Alkynyl polysaccharides: synthesis of propargyl potato starch followed by subsequent derivatizations

Potato starch propargyl ethers (PgS) with degrees of substitution (DS) from 0.1 to 2.2 have been prepared by etherification of starch with sodium hydroxide or Li dimsyl in Me2SO and propargyl bromide. DS values and substituent distribution were determined after hydrolysis and acetylation by GC–MS. The order of reactivity was 2 > 6 ≫ 3, with O-3 substitution being preferably observed in the trisubstituted units. Repeated analysis of the starch derivatives revealed that propargyl residues were lost during storage, a phenomenon that was not fully understood until now. Selected PgS were further functionalized: (a) O- and C-methylated to O-(2-butynyl)-O-methyl starch (BMS), (b) in a Mannich type reaction with diethylamine and formaldehyde to yield O-(4-diethylamino)-2-butinyl starch (DEABiS), (c) in a 1,3-dipolar cycloaddition with benzyl azide (‘click-chemistry’) to a N-benzyltriazole derivatized starch (BTrS), and (d) with carbon dioxide to O-(3-carboxy)-2-butinyl starch (CBiS). While the yield of carboxylation was only poor, conversion was high or nearly quantitative for reactions a–c. Thus, it is demonstrated that starch propargyl ethers are valuable intermediates for the preparation of functional polysaccharides.

Graphical abstractPropargyl starches (PgS) with degrees of substitution (DS) from 0.1 to 2.2 have been prepared. The reactivity was 2 > 6 ≫ 3. Terminal alkynes were further modified by methylation, carboxylation, diethylaminomethylation and 1,3-dipolar cycloaddition of benzylazide.Figure optionsDownload full-size imageDownload as PowerPoint slide