The Cu(OTf) 2 catalysed microwave assisted synthesis of a new scaffold, 7-aryl-7,8-dihydropyrido[4,3-c ]pyridazin-5(6 H )-one

The synthesis of novel 7-aryl-7,8-dihydropyrido[4,3-c ]pyridazin-5(6 H )-ones is described including a one-step Mannich-type reaction followed by intramolecular ring closure of ethyl 3-methylpyridazine-4-carboxylate and aldimines, catalysed by the Lewis acid Cu(OTf) 2 under microwave heating. This synthesis opens up possibilities to access this unexplored scaffold for medicinal chemistry.


Introduction
In the search for new biologically active compounds and drugs, extensive research is based on the synthesis of natural-like or small molecules.In this regard, the theory of privileged scaffolds, structures which can interact with high affinity to a broad range of (unrelated) receptors, provides new insights and hope for the synthesis of new active compounds. 1They are typically rigid and polycyclic heteroatomic systems, able to orient numerous substituents in the threedimensional space surrounding these scaffolds. 2Because these privileged structures furnish activities towards different receptors, they are considered excellent lead compounds, especially when only little is known about the structure of the receptors.Because the amount of new drugs is declining, despite the large amount of sources and research that are invested in this research, new scaffolds need to be explored.Therefore, increasing the chemical diversity in the field of heterocyclic chemistry is of great interest for the pharmaceutical industry.

