Regioselectively substituted cellulose esters and efficient methods of preparing them

Abstract

Embodiments of the invention provide regioselectively substituted carbohydrate and polysaccharide derivatives, such as cellulose esters, and methods for preparing them. Particular methods of the invention include deacylation of esters using tetrabutylammonium fluoride to obtain selective substitution at desired hydroxyl position(s) of the ester. Preferred is deacylation of an ester, which shows selectivity for removal of acyl groups from the ester of the secondary alcohols at C-2 and C-3, and which affords cellulose-6-O-esters with high regioselectivity by a simple one-step process employing no protective groups. Inventive regioselectively substituted cellulose esters can be prepared by such methods to obtain esters with the following anhydroglucose repeating units: wherein R 1 , R 2 , and R 3 are each independently chosen from a hydrogen atom, and, whether substituted or unsubstituted, branched or unbranched, an alkanoyl group, an aroyl group, and a heteroaroyl group. In embodiments, the alkanoyl, aroyl and heteroaroyl groups can comprise from 1-20 carbon atoms.

1 . A method of preparing a regioselectively substituted ester comprising:
 providing or synthesizing an ester from a selected carbohydrate or polysaccharide;   providing a solvent capable of dissolving the carbohydrate or polysaccharide;   providing a fluoride or hydroxide salt capable of selectively deacylating the ester in the solvent; and   reacting the ester and cation in the solvent for a time and under conditions sufficient to obtain a desired ester product which has been regioselectively deacylated at the secondary alcohol positions, substantially retaining its ester substituent at the primary alcohol position.   
     
     
         2 . The method of  claim 1 , wherein the carbohydrate or polysaccharide is chosen from monosaccharides, disaccharides, and trisaccharides, or linear or branched polysaccharide or oligosaccharide molecules comprising monosaccharide, disaccharide, or trisaccharide repeating monomer units. 
     
     
         3 . The method of  claim 2 , wherein the saccharide is chosen from glucose, sucrose, lactose, cellobiose, mannose, amylose, xylose, ribose, galactose, arabinose, fructose, sorbose, cellobiose and raffinose. 
     
     
         4 . The method of  claim 1 , wherein the ester product is one or more ester chosen from cellulose acetates, cellulose propionates, cellulose butyrates, cellulose hexanoates, and cellulose benzoates. 
     
     
         5 . The method of  claim 1 , wherein the solvent is chosen from one or more of methyl ethyl ketone (MEK), tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), 1,3 dimethyl-2-imidazolidinone (DMI), dimethylacetamide (DMAC), dioxane, dimethylformamide (DMF), ethyl acetate (EtOAc), butyl acetate (BuOAc), trichloromethane (CHCl 3 ), dichloromethane (CH 2 Cl 2 ), or combinations thereof. 
     
     
         6 . The method of  claim 1 , wherein the fluoride or hydroxide salt is provided by one or more of tetramethylammonium fluoride or hydroxide; or tetraethylammonium fluoride or hydroxide; tetrabutylammonium fluoride or hydroxide; tetraoctylammonium fluoride or hydroxide; or an alkali metal fluoride or hydroxide. 
     
     
         7 . The method of  claim 1 , wherein the regioselectively substituted ester product is one or more esters comprising the following anhydroglucose repeating unit: 
       
         
           
           
               
               
           
         
         wherein R 1 , R 2 , and R 3  are each independently chosen from a hydrogen atom, and, whether substituted or unsubstituted, branched or unbranched, an alkanoyl group, an aroyl group, and a heteroaroyl group comprising from 1-20 carbon atoms. 
       
     
     
         8 . The method of  claim 7 , wherein the solvent is THF or DMSO, the cation is provided by TBAF or TBAOH, and the ester product is a cellulose acetate. 
     
