Chemo- and regioselective defluorinative annulation of (trifluoromethyl)alkenes with pyrazolones: synthesis and insecticidal activity of 6-fluoro-1,4-dihydropyrano[2,3-c]pyrazoles

Abstract

The chemo- and regioselective defluorinative [3 + 3] annulation of (trifluoromethyl)alkenes and pyrazolones is reported that gives rise to various useful 6-fluoro-1,4-dihydropyrano[2,3-c]pyrazoles in high yields. This reaction distinguishes the different nucleophilic sites of pyrazolones and features mild conditions, a broad substrate scope, and gram-scalability. A simple derivation of the obtained 6-fluoro-1,4-dihydropyrano[2,3-c]pyrazoles efficiently afforded diverse useful compounds. Significantly, compound 3w exhibits up to 100% insecticidal activity against Plutella xylostella, which is a destructive pest worldwide. Mechanism studies indicate that this reaction proceeds via a chemo- and regioselective S_N2′/S_NV pathway and that the double defluorinative alkylation of pyrazolones with (trifluoromethyl)alkenes is completely inhibited.