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The assignment of the carbon‐13 nmr resonances of 3‐(aryl)‐5‐phenylisoxazoles (I) and 3‐phenyl‐5‐(aryl)‐isoxazoles (II) in dimethylsulfoxide‐d6 has been made. The assignments were made by using model compounds, chemical shift arguments, coupling with fluorine substituents, and shifts resulting from quaternization of the isoxazole ring. A carbon‐13 nmr method for distinguishing between I and II which can be produced simultaneously by certain synthetic methods is presented.

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The carbon-13 nmr spectra of a series of 3-aryl-5-phenylisoxazoles (I) and 3-phenyl-5-arylisoxazoles (11) have been recorded and the signals assigned. Carbon-13 data for series I show little effect of substituent on the chemical shift of the isoxazole ring carbons. However, a plot of the carbon-13 chemical shift of carbon-5 in the isoxazole system I versus the chemical shift of carbon-3 in the 3-(4'-aryl~l-phenylpropenones gives a straight line (r = .989) with a slope of 0.35. In series 11, the chemical shifts of both carbon4 and -5 are relatively sensitive to substituent effects. Fair correlations between Hammett sigma values and the chemical shifts of these two carbons are found; dual substituent parameter treatment improves the correlations. The results obtained from correlations with carbon-4 in series 11 are similar to those obtained from 0-carbons of a number of styrene systems. The data show that carbon-4 in series I1 is approximately 20% less sensitive to substituent effects than the previously reported data for carbon-3 of 2-arylfurans. Transmission of substituent effects in the isoxazole system compare well with those of the benzothiazole system. J. Heterocyclic Chem., 18, 795 (1981).Carbon-13 nmr has been successfully used to examine the transmission of substituent effects in a variety of aromatic (1) and heteroaromatic (2) carbon frameworks in-