**2. Results and Discussion**

Isomeric allylic hydroperoxides **1a**, **1b**, **2a**, **2b**, **3c**, **3d**, related isomeric allylic alcohols **1a**/ , **1b**/ , **2a**/ , **2b**/ , **3c**/ , **3d**/ , and isomeric enones **1a**//, **1b**//, **2a**//, **2b**//, **3c**//, **3d**// (Figure 1) were constituents of a single RP-HPLC fraction. In the beginning of this study, the fraction contained 46% allylic hydroperoxides, 43% allylic alcohols, and 11% enones, calculated using relative intensities of the characteristic, isolated

<sup>1</sup>Н-NMR signals of *trans*-olefinic protons (see below). The final results of this study yielded 25%, 54%, and 21% of these allylically oxygenated compounds, respectively. Therefore, further transformation of allylic hydroperoxides to allylic alcohols and enones occurred upon analysis (registrations of NMR and MS spectra, preparation of samples, storage, etc.).

In UPLC-MS (ultra-performance liquid chromatography–mass spectrometry) analysis of oxidized cerebrosides, base peak chromatogram, and extracted-ion chromatograms provided limited information because many components eluted simultaneously. In particular, peaks for major isomeric allylic hydroperoxides **1a** and **1b** were not well-resolved (Supplementary Materials, Figure S1a,b).
