.6. 2-Allyl-6-styryl-4,5-dihydro-2H-pyridazin-3-one (5b)

Yield: 72%; mp: 178–180°C; IR (KBr, cm−1): 1670 (CO); 1H NMR (CDCl3): δ 2.57 (t, 2H, CH2, J=7.5 Hz), 2.81 (t, 2H, CH2, J=7.5 Hz), 4.41–4.44 (m, 2H, NCH2), 5.18–5.26 (m, 2H, =CH2), 5.86–5.97 (m, 1H, =CH), 6,86 (d, 1H, J=16.5 Hz), 6.94 (d, 1H, J=16.5 Hz), 7.31–7.39 (m, 3H), 7.46–7.50 (m, 2H); 13C NMR (CDCl3): δ 21.1 (CH2), 26.9 (CH2), 50.9 (NCH2), 117.0 (=CH2), 126.1 (CH), 127.0 (2CH), 128.4 (CH), 128.9 (2CH), 132.8 (CH), 134.4 (CH), 135.8 (C), 151.9 (C), 165.4 (CO); HRMS (ESI-TOF) m/z: calculated for C15H17N2O [M + H]+: 241.13950 found: 241.13936.


### 2.7. 2-(2-Oxo-2-phenylethyl)-6-styryl-4,5-dihydro-2H-pyridazin-3-one (5c)

Yield: 60%; mp: 88–90°C; IR (KBr, cm−1): 1675 (CO), 1690 (CO); 1H NMR (CDCl3): δ 2.67 (t, 2H, CH2, J=7.6 Hz), 2.91 (t, 2H, CH2, J=7.6 Hz), 5.25 (s, 2H, NCH2), 6.85 (d, 1H, J=16.5 Hz), 6.92 (d, 1H, J=16.5 Hz), 7.29–7.38 (m, 3H), 7.45–7.52 (m, 4H), 7.57–7.62 (m, 1H), 7.96–8.00 (m, 2H); 13C NMR (CDCl3): δ 21.2 (CH2), 26.6 (CH2), 55.2 (NCH2), 125.9 (CH), 127.1 (2CH), 128.0 (2CH), 128.4 (CH), 128.7 (2CH), 128.9 (2CH), 133.6 (CH), 134.6 (CH), 135.0 (C), 135.8 (C), 152.2 (C), 166.5 (CO), 192.9 (CO). HRMS (ESI-TOF) m/z: calculated for C20H19N2O2 [M + H]+: 319.14465 found: 319.14438.


### 2.8. General Procedure for the Preparation of Triazolo[4,3-b]pyridazinones8a–e

To a solution of pyridazin-3(2H)-one (4a) (1.0 g, 5 mmol) and ethyl hydrazono-α-bromoglyoxylate (6a–e)(5 mmol) in dry THF (50 mL), K2CO3 (2.1 g, 15 mmol) was added. The mixture was refluxed in each case for 5–8 h. After evaporation of the solvent, the residue was purified by column chromatography on silica gel using Hexane-EtOAc 80 : 20 as eluent.


### 2.9. 6-Oxo-8a-styryl-1-p-tolyl-1,5,6,7,8,8a-hexahydro-[1,2,4]triazolo[4,3-b]pyridazine-3-carboxylic Acid Ethyl Ester (8a)

Yield: 56%; mp: 182–184°C; IR (KBr, cm−1): 1685 (CONH), 1710 (CO ester), 3050 (NH); 1H NMR (CDCl3): δ 1.40 (t, 3H, CH3, J=7.2 Hz), 2.28 (s, 3H, CH3), 2.26–2.39 (m, 3H), 2.78–2.89 (m, 1H), 4.37 (q, 2H, CH2O, J=7.2 Hz), 6,49 (d, 1H, J=16.1 Hz), 6.83 (d, 1H, J=16.1 Hz), 7.08 (d, 2H, J=8.4 Hz), 7.22 (d, 2H, J=8.4 Hz), 7.28–7.39 (m, 3H, ArH), 7.43–7.46 (m, 2H, ArH), 7.53 (s, 1H, NH); 13C NMR (CDCl3): δ 14.2 (CH3), 20.7 (CH3), 28.6 (CH2), 29.5 (CH2), 62.2 (CH2O), 88.1 (C-8a), 117.8 (2CH), 127.1 (2CH), 128.7 (2CH), 128.9 (CH), 129.8 (2CH), 133.0 (C), 133.5 (CH), 135.3 (C), 138.4 (C), 140.6 (C), 146.1 (C), 158.2 (CO), 174.3 (CO ester); HRMS (ESI-TOF) m/z: calculated for C23H25N4O3 [M + H]+: 405.19212 found: 405.19201.


