Substituent effects of the orthoester group on ring-opening polymerization of alpha-D-glucopyranose 1,2,4-orthoester derivatives

The first chemical synthesis of cellulose derivatives, (1-->4)-beta-D-glucopyranan derivatives has been accomplished by cationic ring-opening polymerization using 3,6-di-O-benzyl-alpha-D-glucopyranose 1,2,4-orthopivalate (1) as a star-ring monomer, taking into account substituent effects.(1) Here, three orthoester derivatives as starting materials for the polymerization, 3,6-di-O-benzyl-alpha-D-glucopyranose 1,2,4-orthopropionate (2), 3,6-di-O-benzyl-alpha-D-glucopyranose 1,2,4-orthoacetate (3), and 3,6-di-O-benzyl-alpha-D-glucopyranose 1,2,4-orthobenzoate (4), were selected to investigate the substituent effects of orthoester group on the ring-opening polymerization. These three monomers were polymerized under the same reaction conditions as those of monomer 1, yielding stereoregular (1-->4)-beta-D-glucopyranan derivatives, previously reported. As the result, monomers 2-4 gave non-regioregular polymers consisting of (1-->4)and (1-->2)-beta-pyranose units, although they gave high stereoregularity, i.e., beta-glucosidic linkage. Thus, it was concluded from the polymerizations of the monomers 1-4 that the orthopivaloyl group of the starting monomer is indispensable for regiospecificity of the polymerization, yielding only the (1-->4)-glycosidic bond, not the (1-->2)-bond.