Results and Discussion
The synthesis started with the synthesis of ethyl 3-methylpyridazine-4-carboxylate 7 (Scheme 1).Two routes were explored, one starting from ethyl acetoacetate 4, the other starting from ethyl 2-chloroacetoacetate 8.In a first entry, ethyl acetoacetate was mono-allylated with 1.2 equivalents of allyl iodide and 1.1 equivalents of sodium hydride in dry THF for 1 hour 30 minutes at room temperature, leading to ethyl 2-allylacetoacetate 5 by column chromatography in 79% yield. 17Allylation with allyl bromide and sodium hydride or a palladium catalyzed Trostallylation with allyl bromide resulted in either diallylation or a mixture of mono-and diallylated product.
Having the alkene in hand, an ozonolysis was performed to obtain the free aldehyde ethyl 2-acetyl-4-oxobutanoate 6.This was performed by bubbling ozone through a solution of ethyl 2-allylacetoacetate 5 in dichloromethane and methanol (ratio 10:1) at -78°C for 30 minutes, to which a trace amount of Sudan III was added as an indicator.An equimolar amount of polymer-bound PPh 3 was added for the reductive work-up of the intermediate ozonides and after 1 hour of slow stirring at room temperature, the reaction mixture was filtered and evaporated. 18nalysis of the reaction mixture by 1 H-NMR demonstrated a 52% conversion of the alkene towards the aldehyde.Because no full conversion of 5 and no pure aldehyde 6 could be obtained, the crude mixture of 5 and 6 was used as such in the next step for the synthesis of ethyl 3-methylpyridazine-4-carboxylate 7.
The synthesis of ethyl 3-methylpyridazine-4-carboxylate 7 was performed by the reaction of crude 6 with hydrazine. 18A solution of the reaction mixture in ethanol was slowly treated with 0.7 equivalents of hydrazine monohydrate at 0°C.The reaction mixture was allowed to warm to room temperature and was stirred for 2 hours 30 minutes.Thereafter, 1.5 equivalents of sodium nitrite, dissolved in water, and acetic acid were added and the mixture was again stirred for 1 hour at room temperature.After column chromatography, the end product was obtained in 39% yield (two steps) as yellow-brownish crystals.Taken this low (overall) yield, the small scale and the troublesome synthesis of pyridazine 7 into account, this pathway was abandoned and a new pathway starting from ethyl 2-chloroacetoacetate was investigated.
The new synthesis pathway towards ethyl 3-methylpyridazine-4-carboxylate 7 started with the formation of ethyl 2-chloroacetoacetate N-methoxycarbonylhydrazone 10. 19 Ethyl 2-chloroacetoacetate 8 was dissolved in dry diethyl ether and 1 equivalent of methyl hydrazinocarboxylate 9 was added.After stirring for 24 hours at room temperature, the precipitate was filtered off and washed with petroleum ether, resulting in pure ethyl 2-chloroacetoacetate N-methoxycarbonylhydrazone 10 as a white powder in 94% yield.
The hydrazone 10 was subsequently converted into 4-ethyl 1-methyl 6-ethoxy-3-methyl-5,6dihydropyridazine-1,4(4H)-dicarboxylate 12 via treatment with sodium bicarbonate and a subsequent inverse electron demand hetero Diels-Alder reaction. 20ydrazone 10 was dissolved in a 2:1 diisopropyl ether:water mixture and 1.06 equivalents of sodium bicarbonate were added.The reaction mixture discoloured immediately and release of gas was noticed and the 1,2-diaza-1,3-diene 11 was formed.After 2 hours of stirring at room temperature, the aqueous phase was discarded and the red organic phase was dried over magnesium sulfate.Because the isomeric mixture of 11 (of which the E-isomer is the major component) polymerises at room temperature and has explosive properties upon heating, thereby prohibiting purification by distillation, 11 was used as an unpurified mixture of isomers in diisopropyl ether in the next step. 21or the inverse electron demand hetero Diels-Alder reaction, 2.3 equivalents of ethyl vinyl ether were added to the dried solution of 11 in diisopropyl ether.The mixture was brought to reflux temperature and heated overnight.The reaction mixture was evaporated, resulting in a yellow oil of 4-ethyl 1-methyl 6-ethoxy-3-methyl-5,6-dihydropyridazine-1,4(4H)-dicarboxylate 12 in 91% yield over two steps.This Diels-Alder product 12 was obtained as a cis-trans mixture.Since 12 had to be converted to ethyl 3-methylpyridazine-4-carboxylate 7 in the next step, no purification nor separation of the isomers was necessary and 12 was used as such in the next step.
The last step in the synthesis of ethyl 3-methylpyridazine-4-carboxylate 7 is the oxidation of 12 with bromine in acetic acid. 20The crude 12 was dissolved in acetic acid and 1.2 equivalents of bromine were slowly added.This resulted in the formation of a brown reaction mixture and the production of gas.After stirring for 24 hours at room temperature, diisopropyl ether was added, resulting in the formation of a precipitate which was filtered and washed with diisopropyl ether.Toluene was added and evaporated to remove residual acetic acid in an azeotropic distillation.The residue was dissolved in water and trace amounts of small impurities were filtered off.The filtrate was neutralized by sodium bicarbonate and sodium chloride was added.The mixture was extracted with diisopropyl ether and the organic phase was dried over magnesium sulfate and evaporated, resulting in red-brown crystals of 7. According to 1 H-NMR and LC-MS analysis, these crystals consisted of 7 in very high purity.Recrystallization from diethyl ether did not improve the purity.After column chromatography and subsequent recrystallization from diethyl ether, colorless crystals of 7 were obtained in 72% yield, having the same 1 H-NMR and LC-MS purity as the red-brown crystals.
The latter synthesis for ethyl 3-methylpyridazine-4-carboxylate was used starting from ethyl 2-chloroacetoacetate 8 because the first synthesis route, starting from ethyl acetoacetate 3, did not offer satisfactory results.This second route not only comprised a more practical synthesis, it could also be executed on gram scale and in high (overall) yields.
