Disic acids. A study of a new series of strong organic acids and their condensation products with aldehydes

4-(p-Nitrophenyl)-3,5-dihydroxyisoxazole is a remarkably strong acid which possesses unique chemical reactivity towards carbonyl compounds. It has been shown that these properties are encountered in a series of related 3,5-dihydroxyisoxazole derivatives. It appears that this isoxazole system is very electron deficient, a fact that together with a neighboring group effect is responsible for the exceptional acidity and reactivity. The electron deficiency is also reflected in the deshielding of the aromatic protons. Comparison of the NMR spectra served as a criterion for the electron deficiency of the isoxazole moiety. This new class of acids yield a variety of novel systems. The influence of substitution on the visible spectrum of the aldimonium system obtained with derivatives of benzaldehyde is discussed.