Borate‐Mediated Stereo‐ and Topo‐Selective Methylation of 1,4‐β‐Glucomannan

Konjak glucomannan (KGM) is methylated with NaOH/MeI in water in the presence of borate and as a reference without this additive. With increasing equiv. of borate increasing suppression of 2‐ and 3‐ O ‐methylation of mannosyl residues (M) is observed, while glucosyl units (G) are mainly affected at O‐6, but to much lower extent. Raising the temperature and/or addition of acetone as a co‐solvent enhance reactivity, but at the cost of M/G regioselectivity. O ‐Methyl konjac glucomannans (M‐KGM) with an average degree of substitution (DS) up to 0.8 and a DS ratio for G and M up to 2 are obtained. From liquid chromatrography–electrospray ionization–mass spectrometry (LC‐ESI‐MS) of the oligosaccharides obtained from M‐KGM, it is concluded that borate‐mediated transient protection might also depend on the location of M within KGM. Comparison with random and block models supports random distribution of M and G in KGM. Thermogravimetric analysis shows higher decomposition temperature of M‐KGMs with increasing DS.