Acylation of 6-Deoxy-L-Hexoses: Regioselectivity in the Enzymatic Transesterification as Compared to Chemical Esterification

INTRODUCTION Regioselective esterification of sugars may represent a valuable tool for providing useful intermediates in the synthesis of more complex structures. A relatively easy solution to the problem can be found when discrimination between a primary vs secondary hydroxyl function is needed, but it seems to be quite unsolvable in a single chemical step when two or more secondary alcoholic groups are competing in the reaction with a given acylating reagent. Hydrolytic enzymes were found to be catalysts of great utility in this respect. Several cases are described in the literature concerning the selective deprotection of acylated sugars in aqueous media,1 and the esterification of free sugars or their alkyl glycosides in non-polar organic solvents1a,2 which first involve the primary alcohol function.3 Enzyme-catalyzed acylation in organic media at secondary alcohol positions was also studied4 in the case of 6-O-butyryl-glucose, -mannose, and -galactose and the 2-OH and/or 3-OH esterifications were shown to be preferred, with no reactivity observed at the 4 position.