A comparison of the reduction of alginic acid by different methods

Abstract

Comparison of the percent reduction of the functional groups on algin polymers showed a marked difference between reduction with a Lewis base, lithium borohydride, and with a Lewis acid, diborane. Towards diborane the order of reactivity is: uronic acid carboxy-group > n-propionyl ester group > hemiacetal end group. Towards lithium borohydride the order of reactivity is reversed: hemiacetal end groups > n-propionyl ester group > uronic acid carboxy-group. In addition, only diborane reduces a portion of the n-propionyl esters to n-propyl ethers. These results are explained by the hypothesis that the reduction with the nucleophilic borohydride involves transfer of the hydride ion to the electron-deficient centre of the functional group, whereas reduction with the electrophilic diborane involves attack at the centres of highest electron density. The lithium borohydride procedure for the reduction of methyl di-O-propionyl alginate is recommended for the reduction of acidic polysaccharides. This procedure reduced all the esterified uronic acid carboxy-groups, no ethers were formed, and all the hemiacetal end groups were reduced.