Isolement et caractérisation des constituants chimiques de deux plantes médicinales camerounaises Lippia rugosa A. Chev (Verbenaceae) et Plectranthus glandulosus Hook (Lamiaceae) et évaluation de leurs activités insecticide et antioxydante

Abstract

This thesis is inscribed in the theme of research developed at the Structural Chemistry Laboratory, in the Department of Organic Chemistry, of the University of Yaounde I. It is focused on bioactive natural molecules of plant origin, potentially of insecticdal and antioxidant activities. Lippia rugosa (Verbenaceae) and Plectranthus glandulosus (Lamiaceae) used in the protection of foodstuffs especially, cereals such as maize, sorghum, wheat and rice against pests. In addition, these plants are also used in traditional medicine due to their therapeutic properties in the treatment of many diseases including rheumatism, fever, cough, jaundice, colds, headaches, malaria. Thus, by means of liquid chromatographic technique, twenty-four (24) compounds were isolated from the extracts of L. rugosa and P. glandulosus. Of these 24 compounds, sixteen (16) were isolated from the leaves and stems of L. rugosa and eight (8) from the entire plant of P. glandulosus. Of the 16 compounds isolated from L. rugosa, there are: five (5) flavonoids including a new derivative to which the trivial name plectranmicin was giving, together with 4 known compounds (7-hydroxy 5,6,4'-trimethoxyflavone, 5,4'-dihydroxy -3,7-dimethoxyflavone, 5-hydroxy-6,7,4'- trimethoxyflavone and 5-hydroxy-3,7,4'-trimethoxyflavone), six (6) known pentacyclic triterpenes (lup-20 (29) -en -3β, 28-diol, lup-20 (29) -en-3β-ol, betulinic acid, lupeol acetate, 3β-hydroxy-urs-12-en-3-ol, fridelan-3-one), one (1) known alcaloïd (2,4-diméthylpyridine-3,5-diol), one (1) fatty alcohol (cosanol) one (1) sugar (sucrose) two (2) sterols stigmasterol glucoside, a mixture of stigmasterol and β sitosterol. While among the eight (8) isolated from P. glandulosus they were three (3) flavonoids: plectranmicin, 5-hydroxy-3,7,2',4'-tetramethoxyflavone, 5,7-dihydroxy 3,2',4'-trimethoxyflavone, 1 new monoterpene derivetive to which trivial name plectranmicinol was assigned, 3 known pentacyclic triterpenes: fridelan-3-one, lup-20 (29) -en-3β, 28-diol, 3β-hydroxy-urs-12-en-3-ol and 1 sterol glucoside: 3-O--D glucopyranosyl stigmasterol. The structures of all these compounds were established from the analyses on the interpretation of their spectral data, in particular 1D and 2D NMR in conjonction to mass spectrometry. Some of these compounds, such as fridelan-3-one and 2,4-diméthylpyridine-3,5-diol, were submitted to a few chemical transformations, in particular Beckmann rearrangement and methylation, which led to caprolactam and 3, 4-dimethyl-3,5-dimethoxylpyridine, all confirmed and characterized. Concerning the biological aspect, extracts of L. rugosa, P. glandulosus and some isolated pure compounds were evaluated for their insecticidal and antioxidant activities. With regards to insecticidal activity, the results obtained showed a good activity for both plants with a mortality rate of 83.75 ± 11.25 % for the ethyl acetate extract of L rugosa and a mortality rate of more than 90% for extracts of P. glandulosus against the maize destructive pest Sitophulus zeamais. For antioxidant activity against DPPH°, the ethanol extract of L. rugosa is the most active with a percentage inhibition of 85.668 ± 1.233 % at 100 μg/ml concentration. On the other hand, all the extracts of P. glandulosus showed good activity, but the ethanol extract is the most active with an inhibition of 94.586 ± 1.071 %. Some pure compounds have been tested for their reducing power using DPPH° and chelating by FRAP method. The results show that the new flavonoid plectranmicin and the new monoterpenol plectranmicinol are the most active with EC50 of (0.322 ± 0.026) and (0.339 ± 0.020) μM / ml for the FRAP method and (0.208 ± 0.029) and (0.185 ± 0.012) μM / ml for DPPH° scavenging, respectively.