A panel of lipases was screened for the selective acetylation and alcoholysis of silychristin and silychristin peracetate, respectively. Acetylation at primary alcoholic group (C-22) of silychristin was accomplished by lipase PS (Pseudomonas cepacia) immobilized on diatomite using vinyl acetate as an acetyl donor, whereas selective deacetylation of 22-O-acetyl silychristin was accomplished by Novozym 435 in methyl tert-butyl ether/ n-butanol. Both of these reactions occurred without diastereomeric discrimination of silychristin A and B. Both of these enzymes were found to be capable to regioselective deacetylation of hexaacetyl silychristin to afford penta-, tetra- and tri-acetyl derivatives, which could be obtained as pure synthons for further selective modifications of the parent molecule.

Istituto di Chimica del Riconoscimento Molecolare Consiglio Nazionale delle Ricerche Via Mario Bianco 9 1 20131 Milano Italy

Laboratory of Biotransformation Institute of Microbiology Academy of Sciences of the Czech Republic Vídeňská 1083 CZ 142 20 Prague Czech Republic

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Chirality Matters: Biological Activity of Optically Pure Silybin and Its Congeners