ANRORC rearrangement in tetrahydro-2H-chromenones. Synthesis and structural assignment by NMR, MS, X-ray and DFT calculations of 2-[3(5)-trifluoromethyl-1H-pyrazol-4-yl)arylmethyl]cyclohexenones and derivatives

•Hydrazine promotes ANRORC reaction in 2-(trifluoromethyl)-2H-chromen-5(6H)-ones.•New series of 2-[(1H-pyrazol-4-yl)arylmethyl]-cyclohex-2-en-1-ones is synthesized.•The respective ANRORC reaction type mechanism is presented and discussed.•Structures solved by MS, X-ray, DFT calculations and 1H, 13C and 19F NMR data.•19F NMR assignments the trifluoromethyl group position at pyrazolyl-cyclohexenones.

This paper describes firstly the synthesis of a new series of 3-hydroxy-2-[(3(5)-(methyl/phenyl)-5(3)-(trifluoromethyl)-1H-pyrazol-4-yl)arylmethyl]-cyclohex-2-en-1-ones (2), where aryl = C6H5, 4-NO2C6H4, 4-OCH3C6H4, from an ANRORC ring transformation reaction of 3-acyl-4-aryl-2-(trifluoromethyl)-2-hydroxy-3,4,7,8-tetrahydro-2H-chromen-5(6H)-ones (1), where acyl = acetyl and benzoyl, in the presence of hydrazine hydrate, at 63–90% yields. In subsequent steps, an oxidative aromatization reaction from 2 was done in an iodine/methanol medium for the preparation of 3(5)-trifluoromethyl-5(3)-methyl-4-[(2,6-dimethoxyphenyl)-(4-methoxyphenylmethyl)]-1H-pyrazole (3) at 72% yields. Also, the alkylation reaction using benzyl chloride easily converted a pyrazole like 2 to its N-benzyl derivative (4), at 69% yield. Finally, the structural assignment of compounds 2–4 was deduced by mass spectrometry, X-ray crystal diffraction and density functional theory (DFT) calculations, which clearly and unambiguously furnished values very close to those determined from 1H, 13C and 19F NMR data.

Graphical abstractA new series of 5(3)-(trifluoromethyl)-1H-pyrazol-4-yl)arylmethyl]-cyclohex-2-en-1-ones from an ANRORC ring transformation reaction of 2-(trifluoromethyl)-3,4,7,8-tetrahydro-2H-chromen-5(6H)-ones, in the presence of hydrazine hydrate, at 63–90% yields, is described.Figure optionsDownload full-size imageDownload as PowerPoint slide