Correlations between steric/thermochemical parameters and O-/N-acylation reactions of cellulose

Abstract

N(α)-t-Butyloxycarbonyl (Boc)-amino acids (Xaa = Gly, Ala, or β-Ala) were reacted with the cellulose hydroxyl groups (O-acylation) using N,N'-carbonyl diimidazole. The degrees of substitution toward the total hydroxyl groups (DS%(/OH)s) were 38% for O-(Boc-Gly)-Cellulose, 29% for O-(Boc-Ala)-Cellulose and 53% for O-(Boc-β-Ala)-Cellulose. The one-by-one N-acylation between the O-(Xaa)-Celluloses and Boc-Ala-Gly using a water-soluble carbodiimide yielded the conjugates N-(Boc-Ala-Gly)-Xaa-Celluloses with DS%(/NH2) values of 25% (Xaa = Gly), 35% (Ala), and 48% (β-Ala), respectively. The results were well correlated with ΔG and ΔEstrain profiles, which were predicted by semi-empirical thermochemical parameter calculation coupled with conformer search (R(2)>0.90). N-acylation of the O-(β-Ala)-Cellulose using various length of oligo-peptides, Boc-(Ala-Gly)n and Boc-(Gly-Ala)n (where, n = 0.5, 1.0, 1.5, 2.0, 3.0), suggested that the DS%(/NH2) was dependent on the structural features of the symmetric anhydrides as the N-acylating agents, including conformer populations and their transition energy.