Studies About the Solvent‐Dependent Substitution Pattern of Starch Acetates

Abstract

Starch acetates bearing the ester moiety predominantly at position 2 of the anhydroglucose unit (AGU) could be synthesized by converting starch with acetic anhydride and imidazole in dimethyl sulfoxide (DMSO). Degraded starch (weight average molar mass, Mtrue‾normalw < 16 460 g mol−1) is additionally acetylated at position 6 exceeding a total degree of substitution (DS) of 0.5. Increasing the concentration of starch up to 50 wt% leads to higher partial DS at position 6 under comparable reaction conditions. Using molten imidazole as solvent, an initial regioselective 6‐O‐acylation is achieved. At higher DS values (>0.3), a random distribution of the substituents appears. The substitution pattern can be revealed precisely by means of 13C and 1H NMR spectroscopy after perpropionylation. Hydrolytic stability of a selected 2‐O‐acetyl starch in aqueous solution is evaluated by time‐dependent capillary viscosimetry.

Synthesis and characterization of novel water-soluble and bactericidic cationic starch esters

“…The increase of the molar ratio to 1:1:1 and 1:2:2 (1 g starch, 5% in DMSO) resulted in a slight increase of DS (sample 2n , DS = 0.32; 2o , DS = 0.50). It became obvious that the esterification of starch with CPTMACl imidazolide leads to similar DS values like the acetylation of starch under comparable conditions using acetic acid imidazolide up to a molar ratio AGU:reagent of 1:1 . However, a higher concentration of starch in DMSO yielded lower DS values of the cationic substituent while during starch acetylation the opposite was observed.…”

“…It turned out that the cationic starch esters showed high hydrolytic stability in aqueous solution. A comparable stability was found for 2-O-acetyl starch of comparable DS [24]. They are stable over a range of 180 days at room temperature determined by time-dependent measurements of capillary viscometry.…”

“…Interestingly, the starch acetates of low DS of up to 1.0 are well water‐soluble and hydrolytically rather stable depending on the distribution of ester moieties. Products with a preferred functionalization at O ‐2 are most stable . Starch ethyl carbonates synthesized with chlorocarbonic acid ethyl ester and ethyl 1H‐imidazole‐1‐carboxylate as reagent are soluble in polar aprotic solvents or even in water depending on the type of starch and DS.…”

Production of Esterified Starches with Increased Resistant Starch Content by an α‐Hydroxy Acid‐Catalyzed Route

“…Spectra of KBr pellets were obtained using 40 scans in the range of 4000–650 cm −1 with a resolution of 4 cm −1 . Finally, spectra were normalized using the signal at 1025 cm −1 , corresponding to the stretching vibration of the acetal of the anhydroglucose unit …”

Synthesis and structure characterization of soluble starch ethyl carbonates

“…However, a starch ethyl carbonate with DS 0.38 ( 2j ) could be obtained by using triethylamine as base. Interestingly, the conversion of starch with carboxylic acid imidazolides applying imidazole as base under comparable conditions is known to be very efficient . These results show that stronger bases are necessary for the reaction of starch with carbonic acid imidazolides due to the nucleophilicity of the hydroxyl groups of starch and donor‐acceptor interactions of starch, reagent, base, and solvent.…”