Synthesis of bidesmosidic lupane saponins - comparison of batch and continuous-flow methodologies

Synthesis of lupane bidesmosides was optimized. The title compounds were obtained by glycosylation of 3-O- or 28-O-substituted betulin monodesmosides with Schmidt donors catalyzed by TMSOTf. Classical batch procedure and microreactor technique were used and compared in the above synthesis. Experimental results clearly showed that both methods are comparable, although any particular outcome strongly depends on the structure of the reagents. Undesired allobetulin derivatives formed by the Wagner-Meerwein rearrangement were usually isolated in minute amounts. In the case of batch reaction, shorter reaction time significantly decreased formation of side-products.