Synthesis and NMR characterization of regiocontrolled starch alkyl ethers

Abstract

Selective polysaccharide functionalization is of importance in molecular and supramolecular structure design and the development of new products and properties. As a typical partially branched polysaccharide, starch (72% amylopectin) was selectively and completely alkylated in the secondary positions. Starting from 6-O-thexyldimethylsilyl starch (6-O-TDS starch) prepared regio-controlled, an effective alkylation takes place by using methyl iodide and benzyl bromide, respectively, and sodium hydride as a base in THF solution. Under these conditions, the silyl ether linkages are stable. The subsequent desilylation proceeded with tetra-n-butylammonium fluoride (TBAF) under mild conditions to form the alkyl starches without cleavage of the alkyl ether linkage. The pattern of functionalization could be analyzed by using the silyl alkyl ethers or subsequently 6-O-acetylated derivatives and multidimensional NMR techniques. In the case of the methyl ethers, the substituent distribution could be detected not only in the anhydroglucose units (AGUs), but also in the non-reducing end-groups (NEGs). As a result, the starch ethers are alkylated regio-controlled in 2,3-di-O (AGU) and 2,3,4-tri-O (NEG) positions.