Synthesis of a novel N‐selective ester functionalized chitin derivative and water‐soluble carboxyethylchitin

A novel chitin derivative having a pendant ester function, 2‐N‐(2‐ethoxycarbonylethyl)chitin ( 2 ), was synthesized by a Michael‐type nucleophilic addition of an amino group of partially deacetylated chitin ( 1 ) to ethyl acrylate in phosphate buffer/methanol (5 : 3, v/v) at 40°C. N‐Selective monosubstitution occurred exclusively in the polymer reaction, which was supported by a reaction of methyl 2‐amino‐2‐deoxy‐D ‐glucopyranoside with ethyl acrylate to afford methyl 2‐N‐(2‐ethoxycarbonylethyl)‐2‐amino‐2‐deoxy‐D ‐glucopyranoside. The degrees of substitution (DSs) of 2 were determined by ¹H NMR spectroscopy. T₁ analysis of 2 was carried out in order to clarify differences of signal intensities of the pendant ester protons and the pyranose ring protons. The result of the T₁ measurement suggested a relatively restricted molecular motion of the chitin backbone in comparison with the flexible pendant ethyl ester groups. Furthermore, 2‐N‐(2‐carboxyethyl)chitin sodium salt ( 3 ) was synthesized from ethyl acrylate and 1 by the Michael addition followed by hydrolysis in 0.1 N NaOH aq. at 40°C. The DSs of 3 were varied from 0.26 to 0.88, which were almost controlled by the reaction period of the Michael reaction from 6 to 168 h. 3 showed good solubility in water. Viscosity measured on a cone‐plate viscometer for the 1.0 wt.‐% aqueous solution of 3 (DS, 0.26) was 0.074 Pas·sec.