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We report the first synthesis of a series of bisdesmosidic oleanolic acid saponins using microflow reactor Comet X-01 via a continuous flow glycosylation-batch deprotection sequence. The main results of this study can be summarized as follows: (1) The microfluidic glycosylation of oleanolic acid at C-28 was achieved in quantitative yield and was applied to the synthesis of six C-28-monoglycosidic saponins. (2) The microfluidic glycosylation of oleanolic acid at C-3 was achieved in good yield without orthoester byproduct formation and was applied to the synthesis of three bisdesmosidic saponins. (3) The continuous synthesis of saponins via a microfluidic glycosylation-batch deprotection sequence was achieved in four steps involving two purifications. Thus, the continuous microfluidic glycosylation-deprotection process is expected to be suitable for the preparation of a library of bisdesmosidic oleanolic acid saponins for in vivo pharmacological studies.

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An improved process for preparing tenuifolin (presenegenin 3-β-d-glucopyranoside) from the root of Polygala senega L. was developed. A crude saponin mixture extracted from P. senega was subjected to hydrolysis, and the reactivity of compounds in the extract was controlled by utilizing the combination of a flow reactor and experimental design. In addition, column chromatography with HP 20, a synthetic polystyrenic adsorbent, allowed the gram-scale preparation of tenuifolin in a continuous manner with fewer steps. This approach shortens the total time required for gram-scale preparation from 16 to 5 h in a continuous manner while improving the yield from 0.59% to 2.08% (w/w).