Efficient differentiation of the hydroxyl groups of 3,4-O-isopropylidene-D-galactopyranosides by lipase catalyzed esterification and de-esterification

The Pseudomonas sp. (LPS) promoted acyl transfer from vinyl acetate to selected 3,4-O-isopropylidene-D-galactopyranosides takes place in a completely selective manner giving in high yield the corresponding 6-O-acetates. The acetylation rate is strongly dependent on the type and the orientation of the aglycon, varying from a maximum of reactivity for the 1-deoxy derivative, 1,5-anhydro-3,4-O-isopropylidene-D-galactitol (1d), to a minimum for beta configurated alkyl glycosides and showing a complete loss of reactivity for 3',4':2,3:5,6-tri-O-isopropylidenelactose dimethyl acetal (1e). The latter compound is, however, selectively 6'-O-esterified in good yield by lipase from Candida Antarctica and vinyl acetate. Also the course of the enzymatic hydrolysis of 2,6-di-O-acetyl-3,4-O-isopropylidene-D-galactopyranosides 2 is dependent on the type of the aglycon, both for the reaction rate and the selectivity. The 2-O-acetates 4 are selectively obtained in good yields with porcine pancreatic lipase (PPL) promoted hydrolysis in the case of beta- and alpha-methyl, and 1-deoxy derivatives (2a, 2b and 2d), while for beta-benzyl (2c) and lactose (2e) analogues satisfactory results are obtained with lipase from Mucor miehei (IM20).