Development of a One-Step Method for Producing Allobetulin from Birch Outer Bark and Study of Its Antioxidant Activity

Objective: Birch bark contains extractives of lupane series, that have pharmacological activity and are of interest for obtaining new medicines. The predominant component of the birch outer bark extracts is pentacyclic triterpene alcohol, betulin. Allobetulin (19,28-epoxyoleanan-3-ol), a pentacyclic oleanane-type triterpenoid, is an isomer of betulin. Allobetulin and its derivatives are nontoxic compounds having antiviral, antimicrobial, and antitumor activities. Methods: The composition of the allobetulin sample after recrystallization from isopropanol was confirmed by elemental analysis, and its structure, by IR and ¹H NMR spectroscopy. The antioxidant properties of allobetulin were tested in vivo and in vitro. Results and Discussion: In the present study, a new one-step method was developed for producing allobetulin directly from birch outer bark by combining the stages of betulin extraction with ethanol and isomerization into allobetulin in the presence of an orthophosphoric acid catalyst. The effect of the duration of the birch outer bark treatment with ethanol and of the H3PO4 content on the allobetulin yield was evaluated. The highest yield of crude allobetulin, 35% of the weight of absolutely dry bark (a.d.b.), was achieved at a process duration of 8 h in the presence of 5% H₃PO₄ and also at a process duration of 15 h in the presence of 3% H₃PO₄. Conclusions: The yield of recrystallized allobetulin was ~22% of the weight of a.d.b.