13C nuclear magnetic resonance studies of sesquiterpenes,anisatin and neoanisatin: The structure of anisatinic acid,a novel isomerization product of anisatin

¹³C NMR spectra are reported for derivatives of toxic sesquiterpenes, anisatin (I) and neoanisatin (II). Chemical shifts for each compound have been assigned on the basis of off-resonance decoupling experiments, known chemical shift rules, and comparison of the spectra among the compounds examined. Toxic anisatin (I) is known to isomerize under mild conditions to a non-toxic compound, anisatinic acid. The structure of anisatinic acid has been determined unambiguously to be IIIa by the ¹³C NMR spectral analysis of a derivative 8 of anisatinic acid. Some aspects of the substituent effects on the ¹³C chemical shifts obtained in the present investigation are described.