### 2.10. 1-(4-Chloro-phenyl)-6-oxo-8a-styryl-1,5,6,7,8,8a-hexahydro-[1,2,4]triazolo[4,3-b]pyridazine-3-carboxylic Acid Ethyl Ester (8b)

Yield: 65%; mp: 185–187°C; IR (KBr, cm−1): 1680 (CONH), 1720 (CO ester), 3035 (NH); 1H NMR (CDCl3): δ 1.40 (t, 3H, CH3, J=7.1 Hz), 2.29–2.44 (m, 3H), 2.83–2.90 (m, 1H), 4.38 (q, 2H, CH2O, J=7.1 Hz), 6,48 (d, 1H, J=16.1 Hz), 6.81 (d, 1H, J=16.1 Hz), 7.21–7.30 (m, 4H, ArH), 7.33–7.44 (m, 5H, ArH), 7.54 (s, 1H, NH); 13C NMR (CDCl3): δ 14.2 (CH3), 28.7 (CH2), 29.4 (CH2), 62.4 (CH2O), 87.7 (C-8a), 118.1 (2CH), 127.1 (2CH), 128.5 (CH), 128.8 (2CH), 129.0 (CH), 129.8 (2CH), 134.1 (CH), 135.0 (C), 139.3 (C), 140.2 (C), 145.9 (C), 158.5 (CO), 174.1 (CO ester); HRMS (ESI-TOF) m/z: calculated for C22H21ClN4O3Na [M + Na]+: 447.11944 found: 447.11900.


### 2.11. 1-(4-Nitro-phenyl)-6-oxo-8a-styryl-1,5,6,7,8,8a-hexahydro-[1,2,4]triazolo[4,3-b]pyridazine-3-carboxylic Acid Ethyl Ester (8c)

Yield: 49%; mp: 168–170°C; IR (KBr, cm−1): 1530, 1320 (NO2), 1670 (CONH), 1725 (CO ester), 3050 (NH); 1H NMR (CDCl3): δ 1.41 (t, 3H, CH3, J=7.2 Hz), 2.24–2.36 (m, 3H), 2.74–2.81 (m, 1H), 4.39 (q, 2H, CH2O, J=7.2 Hz), 6,22 (d, 1H, J=16.3 Hz), 6.85 (d, 1H, J=16.3 Hz), 7.32–7.39 (m, 6H, ArH), 7.71 (s, 1H, NH), 7.73–7.78 (m, 1H, ArH), 8.03–8.13 (m, 2H, ArH); 13C NMR (CDCl3): δ 14.1 (CH3), 29.1 (CH2), 30.4 (CH2), 62.5 (CH2O), 86.8 (C-8a), 122.1 (2CH), 122.8 (CH), 126.4 (2CH), 126.9 (2CH), 128.9 (2CH), 129.1 (CH), 132.5 (CH), 134.8 (C), 140.1 (C), 142.2 (C), 146.4 (C), 157.9 (CO), 172.1 (CO ester); HRMS (ESI-TOF) m/z: calculated for C22H22N5O5[M + H]+: 436.16155 found: 436.16116.


### 2.12. 1-(2-Methyl-3-nitro-phenyl)-6-oxo-8a-styryl-1,5,6,7,8,8a-hexahydro-[1,2,4]triazolo[4,3-b]pyridazine-3-carboxylic Acid Ethyl Ester (8d)

Yield: 51%; mp: 136–138°C; IR (KBr, cm−1): 1545, 1310 (NO2), 1670 (CONH), 1710 (CO ester), 3060 (NH); 1H NMR (CDCl3): δ 1.40 (t, 3H, CH3, J=7.2 Hz), 2.01–2.11 (m, 1H), 2.40 (s, 3H, CH3), 2.45–2.64 (m, 3H), 4.40 (q, 2H, CH2O, J=7.2 Hz), 6,15 (d, 1H, J=16.0 Hz), 6.86 (d, 1H, J=16.0 Hz), 7.29–7.41 (m, 6H, ArH), 7,52 (d, 1H, J=8.1 Hz), 7.76 (s, 1H, NH), 7,80 (d, 1H, J=8.1 Hz); 13C NMR (CDCl3): δ 14.1 (CH3), 16.0 (CH3), 28.6 (CH2), 30.3 (CH2), 62.6 (CH2O), 88.9 (C-8a), 123.8 (CH), 126.2 (CH), 126.7 (CH), 127.0 (2CH), 128.9 (2CH), 129.1 (CH), 131.9 (C), 133.0 (CH), 133 (CH), 134.9 (C), 140.2 (C), 147.4 (C), 157.8 (CO), 171.9 (CO ester).


