Selecting a Green Strategy on Extraction of Birch Bark and Isolation of Pure Betulin Using Monoterpenes

The aim of this work is to study sustainable approaches to pure betulin using green low-polar solvents from natural feedstockpine monoterpenes and limonene. The conventional methods of extracting outer birch bark (OBB) lead to extracts that contain only 50%–75% of betulin, while refining processes of OBB extracts are usually labor consuming and employ toxic solvents. The extractions of OBB using monoterpenes and their hydrogenated analogs at boiling and microwave-assisted conditions turned out to be insufficiently selective and lead to a significant uptake of extractants on the depleted bark. To obtain pure betulin, the sequence including energy-saving extraction of OBB with a green solvent (ethyl acetate, EtOAc), saponification of the extract, and purification of betulin in limonene or hydrogenated monoterpenes was proposed. The protocol of processing primary extracts with stable monoterpeneslimonene, pinane, and hydrogenated turpentine oilensured separation of pure betulin (95%–97%) with high yields (75%–82%) and the effective recycling of extractants (81%–87%). The refining of plant metabolites in monoterpenes can be regarded as a promising method, successfully competing with the known multistep processes and methods employing toxic solvents