Isolation and characterization of related impurities in 24-epibrassinolide

Two extra-hydroxylated analogs of 24-epibrassinolide (2), 17-hydroxy-24-epibrassinolide (4), 25-hydroxy-24-epibrassinolide (5), and two oxo-analogs of 24-epibrassinolide 22-oxo-23S-24-epibrassinolide (6), 22R-23-oxo-24-epibrassinolide (7) as impurities were isolated and identified from the initial material [purity: 2 and its 22S, 23S-isomer (3) >95%] by using various chromatographic methods and repeated crystallization. Among them, compound 4 and compound 6 were first reported. Their structures were established by spectrometric analysis and X-ray crystallography study. Formation of mixed crystals of 6 and 7 in the crystallization process was postulated and further confirmed.