Applications of aziridinium ions: Selective syntheses of pyrazolidin-3-ones and pyrazolo[1,2-alpha]pyrazoles

Reaction of ethyl (2S*,3R*)-3-chloro-2-(dialkylamino)-3-phenylpropanoates ( 4) with hydmzine monohydrate gave pyrazolidin-3-ones (5) via selective open ing of the ill sills generated aziridinium ions, followed by intramolecular amidation. (1Z,4S*,5S*)-1-Arylmethylidene-4-(dialkylamino)-3-oxo-5-phenylp yrazolidinium-1-ide (6), prepared from pyrazolidin-3-ones (5) and aromatic aldehydes under acid catalysis, reacted with a variety of 1,3-dipolarophile s to afford good yields of pure cycloadducts 7-11. The cycloaddition regio- and/or stereoselectivities, in all relevant cases, were also high.