Chemical modification of chitosan: synthesis and biological activity of new heterocyclic chitosan derivatives

BACKGROUND: Numerous works have been published on the chemical modification of chitosan; this polymer is still being modified, leading to various derivatives with improved properties. In the present study, heterocyclic aldehydes including furan‐2‐carbaldehyde, 5‐methylfuran‐2‐carbaldehyde, 3‐pyridine carboxyaldehyde, benzo[ d ][1,3]dioxole‐5‐carbaldehyde and 4‐oxo‐4 H ‐chromene‐3‐carbaldehyde were reacted with chitosan by a reductive alkylation reaction to produce for the first time five new N ‐heterocyclic chitosan derivatives to improve the biological activity of chitosan against the most important economic plant pests including fungi and insects, in particular the cotton leafworm Spodoptera littoralis . RESULTS: The chemical structures of the synthesized compounds were confirmed by 1 H NMR spectroscopy and the degree of substitution ranged from 0.30 to 0.43. The fungicidal assessment was investigated in vitro using a mycelia radial growth inhibition technique against soil‐borne pathogenic fungi Fusarium oxysporum and Pythium debaryanum and the rice leaf blast Pyricularia grisea . The results showed that N ‐[(5‐methylfuran‐2‐yl)methyl] chitosan was the most active against P. grisea with an EC 50 value of 0.919 mg mL −1 while N ‐(benzo[ d ][1,3]dioxol‐5‐ylmethyl) chitosan and N ‐(methyl‐4 H ‐chromen‐4‐one) chitosan exhibited the most potent fungicidal activity against P. debaryanum and F. oxysporum . An insecticidal bioassay against the larvae of S. littoralis showed that N ‐(methyl‐4 H ‐chromen‐4‐one) chitosan exhibited a significant growth inhibition and antifeedant activity among the synthesized compounds. CONCLUSION: The chemical modification of chitosan molecule with a heterocyclic moiety led to an enhancement in the biological activity against the plant pathogenic fungi F. oxysporum, P. debaryanum and P. grisea and the cotton leafworm insect S. littoralis .