Synthesis of 2,3,4‐tri‐<i>O</i>‐benzyl‐[1→6]‐α‐<scp>D</scp>‐glucopyranan and [1→6]‐α‐<scp>D</scp>‐glucopyranan with high molecular weight by polymerization of 1,6‐anhydro‐2,3,4‐tri‐<i>O</i>‐benzyl‐β‐<scp>D</scp>‐glucopyranose

Abstract

Abstract 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 (M̄ n = 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‐% PF 5 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 (M̄ n = 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.