Unusual Reactions between Some 1-Aroyl-4,5-dihydro-4,4-dimethyl-5-methylene-1H-pyrazoles and Ketenes

Article abstract of DOI:10.1021/jo00302a046

5-Methylene-4,5-dihydropyrazoles 1a-d react with diphenylketene (DPK), affording the pyrazolyl-3,3-diphenyl-2-propenyl benzoates 2a-d, very likely via the corresponding pyrazolyl-enol esters 7 as intermediates.By acid hydrolysis of 2 the (diphenyloxopropyl)pyrazole derivative 3 in its enol form is isolated, whereas by cycloaddition with 4-methylbenzonitrile oxide the spiro compound 4 is obtained. 5-Methylene-4,5-dihydropyrazoles 1a-c and 1g also react with dichloroketene (DCK) to give the corrresponding 5-hydroxypyrazolines 5.The structure of 2b has also been confirmed by X-ray crystallography.