### 2.13. 1-(2,4-Dibromo-phenyl)-6-oxo-8a-styryl-1,5,6,7,8,8a-hexahydro-[1,2,4]triazolo[4,3-b]pyridazine-3-carboxylic Acid Ethyl Ester (8e)

Yield: 46%; mp: 160–162°C; IR (KBr, cm−1): 1675 (CONH), 1715 (CO ester), 3060 (NH); 1H NMR (CDCl3): δ 1.38 (t, 3H, CH3, J=7.1 Hz), 2.08–2.18 (m, 1H), 2.45–2.62 (m, 2H), 2.78–2.90 (m, 1H), 4.39 (q, 2H, CH2O, J=7.1 Hz), 6,20 (d, 1H, J=15.9 Hz), 6.79 (d, 1H, J=15.9 Hz), 7,14 (d, 1H, J=8.5 Hz), 7.32–7.38 (m, 5H, ArH), 7,40 (dd, 1H, J=8.1 and 2.2 Hz), 7.61 (s, 1H, NH), 7,80 (d, 1H, J=2.2 Hz); 13C NMR (CDCl3): δ 14.2 (CH3), 28.9 (CH2), 31.9 (CH2), 62.5 (CH2O), 88.0 (C-8a), 122.2 (C), 123.8 (C), 127.0 (2CH), 128.8 (2CH), 128.9 (CH), 131.1 (CH), 131.9 (CH), 133.0 (CH), 136.7 (CH), 138.5 (CH), 142.5 (C), 146.2 (C), 157.9 (CO), 173.1 (CO ester).


## 2.1. Synthesis of Pyridazin-3-ones4a, b

A mixture of the appropriate aldehyde (4 mmol), levulinic acid (4 mmol, 0.46 g), morpholine (3 drops), and glacial acetic acid (9 drops) was heated in toluene at 60°C for 12 h. The solvent was evaporated; the reaction mixture was cooled and washed with acetic acid : water (1 : 4). In each case the formed precipitate was filtered and dried to give the corresponding compound3. A solution of each compound 3a, b(4.3 mmol) in 20 mL of glacial acetic acid containing hydrazine hydrate (0.5 g, 10 mmol) was heated at reflux for 30 h. The acetic acid was evaporated under vacuum and the residue was taken up with cold water. The precipitate was filtered, washed with cold water, dried, and purified by column chromatography (EtOAc/hexane 4/6).


## 2.2. 6-Styryl-4,5-dihydro-2H-pyridazin-3-one (4a)

Yield: 65%; mp: 160–162°C; IR (KBr, cm−1): 3420–3350 (NH), 1680 (CO); 1H NMR (DMSO-d6): δ 2.42 (t, 2H, CH2, J=7.8 Hz), 2.78 (t, 2H, CH2, J=7.8 Hz), 6.88 (d, 1H, J=16.6 Hz), 7.05 (d, 1H, J=16.6 Hz), 7.27–7.41 (m, 3H, ArH), 7.57–7.60 (m, 2H, ArH), 10.89 (s, 1H, NH); 13C NMR (DMSO-d6):  20.6 (CH2), 26.3 (CH2), 126.8 (CH-vinyl), 127.4 (2CH), 128.9 (CH), 129.3 (2CH), 133.9 (CH-vinyl), 136.5 (C), 151.2 (C), 167.7 (CO).


