REGIOSELECTIVE ENZYME-MEDIATED GLYCOSYLATION OF NATURAL POLYHYDROXY COMPOUNDS .1. GALACTOSYLATION OF STEVIOSIDE AND STEVIOLBIOSIDE

Bovine beta-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 kaur ane glycosides stevioside (1) and steviolbioside (2), affording the co rresponding galactosyl derivatives 3 and 4 in high yields. By a combin ation of 2D NMR techniques (COSY, TOCSY, ROESY. HMQC, and HMBC), the s tructure of the products is established as 13-[(beta-D-galactopyranosy l-(1 --> 4)-beta-D-glucopyranosyl-(1 --> 2)-beta-D-glucopyranosyl)oxy] kaur-16-en-19-oic acid beta-D-glucopyranosyl ester (3) and 13-[(beta-D -galactopyranosyl-(1 --> 4)-beta-D-glucopyranosyl-(1 --> 2)-beta-D-glu copyranosyl)oxy]kaur-16-en-19-oic acid (4).