Synthesis and Characterization of New Pyrano[2,3-]pyrazole Derivatives as 3-Hydroxyflavone Analogues.

In this paper, an efficient synthetic route from pyrazole-chalcones to novel 6-aryl-5-hydroxy-2-phenylpyrano[2,3-]pyrazol-4(2)-ones as 3-hydroxyflavone analogues is described. The methylation of 5-hydroxy-2,6-phenylpyrano[2,3-]pyrazol-4(2)-one with methyl iodide in the presence of a base yielded a compound containing a 5-methoxy group, while the analogous reaction of 5-hydroxy-2-phenyl-6-(pyridin-4-yl)pyrano[2,3-]pyrazol-4(2)-one led to the zwitterionic 6-(-methylpyridinium)pyrano[2,3-]pyrazol derivative. The treatment of 5-hydroxy-2,6-phenylpyrano[2,3-]pyrazol-4(2)-one with triflic anhydride afforded a 5-trifloylsubstituted compound, which was further used in carbon-carbon bond forming Pd-catalyzed coupling reactions to yield 5-(hetero)aryl- and 5-carbo-functionalized pyrano[2,3-]pyrazoles. The excited-state intramolecular proton transfer (ESIPT) reaction of 5-hydroxypyrano[2,3-]pyrazoles from the 5-hydroxy moiety to the carbonyl group in polar protic, polar aprotic, and nonpolar solvents was observed, resulting in well-resolved two-band fluorescence. The structures of the novel heterocyclic compounds were confirmed by H-, C-, N-, and F-NMR spectroscopy, HRMS, and single-crystal X-ray diffraction data.