Distribution of Methoxyl Groups in the Methylation of Starch Alkoxides Obtained by Metallation with Alkali Metal Naphthalene

The distribution of the alkoxide groups obtained in the metallation of starch in dimethyl sulfoxide solution by alkali metal naphthalenes was studied. The starch alkoxide was reacted with methyl iodide, and the methylated starch was hydrolyzed and analyzed for glucose and O-methyl glucose derivatives. The metallation reaction was found to be random, as seen from the fact that at low alkoxide concentration (D.S = 0.6), 2,3,6-tri-O-methyl glucose was formed, while at relatively high alkoxide concentration (D.S. = 1.5) unreacted glucose was still present. At low alkoxide concentration (D.S. ⋍ 0.6) there was, to a certain extent, preferential metallation at the C₂ hydroxyl groups, and to a lesser extent at the C₆ hydroxyl groups, as seen from the relative molar ratios of about 10:4:1 of the 2-, 6-, and 3-O-methyl glucose derivatives obtained, respectively. An increase in the metallation at the C₃ hydroxyl occurred with increasing alkoxide concentration. The distribution of the methoxyl groups with the three alkali metals potassium, sodium, and lithium was similar; there were differences only in the ease of metallation of the starch by the various alkali metal naphthalenes and in the efficiency of the coupling reaction with methyl iodide.