CHEMOENZYMATIC APPROACH TO THE PREPARATION OF REGIOSELECTIVELY MODIFIED CYCLODEXTRINS - THE SUBSTRATE-SPECIFICITY OF THE ENZYME CYCLODEXTRIN GLUCOSYLTRANSFERASE (CGTASE)

The synthesis of alpha-maltosyl fluorides substituted at the 6- or 6'-position with H, F, Br, OMe, OAc is described, together with the preparation of the 4-thio-alpha-maltosyl fluoride and 5-thio-alpha-D-glucosyl fluoride. These compounds were obtained by chemical or enzymatic procedures and their structures have been established by C-13 NMR spectroscopy. The glycosyl fluorides have been tested as substrates for the enzyme CGTase under coupling and condensation conditions. It has been found that all the compounds tested are substrates in coupling reactions. Only three of them led to higher oligosaccharides with a modified maltosyl residue as repeating unit, but only the 6'-O-Me and 6'-O-acetyl fluorides were transformed into cyclic compounds. Under the conditions used, 6A, 6C, 6E-tri-O-methylcyclomaltohexaose, 6A, 6C, 6E-tri-O-methylcyclomaltoheptaose, and 6A, 6C, 6E, 6G-tetra-O-methylcyclomaltooctaose were isolated in 42, 13 and 16% yield, respectively. The 6'-O-acetylmaltosyl fluoride afforded 6A, 6C, 6E-tri-O-acetylcyclomaltohexaose and a mixture of partially acetylated cyclomaltoheptaoses in only low yields. By this approach, new insights have been obtained on the specificity of the catalytic site of CGTase of Bacillus macerans and new routes for the preparation of regioselectively modified cyclodextrins have been developed.