For the synthesis of the pyrido[4,3-c]pyridazines, ethyl 3-methylpyridazine-4-carboxylate 7 was reacted with aldimines.Different aldimines were synthetized via a straightforward procedure (Scheme 2). 22The appropriate aldehyde 13 was dissolved in dichloromethane and 1.05 equivalents of the corresponding amine 14 was added.After stirring at reflux temperature for 3 hours in the presence of 1.5 equivalents of magnesium sulfate, the precipitate was filtered and the aldimine 15 was obtained in almost always quantitative yield, without the need for purification.Also, the N-benzylidene-p-toluenesulfonamide 16 was synthetized by refluxing 1 equivalent of p-toluenesulfonamide and 1 equivalent of benzaldehyde under Dean-Stark conditions, resulting in 87% of 16 after recrystallization from diethyl ether.The synthetized ethyl 3-methylpyridazine-4-carboxylate 7 and N-benzylidenep-toluenesulfonamide 16 were used in a Mannich-type reaction to furnish the addition product 18 (Scheme 3, Table 1).Ethyl 3-methylpyridazine-4-carboxylate 7 and N-benzylidene-p-toluenesulfonamide 16 were dissolved in dry THF in a pressure vial under an inert argon atmosphere and were heated at 120°C.Copper(II)triflate (9.4 mol%) was added, together with an equal amount of 1,10-phenanthroline as a ligand to solubilize the copper catalyst.This copper salt does not only activate imine 16 by acting as a Lewis acid, but also shifts the equilibrium between 7 and 7a towards the enamine by the formation of a metal enamide species 17. 24 In a first entry, an excess of 7 was used and the reaction was stirred for 1 hour 40 minutes, resulting in a conversion of 9% of 7 to 18.When 5 mol% of catalyst was added and the reaction was performed for 24 hours, the conversion increased to 30% (entry 2).However, the isolated yield of 18 was very low, due to a very difficult purification by column chromatography.Increasing the equivalents of 16 did lead to an increase in yield after 20 hours (entry 3), while an extensive reaction time of 89 hours only offered a moderately improved yield (entry 4).Adding 1.5 equivalents of DIPEA to trap the expelled proton (entry 5) did not improve the reaction outcome as well.Nevertheless, having the Mannich-type addition product 18 in hand, the ring closure towards 7-phenyl-6-tosyl-7,8-dihydropyrido[4,3-c]pyridazin-5(6H)-one 19 could be attempted (Scheme 4).In a first attempt, 18 was dissolved in DMSO and this solution was added to a solution of 2.5 equivalents of sodium hydride in DMSO and was stirred at 80°C, but no cyclized product could be recovered. 25When the solvent was changed to dioxane, no ring closure to 19 proceeded, but instead the saponification to 3-(2-(N-tosyl)-amino-2-phenylethyl)pyridazine-4-carboxylic acid 20 occurred.The last attempt comprised stirring 18 in a 10:1 dichloromethane:glacial acetic acid mixture overnight at room temperature.Also under these acidic reaction conditions, 19 could not be obtained.The desired ring closure most probably did not occur due to the (too) strong electron withdrawing potency of the N-tosyl group.Therefore, alternative aldimines were used in the pursuit of 7,8-dihydropyrido[4,3-c]pyridazin-5(6H)-ones.
An increase of the amounts of copper(II) triflate and 1,10-phenanthroline to 20 mol% and an increase in reaction temperature to 140°C resulted in a conversion of 44% (entry 2).The conversion could be enhanced by the use of 2 equivalents of 15b, resulting in a conversion of 74% and a yield of 54% (entry 3).Because of the very slow reaction rate resulting in an extremely long reaction time, even at 120-140°C, the influence of microwave (MW) heating was evaluated in the next entries (entries 4-6).When microwave heating was applied at 165°C for 25 minutes, no reaction product could be isolated (entry 4).These conditions were too harsh for the microwave vial septum, resulting in the loss of the reaction mixture.When a temperature of 135°C was maintained for 9 hours, a conversion of 85% and the corresponding yield of 55% was obtained (entry 5).The reaction time could be reduced to 3 hours when 3 equivalents of imine 15b were added (entry 6).for 45 minutes.In this way, the unreacted ethyl 3-methylpyridazine-4-carboxylate 7 was hydrolyzed into the corresponding 3-methylpyridazine-4-carboxylate.When the organic phase was evaporated, 22b-g could be extracted with dichloromethane, together with the excess of aldimine 15b-g.The pure compounds 22b-g were now easily obtained via column chromatography in moderate to good yields as yellow viscous oils.The chloro derivatives resulted in somewhat lower yields, probably due to the interaction of the chlorine atoms with the copper catalyst.Attempts to further improve the yields were performed by a) the portion wise addition of the catalysts and aldimines, b) prolonging the reaction time up to 35 hours or c) using a dilute reaction mixture to increase the solubility of the starting ethyl 3-methylpyridazine-4-carboxylate 7. Unfortunately, these attempts were not successful.
The conformational structure and spectroscopic data of the compounds 22b-g did offer some intriguing results.Due to the steric interaction of the aryl groups positioned on the N 6 and C 7 -position, the C 7 -aryl group is forced into the pseudo-axial position (Figure 1). 26Also, the geometrical conformation led to a difference in 1 H-NMR shift of around 2 ppm for the two diastereotopic geminal protons H a and H b in an AB-system from the amide-methylene group, due to the neighbouring amide carbonyl anisotropy. 27By the use of a NOESY-1 H-NMR-experiment, proton H a was assigned as the proton which is most closely positioned at H eq,c (δ ≈ 3.60 ppm).Because of the anisotropy of the carbonyl group, proton H b has a much higher chemical shift (δ ≈ 5.60 ppm).Despite the difficult synthesis of the 7,8-dihydropyrido[4,3-c]pyridazin-5(6H)-ones 22, requiring rather harsh conditions by microwave heating, we did succeed in the synthesis of this new scaffold.

