The diastereoselectivity of diazomethane addition to the conjugated double bond of alpha,beta-unsaturated sesquiterpene lactones was explored using zaluzanin A (1) as a model. Thus, the absolute configuration of 1 was assured by X-ray diffraction analysis including evaluation of Flack and Hooft parameters, and by vibrational circular dichroism spectroscopy of its diacetyl derivative 2, while the absolute configuration of the diazomethane addition product, zaluzanin A pyrazoline (3), was determined by evaluation of the 1H NMR chemical shift changes with respect to 1, and confirmed by X-ray diffraction analysis, again including evaluation of Flack and Hooft parameters.