2,2,6,6-四甲基哌啶氧化物自由基选择性氧化反应用于羟丙基化半乳甘露聚糖C-6位取代度的表征研究

TEMPO (2,2,6,6-四甲基哌啶氧化物自由基)-NaClO-NaBr是一种对伯羟基具有很高选择性的氧化反应体系, 系统研究了该体系对半乳甘露聚糖及其衍生物的氧化反应过程, 通过对氧化反应中NaOH消耗量的适时跟踪测定, 确定了氧化反应进程以及氧化反应终点, 利用FTIR和13C NMR对氧化产物的结构进行了表征, 进一步确证了TEMPO氧化体系可高选择性地将C-6位伯羟基完全氧化为羧酸. 首次将TEMPO氧化法用于羟丙基化半乳甘露聚糖C-6位取代度的测定, 利用氧化反应NaOH的消耗量计算出不同摩尔取代度的羟丙基半乳甘露聚糖C-6位伯羟基的取代度. 以此为依据, 对半乳甘露聚糖的羟丙基化反应机理进行了初步探讨, 从实验上进一步验证了半乳甘露聚糖羟丙基取代反应优先发生在C-6位伯羟基上.

Determination of the Degree of Substitution at C-6 by 2,2,6,6- Tetramethyl-1-piperidinyloxy Radical-mediated Selective Oxidation of Substituted Galactomannan

TEMPO (2,2,6,6-Tetramethyl-1-piperidinyloxy radical)-mediated oxidation of primary alcohol groups of galactomannan was studied at pH 10.2 and temperature of 0 ℃ with NaOCl as the primary oxidant. The high selectivity of reaction could be proved by using the FTIR and 13C NMR spectra and the end-point of oxidation process could be determined by the NaOH consumption. The oxidation process was investigated for a series of hydroxypropyl galactomannan with different molar degrees of substitution. It was shown that this oxidation could be used for the determination of the degree of substitution at C-6 position of the substituted galactomannan by comparing the difference of oxidation yields between substituted and natural samples. The formula for determination of the degree of substitution at C-6 position was established according to the consumption of NaOH during the oxidation process. Moreover, the hydroxypropyl reaction mechanism of galactomannan was investigated on the basis of determination of the degree of substitution at C-6 position.