Improved preparation of vitexin from hot water extract of Basella alba, the commercially available vegetable Malabar spinach ("Tsurumurasaki" in Japanese) and the application to semisynthesis of chafuroside B.

Hot water extraction of D-arabinofuranosylvitexin from the raw leaves of commercially available Basella alba "Tsurumurasaki" and subsequent acidic hydrolysis was improved to be a procedure using a high-pressure steam sterilizer to afford vitexin. The amount was estimated to be 14.1 mg from 1 g of dry weight of the raw leaves, whose recovery was calculated to be 95% based on the estimated content of D-arabinofuranosylvitexin in B. alba raw leaves. The product was dehydratively cyclized between hydroxy groups on the carbohydrate and flavone skeletons under modified Mitsunobu reaction conditions in N,N-dimethylformamide to give chafuroside B, which is known to be a bioactive Oolong tea polyphenol. Through these transformations, 10.2 mg of chafuroside B could be semisynthesized from 1 g of dry weight of the raw leaves, and the efficiency was improved compared to that from the extraction from Oolong tea (3.4 μg from 1 g of dry weight).