Insecticidal and fungicidal activity of new synthesized chitosan derivatives

Chitosan, the N ‐deacetylated derivative of chitin, is a potential biopolysaccharide owing to its specific structure and properties. In this paper, we report on the synthesis of 24 new chitosan derivatives, N ‐alkyl chitosans (NAC) and N ‐benzyl chitosans (NBC), that are soluble in dilute aqueous acetic acid. The different derivatives were synthesized by reductive amination and analyzed by 1 H NMR spectroscopy. A high degree of substitution (DS) was obtained with N ‐(butyl)chitosan (DS 0.36) at a 1:1 mole ratio for NAC derivatives and N ‐(2,4‐dichlorobenzyl)chitosan (DS 0.52) for NBC derivatives. Their insecticidal and fungicidal activities were tested against larvae of the cotton leafworm Spodoptera littoralis (Boisduval) (Lepidoptera: Noctuidae), the grey mould Botrytis cinerea Pers (Leotiales: Sclerotiniaceae) and the rice leaf blast Pyricularia grisea Cavara (Teleomorph: Magnaporthe grisea (Hebert) Barr). The oral feeding bioassay indicated that all the derivatives had significant insecticidal activity at 5 g kg −1 in artificial diet. The most active was N ‐(2‐chloro‐6‐fluorobenzyl)chitosan, which caused 100% mortality at 0.625 g kg −1 , with an estimated LC 50 of 0.32 g kg −1 . Treated larvae ceased feeding after 2–3 days; the mechanism of action remains unknown. In a radial hyphal growth bioassay with both plant pathogens, all derivatives showed a higher fungicidal action than chitosan. N ‐Dodecylchitosan, N ‐( p ‐isopropylbenzyl)chitosan and N ‐(2,6‐dichlorobenzyl)chitosan were the most active against B cinerea , with EC 50 values of 0.57, 0.57 and 0.52 g litre −1 , respectively. Against P grisea, N ‐( m ‐nitrobenzyl)chitosan was the most active, with 77% inhibition at 5 g litre −1 . The effect of different substitutions is discussed in relation to insecticidal and fungicidal activity. Copyright © 2005 Society of Chemical Industry