Synthesis of functionalized compounds containing pyridazine and related moieties

3-Aminopyridazines 17 and 3-hydrazinopyridazines 18 were used as building blocks for the preparation of various types of functionalized, pyridazine ring containing compounds. 3-Aminopyridazines were employed in the synthesis of 3-(6-chloroimidazo[1,2-b]pyridazin-2-yl)alanines 26, 27 and for the preparation of 3-amino-4H-pyrimido[1,2-b]pyridazin-4-ones 103, intermediates in the ‘ring switching’ synthesis of alkyl 1-pyridazin-3-yl-1,2,3-triazole-4-carboxylates 106.On the other hand, hydrazinopyridazines 18 were employed in a two-step preparation of 3-functionalized 1,2,4-triazolo[4,3-b]pyridazines via condensation with functionalized aldehydes and their enamino analogs followed by oxidative cyclization of the intermediate hydrazones. In this manner, 1,2,4-triazolo[4,3-b]pyridazin-3-yl substituted alanines 29, 30, polyols 33, 39–48, C-nucleosides 49, 50, and terpenes 58, 62, 64–69 were prepared. In another general approach, 3-hydrazinopyridazines 18 were treated with functionalized enaminones as 1,3-dielectrophiles to give the 1-(substituted pyridazin-3-yl)-1H-pyrazole derivatives containing an ester 72, 73, 75, 76, alanine 79, 84, 85, 87, 2-phenylethylamine 97, 99, and β-amino alcohol functional element 98, 100. In the reaction of 4-oxohomoglutamate 82 with hydrazine hydrate and methyl hydrazine, chiral functionalized tetrahydropyridazinones 88a,b were obtained.