Anionic [4 + 2] cycloaddition reactions of dihydropyrazolin-5-one dienolate with dienophiles: a novel approach for substituted and fused indazolones

Dihydropyrazolin-5-one dienolate 2 generated by deprotonation of 2,3-dimethyl-4-formyl-1-phenylpyrazolin-5-one (4-formylantipyrine) is shown to be an efficient anionic pyrazolone α -oxy-o-quinodimethane analog, which undergoes facile cycloaddition with a variety of dienophiles to give substituted indazolones in good yields after dehydration-dehydrogenation of cycloadducts