Efficient and highly regioselective synthesis of ethyl 1-(2,4-dichlorophenyl)-1H-pyrazole-3-carboxylates under ultrasound irradiation

A series of 14 ethyl 1-(2,4-dichlorophenyl)-1H-pyrazole-3-carboxylates has been synthesized from the cyclocondensation reaction of ethyl 4-methoxy-2-oxoalk[cycloalk]-3-enoates [EtO2CC(O)C(R2) = C(R1)OR, where R = H, Me; R1 = Pr, Ph, 4-MeOC6H4, 4-MeC6H4, 4-FC6H4, 4-ClC6H4, 4-BrC6H4, 4-NO2C6H4, fur-2-yl; R2 = H; R1, R2 = –(CH2)3–, –(CH2)4–, –(CH2)5–, –(CH2)6–, 3,4-dihydronaphth-2-yl] with 2,4-dichlorophenyl hydrazine hydrochloride under ultrasound irradiation with high regioselectivity and in 71–92% yields. The main goal of this methodology was the significant reduction of reaction times. The compounds were obtained after irradiation for 10–12 min. In addition, the structure of the ethyl 1H-pyrazole-3-carboxylates was supported by crystallographic data.