AbstractIntroductionQuantitative nuclear magnetic resonance (qNMR) is one of the effective and reliable quantification tools for natural product research. Myelochroa leucotyliza belongs to the genus Myelochroa, a common foliose lichen genus found in the Korean Peninsula, and has not been quantitatively analysed using NMR. Previous chemical studies on M. leucotyliza have been limited to the main components by traditional thin‐layer chromatography (TLC) experiments.ObjectiveWe explored the stability of atranorin, a major component of M. leucotyliza, in methanol and acetone using NMR and characterised the changes in the chemical profiles of the lichen extracts in methanol and acetone using qNMR.MethodologyAtranorin transformation in the presence of methanol was analysed using time‐dependent proton (1H)‐NMR analysis (600 MHz NMR spectrometer). A 1H qNMR (qHNMR) method was established using dimethyl sulfone as the internal standard for quantifying the selected components isolated from M. leucotyliza. Homogenous mixtures of the samples were dissolved in deuterated chloroform.ResultsTime‐dependent 1H‐NMR experiments revealed that atranorin (5) from lichen M. leucotyliza decomposed into atraric acid (1) and methyl haemmatommate (2) in methanol. Four components were identified from M. leucotyliza: 1, 2, usnic acid (4), and 5, and their respective contents were determined using qHNMR. The percentages (w/w) of 1, 2, and 4 in the methanol extract were calculated as 5.66%, 0.69%, and 0.90%, while those of 1, 4, and 5 in the acetone extract were 1.70%, 1.68%, and 19.11%, respectively.ConclusionWe used qHNMR to effectively analyse quantitative compositional variations in two different M. leucotyliza extracts and reliably determined the chemical conversion of the unstable compound atranorin.