## 2.3. 6-[2-(4-Methoxyphenyl)vinyl]-4,5-dihydro-2H-pyridazin-3-one (4b)

Yield: 62%; mp: 164–166°C; IR (KBr, cm−1): 3450–3350 (NH), 1685 (C=O); 1H NMR (DMSO-d6): δ 2.38 (t, 2H, CH2, J=7.6 Hz), 2.75 (t, 2H, CH2, J=7.6 Hz), 3.78 (s, 3H, CH3O), 6,74 (d, 1H, J=16.8 Hz), 6.92 (d, 2H, J=8.7 Hz), 6.98 (d, 1H, J=16.8 Hz), 7.50 (d, 2H, J=8.7 Hz), 10.80 (s, 1H, NH); 13C NMR (DMSO-d6):  20.6 (CH2), 26.4 (CH2), 55.6 (CH3O), 114.7 (2CH), 124.6 (CH-vinyl), 128.8 (2CH), 129.2 (C), 133.6 (CH-vinyl), 151.4 (C), 160.1 (C), 167.7 (CO).


## 2.4. Synthesis of N-Substituted Pyridazinones5a–c

To a solution of compound4a (1.22 g, 6.13 mmol) in dry THF (30 mL) was added potassium carbonate (2.50 g, 18.30 mmol). The selected alkyl halide (7.40 mmol) was added dropwise. Upon disappearance of the starting material as indicated by TLC, the solvent was evaporated under vacuum. The crude material was dissolved with CH2Cl2 (50 mL), washed with water and brine, dried over MgSO4 and the solvent was evaporated at reduced pressure. The resulting residue was purified by column chromatography (EtOAc/hexane 3/7).


## 2.5. 2-Methyl-6-styryl-4,5-dihydro-2H-pyridazin-3-one (5a)

Yield: 85%; mp: 136–138°C; IR (KBr, cm−1): 1666 (CO); 1H NMR (CDCl3): δ 2.53 (t, 2H, CH2, J=7.4 Hz), 2.79 (t, 2H, CH2, J=7.4 Hz), 3.45 (s, 3H, NCH3), 6.84 (d, 1H, J=16.5 Hz), 6.93 (d, 1H, J=16.5 Hz), 7.24–7.48 (m, 5H); 13C NMR (CDCl3): δ 21.2 (CH2), 26.8 (CH2), 36.6 (NCH3), 126.8 (CH-vinyl), 127.4 (2CH), 128.7 (CH), 128.9 (2CH), 134.5 (CH-vinyl), 135.8 (C), 151.7 (C), 165.8 (CO); HRMS (ESI-TOF) m/z: calculated for C13H15N2O [M + H]+: 215.11844 found: 215.11816.


## 2.6. 2-Allyl-6-styryl-4,5-dihydro-2H-pyridazin-3-one (5b)

Yield: 72%; mp: 178–180°C; IR (KBr, cm−1): 1670 (CO); 1H NMR (CDCl3): δ 2.57 (t, 2H, CH2, J=7.5 Hz), 2.81 (t, 2H, CH2, J=7.5 Hz), 4.41–4.44 (m, 2H, NCH2), 5.18–5.26 (m, 2H, =CH2), 5.86–5.97 (m, 1H, =CH), 6,86 (d, 1H, J=16.5 Hz), 6.94 (d, 1H, J=16.5 Hz), 7.31–7.39 (m, 3H), 7.46–7.50 (m, 2H); 13C NMR (CDCl3): δ 21.1 (CH2), 26.9 (CH2), 50.9 (NCH2), 117.0 (=CH2), 126.1 (CH), 127.0 (2CH), 128.4 (CH), 128.9 (2CH), 132.8 (CH), 134.4 (CH), 135.8 (C), 151.9 (C), 165.4 (CO); HRMS (ESI-TOF) m/z: calculated for C15H17N2O [M + H]+: 241.13950 found: 241.13936.


## 2.7. 2-(2-Oxo-2-phenylethyl)-6-styryl-4,5-dihydro-2H-pyridazin-3-one (5c)

Yield: 60%; mp: 88–90°C; IR (KBr, cm−1): 1675 (CO), 1690 (CO); 1H NMR (CDCl3): δ 2.67 (t, 2H, CH2, J=7.6 Hz), 2.91 (t, 2H, CH2, J=7.6 Hz), 5.25 (s, 2H, NCH2), 6.85 (d, 1H, J=16.5 Hz), 6.92 (d, 1H, J=16.5 Hz), 7.29–7.38 (m, 3H), 7.45–7.52 (m, 4H), 7.57–7.62 (m, 1H), 7.96–8.00 (m, 2H); 13C NMR (CDCl3): δ 21.2 (CH2), 26.6 (CH2), 55.2 (NCH2), 125.9 (CH), 127.1 (2CH), 128.0 (2CH), 128.4 (CH), 128.7 (2CH), 128.9 (2CH), 133.6 (CH), 134.6 (CH), 135.0 (C), 135.8 (C), 152.2 (C), 166.5 (CO), 192.9 (CO). HRMS (ESI-TOF) m/z: calculated for C20H19N2O2 [M + H]+: 319.14465 found: 319.14438.


