O-Acetylation of cellulose and monosaccharides using a zinc based ionic liquid{

The application of ionic liquids as environmentally benign reaction solvents for synthesis and catalysis has received extensive recent attention. In this regard we have developed ionic liquids based on the cheap and readily available components choline chloride (ChCl; HOCH2CH2N(Me)3?Cl) and zinc chloride as alternatives to the more commonly employed alkyl imidazolium-aluminium chloride mixtures. In an analogous manner to the chloroaluminate systems, we have shown that these zinc based liquids form complex anions of the form ZnCl3 2 and Zn2Cl5 . Although these species are less Lewis acidic than their aluminium counterparts the ChCl–ZnCl2 ionic liquids are water insensitive and zinc is environmentally more benign. These Lewis acidic solvents have been successfully employed for a variety of reactions including Diels–Alder, Fisher indole synthesis and polymerisations. In this current study we report that O-acetylation of sugars and cellulose can effectively be carried out using chlorozincate ionic liquids. The O-acetylation of carbohydrates is widely employed not only for the protection of hydroxy groups but also for the purification and structural elucidation of natural products. Per-O-acetylation is most frequently achieved using excess acetic anhydride together with a wide variety of catalysts including, amongst others, pyridine (and its derivatives), sodium acetate, FeCl3, 11 metal triflates, and ZnCl2. 10 The reaction has also been carried out in ionic liquids although only imidazolium based salts have been employed to date. Thus, the acetylation of carbohydrates has been demonstrated in imidazolium systems containing dicyanamide or benzoate anions and in both cases the ionic liquid acted as both solvent and catalyst for the reaction. In addition ionic liquids have also recently been employed as media for the enzymatic acylation of glycosides. Results and discussion