Unexpected Trifluoromethylated Pyrazoles From Ethyl 2-diazo-4,4,4-trifluoroacetoacetate and 1-diethylamino-prop-1-yne

Ethyl 2-diazo-4,4,4-trifluoroacetoacetate reacts with N,N-diethylamino-prop-1-yne via an unusual pathway: the ynamine first reacts with the carbonyl adjacent to CF3, leading to a non-isolated vinyldiazomethane, which undergoes intramolecular cyclisation and then a [1,5]-shift to afford a pyrazole