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A series of aryloxymethylquinoxaline oximes, hitherto unknown and synthesized from the corresponding aldehydes, afforded in only one step pyrazolo[1,5-a]quinoxalines in the presence of acetic anhydride at high temperatures. A formal [3,5]-sigmatropic rearrangement was proposed as the mechanistic rationale for this unprecedented transformation. Saponification with potassium hydroxide furnished the free phenol derivatives which were studied by NMR spectroscopy and accompanying theoretical DFT calculations, establishing intramolecular hydrogen bonding and the spatial magnetic properties. Additionally, mass spectrometric fragmentation was investigated by B/E-linked scans and collision-induced dissociation experiments. The fragmentation pattern devoted a new gas phase rearrangement process, which proved to be unique and characteristic for pyrazolo[1,5-a]quinoxalines.

In the presence of a base the title compounds react to products with ether structure (4, 6), or with ester structure (3), or to structure 5 containing both functionalities in dependence on the mole ratio of the starting substances, on reaction conditions and on the substituent patterns in the hydroxybenzoic acid component. Under the influence of alkali hydroxide the m‐ and p‐substituted compounds (6e–g) are saponified to the alkali salts of the carboxylic acid (4b, c). The o‐substituted compounds (6a–d), however, are cyclized to the benzo[b]furanylquinoxalines (8). 8a, d are also obtained by thermal water elimination of the carboxylic acids 4a, d. The red‐coloured benzofuranols 8 react with acetic anhydride and benzoyl‐chloride/pyridine, resp., to the weakly yellow esters 9. The stucture of the products 8 and 9 is studied by UVVIS derivative spectroscopy, by theoretical calculation of the dihedral angles and by 1H and 13C NMR spectroscopy. The 1H and 13C chemical shifts are completely assigned. The quinoxalines 8a–c and the quinoline 8d only exist in the hydroxy form.