Reaction of ethyl (2S*,3R*)-3-chloro-2-(dialkylamino)-3-phenylpropanoates (4) with hydrazine monohydrate gave pyrazolidin-3-ones (5) via selective opening of the in situ generated aziridinium ions, followed by intramolecular amidation. (1Z,4S*,5S*)-1-Arylmethylidene-4-(dialkylamino)-3-oxo-5-phenylpyrazolidinium-1-ide (6), prepared from pyrazolidin-3-ones (5) and aromatic aldehydes under acid catalysis, reacted with a variety of 1,3-dipolarophiles to afford good yields of pure cycloadducts 7 – 11. The cycloaddition regio- and/or stereoselectivities, in all relevant cases, were also high.
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Chuang et al. (2000) studied this question.