Abstract
Konjac glucomannan, built of β-1,4-linked mannose and glucose, was methylated after treatment with dibutyltin oxide/MeOH in pyridine. Best results were obtained in acetonitrile/DMF in the presence of TBAB and K2CO3. While total DS was only 0.1–0.15 and not significantly enhanced in the tin-promoted reaction, the manno-/gluco- (cis-/trans-diol) and the regioselectivity was completely changed with DSMan/DSGlc increasing from 0.9 to 1.8, a strong preference for secondary over primary OH, and a total order of reactivity being M3 > M2 ≥ G2 > G6 ≥ M6 ≥ G3. The influence of the amount of K2CO3, Bu2SnO, and temperature was studied. By repeated reaction a DS of 0.28 was gained without loss in selectivity. In a solvent-free one-pot stannylation/methylation reaction, a DS up to 0.3 was achieved. Surprisingly, any differences between the two stereoisomeric sugar constituents were leveled under these conditions, and regioselectivity was changed to the order M6 ≈ G6 ≫ M3 ≈ G3 > M2 ≈ G2.
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