Biocatalyzed generation of molecular diversity: Selective modification of the saponin asiaticoside

An array of different derivatives of the complex ursane-type triterpene glycoside asiaticoside (1), a saponin component isolated from the perennial herb Centella asiatica, was generated by exploiting the stereo-, regio- and site-selectivity of four groups of enzymes (glycosidases, glycosyltransferases, lipases and laccases). A library of extracellular alpha-L-rhamnosidases (31 different items) was obtained by screening 16 different fungal strains under different cultivation conditions, and several of these preparations catalyzed the derhamnosylation of asiaticoside. Specifically, the enzymes produced by Fusarium oxysporum were selected for the synthesis of derhamno-degluco-asiaticoside (2) and also of derhamno-asiaticoside (3), by in situ glucose-inhibition of contaminating beta-D-glucosidases. beta-1,4-Galactosyltransferase from bovine milk was subsequently used for the galactosylation of the two asiaticoside derivatives 2 and 3, using 20% v/v DMSO as a cosolvent in order to increase substrate solubility. Finally, new acylated and oxidized derivatives of asiaticoside were also prepared by exploiting the lipase from Candida antarctica (Novozym 435) and the laccase from Trametes pubescens, respectively.