Relative reactivities of amylose hydroxyl groups in the reaction with diketene

¹³C nuclear magnetic resonance (n.m.r.) spectra at 75.4 MHz of partially modified amylose with β-keto ester groups (degree of substitution (DS) ranging from 0.52 to 2.42) were studied in order to evaluate the selectivity of the reaction of amylose with diketene in the homogeneous phase. Analysis of the spectra of ring carbons in the anhydroglucose units shows that the reactivity of individual hydroxyl groups decreases in the order C-6>C-3>C-2 for DS values up to ∼1.8. For higher DS values a negative deviation is observed for the hydroxyl group at C-6 and a positive deviation for the hydroxyl groups at C-2 and C-3. The DS values determined by ¹³C n.m.r. spectra are in good agreement with those found by chemical analysis.