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.

中文翻译：

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选择单萜类化合物提取桦树皮和分离纯桦木的绿色策略

这项工作的目的是研究使用天然原料（松油单萜和柠檬烯）中的绿色低极性溶剂生产纯净桦木素的可持续方法。传统的提取白桦树皮（OBB）的方法导致提取物中只含有50％–75％的白桦蛋白，而OBB提取物的精制过程通常很费力并且使用有毒溶剂。事实证明，在沸腾和微波辅助条件下使用单萜及其氢化类似物萃取OBB的选择性不足，并导致枯竭的树皮大量吸收萃取剂。为了获得纯的桦木素，提出了包括用绿色溶剂（乙酸乙酯，EtOAc）节能萃取OBB，皂化提取物以及在柠檬烯或氢化单萜烯中纯化桦木素的步骤。使用稳定的单萜烯（柠檬烯，pin烷和氢化松节油）加工初级提取物的协议可确保分离出高纯度（75％–82％）的纯桦木素（95％–97％）并有效地回收萃取剂（81％ –87％）。在单萜中精制植物代谢物可以被认为是一种有前途的方法，成功地与已知的多步法和使用有毒溶剂的方法竞争。