Reaction of 2-hydrazinothiazoles 1 with 1-thienyl-and 1-furyl-1,3-butanediones 2a,b in methanol in the presence of hydrochloric acid mainly leads to a mixture of pyrazoles 3 and pyrazolines 4 or pyrazoles 3 and 5 in strong acidic conditions. Isomeric hydrazones 6 and pyrazolines 4 were formed and isolated in these reactions in the absence of hydrochloric acid. It has been shown that the regioselectivity in the reaction of diketones 2 with hydrazine 1 is governed by both the concentration of acid and the nature of substituents in the 1,3-diketones 2. Cyclization of hydrazones 6 is shown to occur under milder conditions than dehydration for pyrazolines 4. The new heterocyclic compounds were prepared and fully characterized by NMR spectra and by X-ray analysis for 3c. © 2002 Elsevier Science B.V. All rights reserved.
Denisova, A. B., Sosnovskikh, V. Y., Dehaen, W., Toppet, S., Van Meervelt, L., & Bakulev, V. A. (2002). The regioselectivity of the formation of 2-pyrazolylthiazoles and their precursors from the reaction of 2-hydrazinothiazoles with 4,4,4-trifluoro-1-hetaryl-1,3-butanediones. Journal of Fluorine Chemistry, 115(2), 183–192. https://doi.org/10.1016/S0022-1139(02)00060-X