Toluene → tert-butyl 4,5-dihydro-1H-pyrazole-5-carboxylate

1,1-Dimethylethyl 2-propenoate (10.25 ml, 70 mmol) was added to a round-bottom flask equipped with a stirring bar and charged with toluene (100 ml) and tetrahydrofuran (THF) (100 ml) under nitrogen, and then TMS-diazomethane (42.0 ml, 84 mmol) was added. After 4 h, the reaction solvents and excess TMS-diazomethane were removed in vacuo. The residue was dissolved in dichloromethane (150 mL). TFA (6.26 ml, 84 mmol) was added at 0° C. and the resulting mixture was stirred at room temperature overnight. After this time, the reaction was quenched with sat. Na2CO3 and extracted with dichloromethane. The organic layer was dried over Na2SO4. The solvent was removed under reduced pressure to give the crude product as yellow oil, which was purified by silica gel chromatography (Combiflash, 120 g column, hexane/ethyl acetate 0-30% 60 min) to yield the desired product 1,1-dimethylethyl 4,5-dihydro-1H-pyrazole-5-carboxylate (9.7 g, 81% yield) as yellow oil.