Facile Synthesis of Chalcone Glycosides Isolated from Aerial Parts of Brassica rapa l. ‘hidabeni’.

Background: The Cruciferous family of vegetables which includes Brassica Turnips showed antioxidant and hepatoprotective effects. The phytochemical investigations of the aerial parts of the traditional Japanese turnip vegetable (B. rapa L. 'hidabeni') revealed the presence of three chalcone glycosides, along with other glycoside components. As many natural products inhibited Ag-stimulated degranulation in cellular system, those chalcone glycosides have biological significance of suppressing antigen-stimulated degranulation in rat basophilic leukemia RBL-2H3 cells. Aim and Objective: Further investigation on the biological importance of those chalcone glycosides demands ample quantities of well-defined compounds. Therefore, we report herein a convenient and concise synthetic approach for the preparation of those chalcone glycosides. Materials and Methods: 4'-O-β-D-Glucopyransoyl-4-hydroxy-3'-methoxychalcone and 4'-O-(β-D-Glucopyransoyl)- 3',4-dimethoxychalcone were synthesized using a three-step strategy includes: i) O-glucosylation of 4-OH of 4'-hydroxy-3'-methoxy acetophenone (Acetovanillone); ii) introduction of the cinnamoyl residue by aldol condensation with p-benzyloxy benzaldehyde and p-methoxy benzaldehyde respectively; iii) full debenzylation of all the sugar hydroxyl groups. Meanwhile, 4,4'-Di-O-β-D-glucopyransoyl-3-methoxychalcone was synthesized by an alternative way where a double armed aglycon acceptor was utilized in a one pot double glycosylation reaction. Results: Constructing the target chalcone glycosides: 4'-O-β-D-Glucopyransoyl-4-hydroxy-3'-methoxychalcone, 4'-O-(β-D-Glucopyransoyl)-3',4-dimethoxychalcone and 4,4'-Di-O-β-D-glucopyransoyl-3-methoxychalcone were achieved in 13%, 14%, and 90% yields. Conclusion: A simple and practical synthetic procedure by which the target chalcone glycosides were synthesized could be a promising and viable method. Furthermore, this strategy could be utilized in the synthesis of various O-diglycosyl chalcones having more complicated structures.