Selective Acetylation of Unprotected Thioglycosides and Fully Unprotected Monosaccharides with <i>Candida antarctica</i> Lipase-B

A selective enzymatic acetylation method for the protection of the second and the sixth positions of thio-d-galactopyranoside was found using immobilized Candida antarctica lipase-B (CAL-B). Unfortunately, it was determined that the immobilized enzyme cannot be recycled effectively. The optimized acetylation method was screened with different thioglycosides and with fully unprotected saccharides. New methods for several new partially protected saccharides were found, while the synthesis of some known saccharides, e.g., the third and the sixth position-protected d-glucose or the fourth position-protected l-rhamnose, was improved. Furthermore, an enzymatic acetal formation between the fourth and the sixth positions was discovered. The main limitation for acetylation reactions with CAL-B has been determined to be the substrate solubility.