Regioselective Acetylation of Secondary Hydroxyl Groups of Carbohydrates by Lipase-Catalyzed Transesterification

In the chemical transformation of the sugars, their hydroxyl groups are frequently protected by acylation. Ester is useful for the protection of the hydroxyl group because of ease of formation and cleavage. Selective acylation of different hydroxyl groups in sugars is important for the prepara tion of various partially substituted sugar derivatives and oligosaccharides which contain the glycosidic bond at a specific position. Partially acylated sugar derivatives have been chemically prepared with benzoyl chloride through dialkyl stannylene acetals, trialykyltin ethers,1,2) and phase transfer conditions.3-6) Some bulky acylating agents have also been used for their preparation. These methods exploit steric effects and differences of acidity between each hydroxyl group. In recent years, hydrolytic enzymes (lipase and protease) have been used as catalysts for the stereospecific transesterification using some active esters as acylating agents.7,8) We also confirmed that some Pseudomonas lipases revealed high stereoand enantioselectivity on the acetylation of some alcohols using vinyl acetate as an acylating agent.9-11) In applica tion of the reaction to carbohydrates, these hydrolytic enzymes have shown remarkable regioselectivity, often allowing the isolation of monoacyl derivatives in good yields.12-19> We planned to prepare the partially acety lated derivatives of methyl 6-0(p-tolylsul fonyl) -D-glycopyranosides by the lipase-cata lyzed regioselective reaction. These compounds are convenient for the preparation of various sugar derivatives, because p-tolylsul f onyl (tosyl) group is useful for protection, deoxygenation, and substitution of the C-6 hydroxyl group.