## 2.8. General Procedure for the Preparation of Triazolo[4,3-b]pyridazinones8a–e

To a solution of pyridazin-3(2H)-one (4a) (1.0 g, 5 mmol) and ethyl hydrazono-α-bromoglyoxylate (6a–e)(5 mmol) in dry THF (50 mL), K2CO3 (2.1 g, 15 mmol) was added. The mixture was refluxed in each case for 5–8 h. After evaporation of the solvent, the residue was purified by column chromatography on silica gel using Hexane-EtOAc 80 : 20 as eluent.


## 2.9. 6-Oxo-8a-styryl-1-p-tolyl-1,5,6,7,8,8a-hexahydro-[1,2,4]triazolo[4,3-b]pyridazine-3-carboxylic Acid Ethyl Ester (8a)

Yield: 56%; mp: 182–184°C; IR (KBr, cm−1): 1685 (CONH), 1710 (CO ester), 3050 (NH); 1H NMR (CDCl3): δ 1.40 (t, 3H, CH3, J=7.2 Hz), 2.28 (s, 3H, CH3), 2.26–2.39 (m, 3H), 2.78–2.89 (m, 1H), 4.37 (q, 2H, CH2O, J=7.2 Hz), 6,49 (d, 1H, J=16.1 Hz), 6.83 (d, 1H, J=16.1 Hz), 7.08 (d, 2H, J=8.4 Hz), 7.22 (d, 2H, J=8.4 Hz), 7.28–7.39 (m, 3H, ArH), 7.43–7.46 (m, 2H, ArH), 7.53 (s, 1H, NH); 13C NMR (CDCl3): δ 14.2 (CH3), 20.7 (CH3), 28.6 (CH2), 29.5 (CH2), 62.2 (CH2O), 88.1 (C-8a), 117.8 (2CH), 127.1 (2CH), 128.7 (2CH), 128.9 (CH), 129.8 (2CH), 133.0 (C), 133.5 (CH), 135.3 (C), 138.4 (C), 140.6 (C), 146.1 (C), 158.2 (CO), 174.3 (CO ester); HRMS (ESI-TOF) m/z: calculated for C23H25N4O3 [M + H]+: 405.19212 found: 405.19201.


## 2.10. 1-(4-Chloro-phenyl)-6-oxo-8a-styryl-1,5,6,7,8,8a-hexahydro-[1,2,4]triazolo[4,3-b]pyridazine-3-carboxylic Acid Ethyl Ester (8b)

Yield: 65%; mp: 185–187°C; IR (KBr, cm−1): 1680 (CONH), 1720 (CO ester), 3035 (NH); 1H NMR (CDCl3): δ 1.40 (t, 3H, CH3, J=7.1 Hz), 2.29–2.44 (m, 3H), 2.83–2.90 (m, 1H), 4.38 (q, 2H, CH2O, J=7.1 Hz), 6,48 (d, 1H, J=16.1 Hz), 6.81 (d, 1H, J=16.1 Hz), 7.21–7.30 (m, 4H, ArH), 7.33–7.44 (m, 5H, ArH), 7.54 (s, 1H, NH); 13C NMR (CDCl3): δ 14.2 (CH3), 28.7 (CH2), 29.4 (CH2), 62.4 (CH2O), 87.7 (C-8a), 118.1 (2CH), 127.1 (2CH), 128.5 (CH), 128.8 (2CH), 129.0 (CH), 129.8 (2CH), 134.1 (CH), 135.0 (C), 139.3 (C), 140.2 (C), 145.9 (C), 158.5 (CO), 174.1 (CO ester); HRMS (ESI-TOF) m/z: calculated for C22H21ClN4O3Na [M + Na]+: 447.11944 found: 447.11900.


