A simple and high-yielding protocol for the malonylation of some flavonoid glycosides is described. The two-step synthesis is based on the regioselective enzymatic introduction of a benzylmalonyl group by catalysis with the lipase from Candida antarctica, followed by Pd/C hydrogenolysis of the benzyl moiety.
Bovine ~-1,4-galactosyltransferase is an efficient catalyst for the regioselective transfer of galactose from UPD-galactose, generated in situ with the UDP-glucose/UDP-glucose-4-epimerase system, to the kaurane glycosides stevioside (1) and steviolbioside (Z), affording the corresponding galactosyl derivatives 3 and 4 in high yields. By a combination of 2 D NMR techniques (COSY, TOCSY, ROESY, HMQC, and HMBC), the structure of the products is established as 13-[(P-~-galactopyranosyl-(l -+ 4)-~-~-glucopyranosyI-(1 + 2)-fi-o-glucopyranosyl)oxy]kaur-16-en-19-oic acid P-D-ghCOpyranOSyl ester (3) and 13-[(~-~-gaIactopyranosyl-(l + 4)-P-~-glucopyranosyl-(1 + 2)-~-o-glucopyranosyl)oxy]kaur-16-en-19-~~~ acid (4).
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