Enhancement of fungicidal and insecticidal activity by reductive alkylation of chitosan

A series of N ‐alkyl chitosan (NAC) derivatives were synthesized using a reductive alkylation reaction to examine their fungicidal and insecticidal activity. The chemical structures were characterized by IR and 1 H NMR spectroscopy, and the degree of substitution (DS) ranged from 0.02 to 0.37. Their fungicidal activity was evaluated against the grey mould Botrytis cinerea Pers ex Fr (Leotiales: Sclerotiniaceae) and the rice leaf blast pathogen Pyricularia grisea Sacc [Teleomorph: Magnaporthe grisea (Hebert) Barr] by a radial growth bioassay. It was of interest that most of the NAC derivatives were more active against both fungi than chitosan itself. The most active derivative was N ‐(2,2‐diphenylethyl)chitosan with EC 50 values of 0.031 and 0.23 g L −1 against B. cinerea and P. grisea respectively. In addition, some derivatives, at higher concentrations up to 1.0 g L −1 , inhibited the mycelial growth and spore formation of P. grisea . Bioassays against larvae of the cotton leafworm, Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae) with the NAC derivatives at a rate of 5.0 g kg −1 in artificial diet demonstrated that N ‐(3‐phenylbutyl)chitosan was the most active compound. In addition, N ‐propylchitosan, N ‐undecanylchitosan and N ‐(3‐phenylpropyl)chitosan derivatives strongly inhibited larval weight gain in S. littoralis , with respective reductions of 76, 66 and 65% after 4 days of feeding on treated diet. Copyright © 2006 Society of Chemical Industry