## 2.11. 1-(4-Nitro-phenyl)-6-oxo-8a-styryl-1,5,6,7,8,8a-hexahydro-[1,2,4]triazolo[4,3-b]pyridazine-3-carboxylic Acid Ethyl Ester (8c)

Yield: 49%; mp: 168–170°C; IR (KBr, cm−1): 1530, 1320 (NO2), 1670 (CONH), 1725 (CO ester), 3050 (NH); 1H NMR (CDCl3): δ 1.41 (t, 3H, CH3, J=7.2 Hz), 2.24–2.36 (m, 3H), 2.74–2.81 (m, 1H), 4.39 (q, 2H, CH2O, J=7.2 Hz), 6,22 (d, 1H, J=16.3 Hz), 6.85 (d, 1H, J=16.3 Hz), 7.32–7.39 (m, 6H, ArH), 7.71 (s, 1H, NH), 7.73–7.78 (m, 1H, ArH), 8.03–8.13 (m, 2H, ArH); 13C NMR (CDCl3): δ 14.1 (CH3), 29.1 (CH2), 30.4 (CH2), 62.5 (CH2O), 86.8 (C-8a), 122.1 (2CH), 122.8 (CH), 126.4 (2CH), 126.9 (2CH), 128.9 (2CH), 129.1 (CH), 132.5 (CH), 134.8 (C), 140.1 (C), 142.2 (C), 146.4 (C), 157.9 (CO), 172.1 (CO ester); HRMS (ESI-TOF) m/z: calculated for C22H22N5O5[M + H]+: 436.16155 found: 436.16116.


## 2.12. 1-(2-Methyl-3-nitro-phenyl)-6-oxo-8a-styryl-1,5,6,7,8,8a-hexahydro-[1,2,4]triazolo[4,3-b]pyridazine-3-carboxylic Acid Ethyl Ester (8d)

Yield: 51%; mp: 136–138°C; IR (KBr, cm−1): 1545, 1310 (NO2), 1670 (CONH), 1710 (CO ester), 3060 (NH); 1H NMR (CDCl3): δ 1.40 (t, 3H, CH3, J=7.2 Hz), 2.01–2.11 (m, 1H), 2.40 (s, 3H, CH3), 2.45–2.64 (m, 3H), 4.40 (q, 2H, CH2O, J=7.2 Hz), 6,15 (d, 1H, J=16.0 Hz), 6.86 (d, 1H, J=16.0 Hz), 7.29–7.41 (m, 6H, ArH), 7,52 (d, 1H, J=8.1 Hz), 7.76 (s, 1H, NH), 7,80 (d, 1H, J=8.1 Hz); 13C NMR (CDCl3): δ 14.1 (CH3), 16.0 (CH3), 28.6 (CH2), 30.3 (CH2), 62.6 (CH2O), 88.9 (C-8a), 123.8 (CH), 126.2 (CH), 126.7 (CH), 127.0 (2CH), 128.9 (2CH), 129.1 (CH), 131.9 (C), 133.0 (CH), 133 (CH), 134.9 (C), 140.2 (C), 147.4 (C), 157.8 (CO), 171.9 (CO ester).


## 2.13. 1-(2,4-Dibromo-phenyl)-6-oxo-8a-styryl-1,5,6,7,8,8a-hexahydro-[1,2,4]triazolo[4,3-b]pyridazine-3-carboxylic Acid Ethyl Ester (8e)

Yield: 46%; mp: 160–162°C; IR (KBr, cm−1): 1675 (CONH), 1715 (CO ester), 3060 (NH); 1H NMR (CDCl3): δ 1.38 (t, 3H, CH3, J=7.1 Hz), 2.08–2.18 (m, 1H), 2.45–2.62 (m, 2H), 2.78–2.90 (m, 1H), 4.39 (q, 2H, CH2O, J=7.1 Hz), 6,20 (d, 1H, J=15.9 Hz), 6.79 (d, 1H, J=15.9 Hz), 7,14 (d, 1H, J=8.5 Hz), 7.32–7.38 (m, 5H, ArH), 7,40 (dd, 1H, J=8.1 and 2.2 Hz), 7.61 (s, 1H, NH), 7,80 (d, 1H, J=2.2 Hz); 13C NMR (CDCl3): δ 14.2 (CH3), 28.9 (CH2), 31.9 (CH2), 62.5 (CH2O), 88.0 (C-8a), 122.2 (C), 123.8 (C), 127.0 (2CH), 128.8 (2CH), 128.9 (CH), 131.1 (CH), 131.9 (CH), 133.0 (CH), 136.7 (CH), 138.5 (CH), 142.5 (C), 146.2 (C), 157.9 (CO), 173.1 (CO ester).


