Reduction of pyridazin-3-ones (1a—d) with zinc dust in acetic acid affords selectively 4,5-dihydropyridazin-3-ones (2a—d), but N(2)-substituted (1) react further with excess of zinc to give equal amounts of pyrrolidin-2-ones (3) and 3-pyrrolin-2-ones (4).
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Brown et al. (1984) studied this question.