Synthesis of 2,3,4-tri-O-benzyl-[1→6]-α-D-glucopyranan and [1→6]-α-D-glucopyranan with high molecular weight by polymerization of 1,6-anhydro-2,3,4-tri-O-benzyl-β-D-glucopyranose

The polymerization mechanism of 1,6‐anhydro‐2,3,4‐tri‐O‐benzyl‐β‐D‐glucopyranose (1) was investigated in order to synthesize 2,3,4‐tri‐O‐benzyl‐[1→6]‐α‐D‐glucopyranan (2a) and [1→6]‐α‐D‐glucopyranan (2b) (dextran) with high molecular weight. It was found that the optimum polymerization time to obtain high molecular weights was 40min when the monomer was polymerized in methylene chloride at −60°C. Stereoregular 2a with a \documentclass{article}\pagestyle{empty}\begin{document}$ \overline {DP} _{\rm n} $ of 1800 (M̄n = 777000), which was about twice as high as the highest \documentclass{article}\pagestyle{empty}\begin{document}$ \overline {DP} _{\rm n} $ previously reported, was obtained in 77% yield by polymerizing the monomer with 0.8 mole‐% PF5 as catalyst applying a monomer‐to‐solvent weight/volume ratio of 50%. 2a with a high molecular weight was debenzylated to give 2b with a \documentclass{article}\pagestyle{empty}\begin{document}$ \overline {DP} _{\rm n} $ of 446 (M̄n = 72300). The α‐stereospecificity and the solid state of the synthetic dextran were investigated by means of optical rotation, NMR spectroscopy, and X‐ray diffraction pattern.