Conclusions
In conclusion, a novel approach towards 7-aryl-7,8-dihydropyrido[4,3-c]pyridazin-5(6H)-ones 22b-g is presented.The key step is a one-step Mannich-type reaction of ethyl 3-methylpyridazine-4-carboxylate 7 and aldimines 15b-g followed by an intramolecular ring closure, catalysed by the Lewis acid Cu(OTf) 2 under microwave heating.This ring closure occurred concomitantly with the Mannich-type reaction and the intermediate addition product could not be isolated nor observed.This new scaffold opens up new opportunities in the chemical space and in the field of heterocyclic chemistry for the pharmaceutical industry.

Experimental Section
General.High resolution NMR spectra were run on a Bruker Avance III Nanobay 400 MHz spectrometer 1 H-NMR (400 MHz), 13 C-NMR (100 MHz) and 19 F-NMR (376.5 MHz).Peak assignments were obtained with the aid of DEPT, 2D-HSQC, 2D-COSY spectra.The compounds were dissolved in deuterated solvents and the used solvent is indicated for each compound.Low resolution mass spectra were recorded on an Agilent 1100 Series VS (ES, 4000V) mass spectrometer.HRMS analysis was performed using an Agilent 1100 series HPLC coupled to an Agilent 6210 TOF-Mass Spectrometer, equipped with ESI/APCI-multimode source.IR-spectra were obtained from a Perkin Elmer Spectrum One infrared spectrometer.The purification of reaction mixtures was performed by flash chromatography using a glass column with silica gel (Acros, particle size 0.035-0.070mm, Pore diameter ca.6 nm).All microwave reactions were performed in a CEM Discover Benchmate with a continuous power output from 0 to 300 watt and a self-adjusting, single mode MW cavity.The reactions were performed in 10 mL thick-walled Pyrex reaction vessels, closed with a 'snap-on' septa cap and equipped with a small stirring bar.A ramp time of maximum five minutes was used whereby the temperature was increased from room temperature to the desired one.This temperature was maintained during the course of the reaction for the indicated time.The temperature control system used a non-contact infrared sensor to measure the temperature on the bottom of the vessel and was used in a feedback loop with the on-board computer to regulate the temperature from 25 to 250 °C by adjusting the power output (1 Watt increments).Cu(OTf) 2 and 1,10-phenanthroline were dried at 60-80°C at 2-3 mbar for at least one hour before every use in the reactions.X-ray intensity data were collected on a Agilent Supernova Dual Source (Cu at zero) diffractometer equipped with an Atlas CCD detector using CuKα radiation (λ = 1.54178Å) and ω scans.The images were interpreted and integrated with the program CrysAlisPro (Agilent Technologies).Using Olex2, the structure was solved by direct methods using the ShelXS structure solution program and refined by full-matrix least-squares on F 2 using the ShelXL program package.Non-hydrogen atoms were anisotropically refined and the hydrogen atoms in the riding mode and isotropic temperature factors fixed at 1.2 times U(eq) of the parent atoms.The amide and amine hydrogen atoms were located from a difference electron density map and were unrestrained refined.CCDC-1020668 contains the supplementary crystallographic data for this paper and can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44-1223-336033; or deposit@ccdc.cam.ac.uk).