## 3. Results and Discussion

The starting compounds 6-(styryl)-4,5-dihydropyridazinones4a, b used for alkylation reaction and 1,3-dipolar cycloadditions, were prepared from levulinic acid 1 according to Scheme 1. The treatment of compound 1 with the aromatic aldehydes 2a, b produced the intermediate benzylidenelevulinic acid 3a, b. The derivatives 3a, b obtained were then treated with hydrazine hydrate in refluxing acetic acid in order to achieve the desired styrylpyridazinones 4a, b.Scheme 1The structure of compound4a was confirmed for the first time by X-ray crystallography (Figures 1 and 2 and Table 1). The crystal structure of this compound, whose molecular formula is C12H12N2O, was determined by single-crystal diffraction methods. The compound crystallizes in the monoclinic unit cell P21/n space group symmetry with lattice parameters: a=8.0348(6) Å, b=11.1532(6) Å, c=11.9941(8) Å, and β=101.074(2)°; V=1054.82(12) Å3 and D (calc., Z=4) = 1.261 Mg m−3. A total of 11692 data reflections were collected over the range of 5°⩽2θ⩽55°; of these, 1647 (independent and with I ≥2σ(I)) were used in the structural analysis. The final R(F) and Rw(F) residuals were 0.054 and 0.179, respectively.Table 1
Crystal data and structure refinement parameters of compound4a.
Chemical Formula: C12H12N2O
Formula weight: 200.24
Crystal system: monoclinic
Space group (no):P21/n (14)
a = 8.0348 (6) Å
β = 101.074 (2)°
b = 11.1532 (6) Å
V = 1911.5 (3) Å3
c = 11.9941 (8) Å
Z = 4
D
x = 1.261 Mg m−3
MoKα radiation, λ = 0.71073 Å
μ = 0.08 mm−1
F(000) = 424
Crystal sizes = 0.34 * 0.25 * 0.23 mm3
Crystal: Prism, colourless
No. of reflections collected = 11692
No. of independent reflections = 2431
No. of reflections with I  > 2σ(I) = 1647
θ range for data collection: θmax = 27.5°, θmin = 2.5°
Goodness-of-ft onF2 = 1.07
R indices [I > 2s(I)]: R1 = 0.054, wR2 = 0.179
(D/s)max = 0.000
(Dr)max = 0.527 eÅ−3 (Dr)min = −0.349 eÅ−3
Measurement: X8 Apex, Bruker CCD
Program system: Apex 2
Structure determination: WinGX (SHELXL and SHELXSFigure 1
ORTEP III diagram of4a. Compound showing the molecular numbering scheme. Displacement ellipsoids are drawn at 50% probability for all atoms, except for H, for which they have been set to be artificially small.Figure 2
Partial packing view showing the chain formed by the hydrogen bridge N–H…O.In compound4a, the dihydropyridazinone ring is oriented at dihedral angles of 17.11 (9)° with respect to the benzene ring. In the crystal, the molecules are linked by N–H…O hydrogen bonds (Figure 2).TheN-alkylation reaction in the pyridazinone series is generally used for the introduction of pharmacophoric groups; consequently first of all it is necessary to study the alkylation reaction in the presence of 4,5-dihydropyridazinone and base in order to establish their reactivity and possible regioselectivity. The treatment of 6-styryl-4,5-dihydropyridazinone (4a) with alkyl halides (CH3I, BrCH2CH=CH2 and BrCH2COC6H5) in the presence of anhydrous K2CO3 in dry THF gave only the N-substituted-pyridazinones 5a–c in moderate to good yields (Scheme 2).Scheme 2The structures ofN-substituted pyridazinones 5a–c were characterized using 1H NMR and 13C NMR spectra. The exclusive alkylation at the 2-N position was confirmed by X-ray crystallography of compound 5a (Figures 3 and 4 and Table 2). The crystal structure of compound 5a, whose molecular formula is C13H14N2O, was also determined by single-crystal diffraction methods. The compound crystallizes in the monoclinic unit cell P21 space group symmetry with lattice parameters: a=6.1966(3) Å, b=7.3246(4) Å, c=12.6055(7) Å, and β=96.035(4)°; V=568.96(5) Å3 and D (calc., Z=2) = 1.261 Mg m−3. A total of 13216 data were collected over the range of 3.2°≤2θ≤72.8°; of these, 3329 (independent and with I ≥2σ(I)) were used in the structural analysis. The final R(F) and Rw(F) residuals were 0.051 and 0.167, respectively.Table 2
