Ethyl 1,6-dihydropyridazine-3-carboxylate

The base-catalyzed cyclization of ethyl 3-[alkyl(4-cyanopyridazin-3-yl)amino]propanoates gives ethyl 8-alkyl-5-amino-7,8-dihydropyridazine-6-carboxylates 1, which by acidic hydrolysis were converted to the corresponding ends 2. Due to their air sensitivity, ends 2 are partly converted into ethyl 8-alkyl-5-oxo-5,8-dihydiopyiido[2,3-c]pyridazine-6-carboxylates 3. To avoid mixtures, the oxidation is completed by treatment with tetrachloro-l,2-benzoquinone (o-chloranil).20... 

In a related reaction sequence, either arylhydrazones of diethyl 3-amino-2-cyano-4-oxopent-enedioate or their cyclization products, the corresponding ethyl 4-amino-l-aryl-5-cyano-6-oxo-l,6-dihydropyridazine-3-carboxylates react with malononitrile or ethyl cyanoacetate to give pyrido[2,3-d]pyridazine-2,5(1 f/,6//)-diones 2 or 3.71... 

A new route to pyridazine analogues of dihydropyridine calcium antagonists has been devised. Ethyl l,3-dithiolane-2-carboxylate was found to add efficiently, as its lithium salt, to benzylidene derivatives prepared from aryl aldehydes and diethylmalonate or ethyl acetoacetate the dithiolane adducts were readily deprotected using NBS (Scheme 98). The resultant diketo derivatives (121) were cyclized with hydrazine to give either 1,4-dihydropyridazines (122) or l,4,5,6-tetrahydro-6-... 

The 1,3-dipolar addition of secondary diazo-compounds to 1,3,3-trimethylcyclo-propene affords 2,3-diazabicyclo[3,l,0]hex-3-enes [cf. (130)], but with diazoacetic ester this product, with an acidic proton at C-4, undergoes rearrangement to ethyl 3,4,4-trimethyl-l,4-dihydropyridazine-6-carboxylate. Further additions of phenyl azide to methylenecyclopropanes have shown that the presence of a carboxylate group in the three-membered ring causes the initially formed spirodihydrotriazole to undergo rearrangement to a 4-substituted 1,2,3-triazole. ... 

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