UV-assisted rearrangement of substituted 3-arylaminopyrazoles with an allomaltol fragment into tricyclic cyclopenta[4,5]pyrrolo[2,3- <i>c</i> ]pyrazole derivatives

In the present study, we report a novel photochemical rearrangement of substituted 3-arylaminopyrazoles incorporating an allomaltol fragment. Under UV-irradiation, these compounds undergo ESIPT-induced contraction of the pyran-4-one ring, leading to an unstable α-hydroxy-1,2-diketone intermediate. For the first time, this transient intermediate was successfully trapped <i>via</i> intramolecular cyclization involving the nitrogen atom of the aniline moiety. Using this UV-induced cascade transformation, a facile method for the synthesis of tricyclic cyclopenta[4,5]pyrrolo[2,3-<i>c</i>]pyrazole derivatives was elaborated. The applicability of the designed approach was demonstrated by the preparation of 14 photoproducts with yields up to 78%. The structures of the representative starting pyrazole and one of the target photoproducts were unambiguously confirmed by X-ray analysis.