Crystal data and structure refinement parameters of compound5a.
Chemical Formula: C13H14N2O
Formula weight: 214.26
Crystal system: monoclinic
Space group:P21
a = 6.1966 (3) Å
β = 96.035 (4)°
b = 7.3246 (4) Å
V = 568.96 (5) Å3
c = 12.6055 (7) Å
Z = 2
D
x = 1.261 Mg m−3
MoKα radiation, λ = 0.71073 Å
μ = 0.08 mm−1
F(000) = 228
Crystal sizes = 0.25 * 0.25 * 0.18 mm3
Crystal: Prism, colourless
No. of reflections collected = 13216
No. of independent reflections = 4864
No. of reflections with  I > 2σ(I) = 3329
θ range for data collection: θmax = 36.4°, θmin = 1.6°
Goodness-of-ft onF2 = 1.04
R indices [I > 2s(I)]: R1 = 0.051, wR2 = 0.167
(D/s) max < 0.001
(Dr) max = 0.28 eÅ−3 (Dr)min = −0.23 eÅ−3
Measurement: X8 Apex, Bruker CCD
Program system: Apex 2
Structure determination: WinGX (SHELXL and SHELXSFigure 3
ORTEP III diagram of5a. Compound showing the molecular numbering scheme. Displacement ellipsoids are drawn at 50% probability for all atoms, except for H, for which they have been set to be artificially small.Figure 4
Partial packing view showing the chain formed by C–H…O.In compound5a, the dihydropyridazinone ring is oriented at dihedral angles of 20.96 (8)° with respect to the benzene ring. In the crystal, molecules are linked by C–H…O hydrogen bonds (Figure 4).1,3-Dipolar cycloadditions offer a convenient one-step concerted route for the construction of five-membered heterocycles with multiple stereogenic centers [15–21]. In the present work, we report a full account on the examination on 1,3-dipolar cycloaddition reaction of 6-styryl-4,5-dihydropyridazinone 4a with nitrile imines. The former compound has three potential dipolarophilic sites: the C=N double bond, the C=C double bond, and the C=O double bond. The reaction of compound 4awith N-aryl-C-ethoxycarbonyl nitrile imines 7a–e, generated in situfrom ethyl hydrazono-α-bromoglyoxylates 6a–e  [22] and K2CO3, was performed in refluxing dry THF. In all cases, only one type of triazolo[4,3-b]pyridazinone (8a–e) was obtained in moderate to good yields (Scheme 3). No adducts resulting from condensation on the double bonds C=C and/or C=O were detected. The reaction was exclusively site- and regioselective.Scheme 3The structural assignments of the triazolo[4,3-b]pyridazinones8a–e are based on a full characterization by 1H NMR and 13C NMR spectra.The1H NMR spectra of the compounds 8a–e, show in particular the presence of two doublet signals at ranges 6.15–6.49 ppm and 6.79–6.86 ppm corresponding to the vinylic protons of the double bond HC=CH with a coupling constant of ca. 16.1–16.5 Hz; this excludes the addition of the dipole to the double bond HC=CH.The13C NMR spectra of cycloadducts 8a–e, exhibit a signal at 157.8–158.5 ppm assigned to the resonance of carbonyl carbon C=O; this excludes also the addition of the nitrile imine to the double bond C=O. These results demonstrate the site selectivity of the double bond C=N; the 13C NMR spectra of cycloadducts 8a–e, exhibit in each case a signal at 85.9–89.6 ppm due to the resonance of each quaternary carbon C-8.These carbon centres are then slightly deshielded.Such fact confirms the direction of the dipole addition to the C=N double bond; otherwise, the C-8 signals would appear upfield (the value would be <60 ppm).The reaction is thus regioselective and no 1,2,3-triazole is formed.


## 4. Conclusion

In summary, with a simple approach, a series of newN-substituted pyridazinones and triazolo[4,3-b]pyridazinones can be synthesized, from moderate to good yields, by reaction of 6-styryl-4,5-dihydro-2H-pyridazin-3-one 4a with alkyl halides and using N-aryl-C-ethoxycarbonyl nitrile imines as 1,3-dipoles.



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*Source: 636280-2012-11-03.xml*