     
         9 . A regioselectively substituted carbohydrate or polysaccharide ester comprising the following anhydroglucose repeating units: 
       
         
           
           
               
               
           
         
         Wherein R 1 , R 2  and R 3  are each independently chosen from a hydrogen atom, and, whether substituted or unsubstituted, branched or unbranched, an alkanoyl group, an aroyl group, and a heteroaroyl group comprising from 1-20 carbon atoms; and 
         wherein the regioselectively substituted carbohydrate or polysaccharide ester is prepared by deacylation of an ester starting material, has a total degree of substitution at its C-2, C-3, or C-6 positions combined (DS TOT ) that is less than that of the ester starting material, and has at least some degree of substitution at C-6. 
       
     
     
         10 . The regioselectively substituted carbohydrate or polysaccharide ester of  claim 9  comprising a degree of substitution at C-6 that is equal to or up to about 50% less than that of the ester starting material. 
     
     
         11 . The regioselectively substituted carbohydrate or polysaccharide ester of  claim 10  comprising a degree of substitution at C-6 that is equal to or up to about 30% less than that of the ester starting material. 
     
     
         12 . The regioselectively substituted carbohydrate or polysaccharide ester of  claim 11  comprising a degree of substitution at C-6 that is equal to or up to about 10% less than that of the ester starting material. 
     
     
         13 . The regioselectively substituted carbohydrate or polysaccharide ester of  claim 9 , wherein the alkanoyl groups are chosen from branched or unbranched, substituted or unsubstituted, C 1-4  alkanoyl, C 2-6  alkanoyl, C 3-8  alkanoyl, C 5-10  alkanoyl, C 7-15  alkanoyl, C 9-18  alkanoyl, C 11-19  alkanoyl, C 12-17  alkanoyl, and C 13-16  alkanoyl groups. 
     
     
         14 . The regioselectively substituted carbohydrate or polysaccharide ester of  claim 9 , wherein the aroyl or heteroaroyl group is chosen from branched or unbranched, substituted or unsubstituted, C 1-4  heteroaroyl or aroyl, C 2-6  heteroaroyl or aroyl, C 3-8  heteroaroyl or aroyl, C 5-10  heteroaroyl or aroyl, C 7-15  heteroaroyl or aroyl, C 9-18  heteroaroyl or aroyl, C 11-19  heteroaroyl or aroyl, C 12-17  heteroaroyl or aroyl, and C 13-16  heteroaroyl or aroyl groups. 
     
     
         15 . The regioselectively substituted carbohydrate or polysaccharide ester of  claim 14 , wherein the aroyl or heteroaroyl group comprises at least one heteroatom chosen from phosphorus, sulfur, oxygen, nitrogen, chlorine, bromine, and iodine. 
     
     
         16 . The regioselectively substituted carbohydrate or polysaccharide ester of  claim 14 , wherein the aroyl or heteroaroyl group is a 3-, 4-, 5-, 6-, 7-, 8-, 9-, 10-, 11-, 12-, 13-, 14-, 15-, 16-, 17-, 18-, 19-, or 20-membered ring, unsubstituted or substituted with one or more group chosen from methyl, ethyl, propyl, or butyl groups, or any C 1-20  substituent, or a halide. 
     
     
         17 . An optical film comprising a regioselectively substituted cellulose ester comprising the following anhydroglucose repeating units: 
       
         
           
           
               
               
           
         
         wherein R 1 , R 2 , and R 3  are each independently chosen from a hydrogen atom, and, whether substituted or unsubstituted, branched or unbranched, an alkanoyl group, an aroyl group, and a heteroaroyl group comprising from 1-20 carbon atoms; and 
         wherein the regioselectively substituted carbohydrate or polysaccharide ester is prepared by deacylation of an ester starting material, has a total degree of substitution at its O-2, O-3, and O-6 positions (DS tot ) combined that is less than that of the ester starting material, and has at least some degree of substitution at C-6. 
       
     
     
         18 . The optical film of  claim 17 , wherein the regioselectively substituted cellulose ester is a cellulose acetate. 
     
     
         19 . The cellulose acetate film of  claim 18 , wherein the cellulose acetate has a degree of substitution at C-6 equal to or up to about 20% less than that of the ester starting material. 
     
     
         20 . The cellulose acetate film of  claim 19  comprising an in-plane retardation of less than about 10 nm.