Pyridazine, 3-vinyl

Amongst miscellaneous UV spectra recorded are those of alkylthio adducts of pyrido[3,2-c]pyridazine (78KGS1272), various pyrido[3,4-d]pyridazines (57AC(R)728) and several vinyl-pyridine-azodicarboxylic ester adducts (79T2027, 79KGS639). 

The other main source of various pyridopyridazines from pyridines are the [4 + 2] cycloaddition reactions, already mentioned (Section 2.15.8.3), between vinylpyridines and azodicarboxylic esters (79T2027, 79KGS639) or triazolidinediones e.g. 78KGS651). 2-Vinyl-pyridines gave reduced pyrido[3,2-c]pyridazines (370), 4-vinylpyridines gave [3,4-c] analogues, whilst 2-methyl-5-vinylpyridine furnishes a mixture of the [2,3-c] and [4,3-c] compounds. Yields are low, however, and these remain curiosities for practical synthetic purposes. 

Phthalides — see Benzo[c]furan-l (3H)-one Phthalimide, 2-amino-pyridazine synthesis from, 3, 53 Phthalimide, N-cyclohexylthio-as vulcanization accelerator, 1, 404 Phthalimide. methylidine-polymerization, 1, 273 Phthalimide, N-(trichloromethylthio)-biocide, 1, 399 Phthalimide, 1-vinyl-polymerization, 1, 273 Phthalimide, N-vinyl-copolymer... 

The cycloisomerization of hydrazide 80, catalyzed by CuCl, gives diazine 81, 5-Ai-aminolactam 82, and, unexpectedly, 6,7-dihydro-4-vinyl-1-methylpyrazolo [3,4-if]pyridazine-7 83 (85MI2) (Scheme 135). 

JOC6503>, and new pyridazino-psoralens 15 were prepared via a furan ring expansion reaction <05T4805>. The reaction of 3-acetylcoumarins with alloxan followed by NH2NH2 easily produced 3-(2-oxo-2//-chromen-3-yl)-6//,8//-pyrimido[4,5-c]pyridazine-5,7-diones <05JHC1223>. Furano- and pyrano[2,3-c]pyridazines 17 and 18a,b as well as substituted quinolines were conveniently prepared from pyridazinone 16 and vinyl- and allyltriphenyl-phosphonium salts <05HAC56>. 

Under classical conditions, the reaction between 3 and styrene required 50 h of heating at 110 °C, and gave the dihydropyridazine adduct 10a [24], After SMWI with 30 W incident power for 5 min (Tmax = 154 °C), the adduct 10a was not detected whereas the totally dehydrogenated product, pyridazine 10b, was isolated in almost quantitative yield (Tab. 7.1, entry 8). Ethyl vinyl ether and 3 gave the same product, pyridazine 11, under both classical heating [25] and MW irradiation conditions (Tab. 7.1, entry 9). In this instance the DA adduct lost nitrogen and ethanol. 

A common method to synthesize pyridazines remains the inverse electron-demand Diels-Alder cycloaddition of 1,2,4,5-tetrazines with electron rich dienophiles. [4 + 2]-Cycloadditions of disubstituted 1,2,4,5-tetrazine 152 with butyl vinyl ether, acrylamide, phenylacetylene, and some enamines were performed to obtain fully substituted pyridazines 153 . This reaction was accelerated by electron withdrawing groups, and is slowed by electron donating groups, R1 and R2on the tetrazine. 

New examples involve the reaction of 5-vinyl-17/-imidazole 293 with 4-phenyl-l,2,4-triazoline-3,5-dione 294. In this way, the imidazo[4,5-i ]pyridazine skeleton was smoothly constructed (Equation 72) <1998TL4561>. Another example using in situ formed 294 can be found in Section 8.01.6.5. 

On standing at 25°C, the spiro-3//-pyrazole 57 is converted via an isolable 3-azopyrazole to a pyrazolotriazine.96 Pyridazines have been prepared both from a 4-vinyl-3//-pyrazole by heating in acetic acid92 and from a 3-phenacyl-3//-pyrazole in strong base.114... 

Pyridazine-3,4-dicarboxylic acid can be conveniently obtained as shown on Scheme 21 (90JHC579). The key step of this conversion is a [4 + 2] cycloaddition of the diazadiene (98) (generated in situ from the precursor (97)) and ethyl vinyl ether. 

Propynyl vinyl sulfide, 1877 Pyrazine, 1428b Pyridazine, 1428c... 

The formal addition of perfluorinated pyridine, pyrimidine, pyridazine or of pentafluorobenzo-nitrile to fluorinated acetylenes in the presence of cesium fluoride in sulfolane leads to fluorinated aryl-substituted alkenes. " In the first reaction step fluoride ion adds to the fluorinated acetylene to give a vinyl carbanion, which substitutes, in a second step, a fluoride ion from the perfluorinated aromatic compound. Some examples of this type of reaction are shown by the formation of... 

Dibromoimidazoles were employed as precursors to syntheses of 6-substituted imidazol[4,5-d]pyridazin-7-ones <03H(60)1329>. 4-Vinyl imidazoles participated in intermolecular Diels-Alder reactions with V-phenylmaleimide <03H(60)1> and in intramolecular Diels-Alder reactions <03OL3623>. 

Upon heating a mixture of benzaldehyde and hydrazine salt in the presence of styrene (2 1 6 molar mixture), in addition to 1,5-diazabicyclo [3.3.0] octane, 3,5,6-triphenyl-l,4,5,6-tetrahydropyridazine was obtained as byproduct. The latter was the only product when a 2 1 2 ratio was used. The pyridazine formation is explained on the basis of cycloaddition of benzaldehyde azine to styrene, followed by tautomerization (87JOC2277). Similarly, other diazadienes add ethyl vinyl ether to give also 1,4,5,6-tetrahydropyridazines [83JCS(PI)1803j. 

Pyridazines were obtained also by photolysis of 1-phenyl-l-vinyl azide in the presence of iron pentacarbonyl (3,6-di-phenylpyridazine was obtained in 1.1% yield) (78HCA589) or by thermal decomposition of an allenic hydrazonate (81JA7011). Acetylenic hydrazides can be transformed into pyridazines [84BSF(2)129], and thermal cyclization of dialkali metal salts of cu-hydroxyketone tosylhydrazones afforded pyridazines in moderate yield (85TL655). Propionyl phenylhydrazine, after reaction with 4-bromobutyronitrile, converts into a pyridazine (87SC1253). 

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