Phytochemical investigation of three leguminosae Plants for larvicidal activity against aedes aegypti

Abstract

The family Leguminosae is known to synthesize a range of flavonoids and isoflavonoids that possess wide varieties of biological activities. Among these, larvicidal activity is associated with the rotenoids; mainly found in the genera Tephrosia, Derris, Millettia and Lonchocarpus. Investigation of these plants could result in the identification of biodegradable and cost effective larvicidal agents to control mosquito borne diseases such as malaria and dengue fever. Three plants from this family namely Millettia oblata spp. teitensis (leaves), Millettia micans (root bark) and Derris trifoliata (leaves) have been phytochemically investigated. A total of seven compounds have been isolated from these plants. The characterization of the isolated compounds was based on spectroscopic evidence including 1D NMR, 2D NMR, UV, ORD, CD, EI-MS and ESI. In this study, the larvicidal activity of the crude extracts and four compounds have also been investigated. The crude extract (DCM/MeOH, 1:1) of M. oblata spp. teitensis was tested against the 3rd instar larvae of Aedes aegypti and showed moderate larvicidal activity with LC50 value of 84.6±7.8 μg/mL, at 24 hr. This crude extract was subjected to column chromatography over silica gel eluting with increasing polarities of mixtures of n-hexane and ethyl acetate (100:0 to 0:100) and led to the isolation of four compounds. These were identified as the rotenoids; tephrosin (125), oblatarotenoid A (126), and millettone (128) and the steroid stigmasterol (127). Among these, oblatarotenoid A (126) is a new compound. Tephrosin (125) showed high larvicidal activity with LC50 value of 1.4±0.2 μg/mL, while oblatarotenoid A (126) was inactive (LC50 > 100 μg/mL). This finding is consistent with the previous suggestion that the two methoxyl groups in ring A of tephrosin are important for larvicidal activity. The crude extract (DCM/MeOH, 1:1) of M. micans (root bark), a plant with no phytochemical report; showed weak larvicidal activity against the 3rd instar larvae of Aedes aegypti with LC50 value of 132±14.4 μg/mL at 24 hr. The crude extract was then subjected to CC and led to isolation of one compound. This compound was characterized as a new pterocarpan, for which the trivial name micanspterocarpan A (129) is assigned. The new pterocarpan was tested against the 3rd instar larvae of Aedes aegypti and it was inactive (LC50 > 100 μg/mL). The crude extract (DCM/MeOH, 1:1) of D. trifoliata (leaves) showed weak larvicidal activity against the 3rd instar larvae of Aedes aegypti with LC50 value of 367.7±162.8 μg/mL at 24 hr. This crude extract was also subjected to CC and led to isolation of two known flavanones namely: lupinifolin (130) and lupinifolin 4’-methyl ether (131). Lupinifolin (130) which is the major compound of the extract did not show significant larvicidal activity (LC50 > 100 μg/mL). However, good antiplasmodial activity has been reported for this compound. xvii O O O R1 R2 O H R3 O O O O MeO O OH H 126 125 R3 = OH R1= R2 = OMe 128 R3 = H R1, R2 = OCH2O OH 127 O O OH OMe OMe 129 OMe H H O O OH R O 130 R = OH 131 R = OMe