Ovicidal Activity of 2-Hydroxy-4-Methoxybenzaldehyde,Derivatives and Structural Analogues on Anopheles gambiae Eggs

Abstract

Efective remedies for disrupting An. gambiae metamorphosis at the egg stage are crucial in suppression of the malaria vectorpopulations that result in the reduction of disease burden. 2-Hydroxy-4-methoxybenzaldehyde (the major component of Mondiawhytei roots), its derivatives, structural analogues and their blends were evaluated against the eggs of An. gambiae in the search forovicidal compounds with potential use in mosquito control programs. Mature roots were harvested from Mondia whytei (Hook)Skeels (Asclepiadaceous) plants grown in the Center for African Medicinal and Nutritional Flora and Fauna (CAMNFF) herbalmedicinal garden and cleaned with distilled water. 2-Hydroxy-4-methoxybenzaldehyde (1) was isolated by steam distillation of thechopped roots. Te selected derivatives and/or analogues were prepared using established chemical procedures and their structuresconfrmed by NMR spectroscopy and ESI-MS. Ovicidal activity of the pure compounds, derivatives, structural analogues and/orformulated blends was tested at 1, 10, 25 and 50 parts per million (ppm) on An. gambie eggs. Eleven mono-substituted (3–7), di-substituted (8–10), tri-substituted (1-2) aromatic compounds were assayed for ovicidal activity against Anopheles gambiae eggs singlyor as blends. Benzaldehyde (4) and 4-methoxybenzaldehyde (9) were further converted into 2-hydroxy-1, 2-diphenylethanone (11),1, 5-diphenylpenta-1, 4-diene-3-one (12) and 1, 5-bis (4-methoxyphenyl) penta-1, 4-diene-3-one (13) and evaluated for ovicidalactivity individually or as blends. Of the 13 compounds evaluated individually, 2-hydroxy-4-methoxybenzaldehyde (1) exhibited thehighest ovicidal activity at LC50 0.7075 ppm while anisole had the lowest activity at LC50 40.342 ppm. Te derivatives exhibitedmoderate activity: 2-hydroxy-1, 2-diphenylethanone (LC50 10.599 ppm), 1, 5-diphenylpenta-1, 4-diene-3-one (LC50 9.019 ppm) and1, 5-bis (4-methoxyphenyl) penta-1, 4-diene-3-one (LC50 15.642 ppm). Te blends exhibited intriguingly high ovicidal efcacy withthe mixture of benzaldehyde and phenol showing the highest (LC50 0.332 ppm) while phenol and anisole exhibited the lowest activity(LC50 9.9909 ppm). From the activity of the blends, it is evident that anisole is antagonistic to the efcacy of phenol and benzaldehyde.It is also apparent that aldehyde and hydroxyl groups, when directly attached to the phenyl ring, provide the critical structuralcharacteristics that contribute to the ovicidal activity of the aromatic compounds.