Synthesis of ethyl 3-methylpyridazine-4-carboxylate (7) starting from ethyl 2chloroacetoacetate (8)
Synthesis of ethyl 2-chloroacetoacetate N-methoxycarbonylhydrazone (10). 19Ethyl 2-chloroacetoacetate (8) (16.21 g, 1eq) and hydrazinocarboxylate(9) (8.90 g, 1eq) was dissolved in 100 ml of dry ethyl ether in a 250 ml round-bottomed flask.After stirring for 24 hours at room temperature, the precipitate was filtered off and washed with 2 times 50 ml petroleum ether, resulting in pure ethyl 2-chloroacetoacetate N-methoxycarbonylhydrazone 10 as a white powder in 94% yield (22.24 g).Synthesis of 4-ethyl 1-methyl 6-ethoxy-3-methyl-5,6-dihydropyridazine-1,4(4H)dicarboxylate 12. 20 Ethyl 2-chloroacetoacetate-N-methoxycarbonylhydrazone (10) (5 g, 1 eq) was dissolved in a 2:1 diisopropyl ether:water mixture (35 ml:15 ml) and 1.06 equivalents (1.88 g) of sodium bicarbonate were added.The reaction mixture discoloured immediately and release of gas was noticed and methyl 2-(-4-ethoxy-4-oxobut-2-en-2-yl)diazene-1-carboxylate 11 was formed.After 2 hours of stirring at room temperature, the aqueous phase was discarded and the red organic phase was dried over magnesium sulfate.For the inverse electron demand hetero Diels-Alder reaction, 2.3 equivalents (4.69 ml) of ethyl vinyl ether were added to the dried solution of 11 in diisopropyl ether.The mixture was brought to reflux temperature and heated overnight.The reaction mixture was evaporated, resulting in 5.23 g of a yellow oil of 4-ethyl 1-methyl 6-ethoxy-3-methyl-5,6-dihydropyridazine-1,4(4H)dicarboxylate 12 in 91% yield over two steps, without purification.Synthesis of ethyl 3-methylpyridazine-4-carboxylate (7). 204-Ethyl 1-methyl 6-ethoxy-3methyl-5,6-dihydropyridazine-1,4(4H)-dicarboxylate (12) (18.48 g, 1eq) was dissolved in 250 ml of acetic acid in a 500 ml round-bottomed flask and 1.2 equivalents (4.05 ml) of bromine were slowly added.This resulted in the formation of a brown reaction mixture and the production of gas.After stirring for 24 hours at room temperature, 250 ml of diisopropyl ether was added, resulting in the formation of a precipitate which was filtered and washed with 2 times 50 ml of diisopropyl ether.100 ml of toluene was added and evaporated to remove residual acetic acid in an azeotropic distillation.The residue was dissolved in 75 ml of water and trace amounts of small impurities were filtered off.The filtrate was neutralized by 7.3 g of sodium bicarbonate and 12 g of sodium chloride was added.The mixture was extracted with 3 times 75 ml diisopropyl ether and the organic phase was dried over magnesium sulfate and evaporated.After column chromatography with ethyl acetate (R f =0.38) and subsequent recrystallization from diethyl ether, 8.03 g of colorless crystals of 7 were obtained in 72% yield.

Figure 2 .
Figure 2. Asymmetric unit of the crystal structure of 22e, showing thermal displacement ellipsoids at the 50% probability level and atom labeling scheme of the non-hydrogen atoms.

Table 1 .
Conditions for the synthesis of 18 a Based on NMR.

Table 2 .
Conditions for the synthesis of 22b a Based on NMR.