Larvicidal Activity of Geranylacetone Derivatives against Culex quinquefasciatus Larvae and Investigation of Environmental Toxicity and Non-Target Aquatic Species

: A grindstone method based on Mannich condensation was used to synthesize geranylacetone derivatives ( 1a – 1f ). The method showed a high yield under milder reaction conditions. Analyses of the synthesized compounds were carried out by FTIR, 1 H, 13 C NMR, mass spec-trometry, and elemental analysis. We synthesized and evaluated the larvicidal and ichthyotoxic activities of six compounds ( 1a – 1f ) in this study. Compound 1f (5,9-dimethyl-1-phenyl-3-(2-(3-phenylallylidene)hydrazinyl)deca-4,8-dien-1-one) was more active (LD 50 : 14.1 µ g/mL) against the second instar larvae of Culex quinquefasciatus than geranylacetone (67.2 µ g/mL), whereas the former caused 13.9% mortality at 100 µ g/mL. Geranylacetone, in an antifeedant screening test, showed 53.1% against Oreochromis mossambicus within 24 h. The compound 1f showed high larvicidal activity against C. quinquefasciatus and was non-toxic to non-target aquatic species.


Introduction
Mosquitoes are important vectors of several diseases, including malaria [1,2]. In several regions, Culex quinquefasciatus is associated with several vector-borne diseases. Insecticides designed to kill larvae are called larvicides. As an insect growth controller, metaprene prevents the larvae from returning to the pupa stage significantly after their growth period has been interrupted [3]. The pesticide methoprene is mildly toxic to crabs, shrimp, lobster, and crayfish, as well as to fish and aquatic herbivores; in fact, it tends to accumulate in the tissues of fishes. Toxins produced by fishes or compounds toxic to them are ichthyotoxins [4]. The euglenophyceae and prymnesium, made by algae can cause large-scale fish deaths [5]. Toxin-producing algae are found both in freshwater and marine environments. It is important to keep in mind that the severity of ichthyotoxic poisoning (in humans) varies depending on how much of the toxin was consumed. An ichthyotoxin poisoning can result in headaches, diarrhea, dizziness, high blood pressure, and other symptoms [6].
In the food industry and medicine, GA is a white or yellowish oily liquid at room temperature that dissolves slowly in water and is volatile. Its application in these fields is limited. GA's water solubility and stability need to be improved in order for its application to expand [15]. The researchers studied the slow-release adsorption and adsorption effects of GA on various adsorption materials, and the improved GA stability. It has also been demonstrated that geranylacetone is an antimicrobial compound when examined as a food preservative. Chemicals are used as insecticides against vector mosquitoes [16], although their usage is limited because of their toxic effects on the environment and other organisms [17,18]. Various larvicidal active targets were published in previous years, but chemical insecticides pose a bigger risk with numerous potential environmental problems, including widespread development of resistance and disruption of natural biological control processes [19,20]. In order to overcome the challenges and limitations described above, we developed a grindstone chemistry methodology which we applied to the synthesis of novel geranylacetone derivatives and examined their larvicidal activity.
In the food industry and medicine, GA is a white or yellowish oily liquid at room temperature that dissolves slowly in water and is volatile. Its application in these fields is limited. GA's water solubility and stability need to be improved in order for its application to expand [15]. The researchers studied the slow-release adsorption and adsorption effects of GA on various adsorption materials, and the improved GA stability. It has also been demonstrated that geranylacetone is an antimicrobial compound when examined as a food preservative. Chemicals are used as insecticides against vector mosquitoes [16], although their usage is limited because of their toxic effects on the environment and other organisms [17,18]. Various larvicidal active targets were published in previous years, but chemical insecticides pose a bigger risk with numerous potential environmental problems, including widespread development of resistance and disruption of natural biological control processes [19,20]. In order to overcome the challenges and limitations described above, we developed a grindstone chemistry methodology which we applied to the synthesis of novel geranylacetone derivatives and examined their larvicidal activity.

Materials and Method
All compounds were analyzed with a Nicolet iS5 FTIR (4000-400 cm −1 ) by Thermo Scientific. Spectra for 1 H and 13 C were analyzed using the Bruker DRX-300 MHz. We analyzed (C, H, N, and S) elements and their percentages (%) using an elemental analyzer (model Vario EL III). Perkin Elmer GCMS model Clarus SQ8 (EI) was used to record mass spectra.

Synthesis of Compound 1a-f: General Procedure
A reaction mixture of cinnamaldehyde (0.01 mole), acetone (or) acetophenone (0.01 mole), substituted amine (0.01 mole) was mixed and ground at room temperature. Separation of the solid material was performed using column chromatography (Ethyl acetate 4: hexane 6). The same method was followed when mixing compounds 1b-1f.

Larvicidal Activity
Culex quinquefasiatus eggs were collected from the drainage system. Hatching eggs were kept in clean water at room temperature. After seven days larval development was tracked. Larvae collected at the tip of a pasture pipette were placed in cotton buds in order to remove excess water. The cotton buds were then transferred to test vials (ten/vial). We observed larval mortality at various concentrations of 10, 25, 50, and 100 µg/mL, including positive (with 2% ethanol) and negative controls (without vehicle) according to a procedure described previously [21,22]. We measured mortality in terms of ratios (%) of dead vs. live larvae caused by the compounds. Probit scale analysis was used to calculate the LD 50 values. After exposure for 24 h, the number of larvae still alive was counted. This was repeated three times to ensure that the results would hold up statistically.

Antifeedant Activity
The antifeedant activity was evaluated to study the effect of larvicides against nontarget aquatic species. In order to evaluate the antifeedant activity, samples were tested at concentrations of 10, 25, 50, and 100 µg/mL, in conjunction with positive and negative controls (with 2% ethanol) and evaluated on marine fingerlings (O. mossambicus).
The tested compounds were dissolved in 1000 mL water containing five fingerlings in each experimental and control glass bowl. After six hours, mortality was measured continuously and at 1 h intervals for the next 12 h. A ratio (%) of the numbers of dead vs. live fingerlings was used to assess the mortality caused by the compounds (as described previously [21,22]).

Statistical Analysis
With Microsoft Excel, we calculated standard deviations (SD) based on at least three independent assessments for each LD 50 value.

Results and Discussion
The grindstone method was used to synthesize geranylacetones of the title compounds. A mixture of citral, aceotone, substituted amine were ground together in a mortar and pestle. This was then followed by purification via column chromatography on silica with Ethyl acetate:hexane (4:6) as the mobile phase. Scheme 1 shows a synthetic route for the preparation of geranylacetone derivatives. The compounds were obtained up to 88-92% yield. FT-IR, 1 H, and 13 C NMR spectroscopy were used to analyze the compounds obtained (see Supplementary Information). Each compound (1a-1f) was analyzed for their solubility in ethanol and water. All compounds were found to be completely soluble in ethanol. The solubility of geranylacetones in water is 8.867 mg/L at 25 • C and 0.1 mg/5 mL in 2% of ethanol. The dissolution rates of compound 1f are 2.13 mg/L of water and 0.15 mg/5 mL in the case of 2% of ethanol.
The compounds were assigned by their IR spectra and were found to have significant resonances at 3350. 23 -NH, -CH, -C=CH, and =C-CH 3 protons, respectively. The 13 C NMR spectrum has peaked in the range of δ 197.0-207.7, 50.0-54.3, 121.5-127.9, and 135.1-135.9 ppm, which conforms to -C=O, -CH, =CH, and -HC= atoms, respectively. The 1 H and 13 C NMR spectrum of compounds (1a-1f) are shown in Figures S1-S12 in the Supplementary Materials. For all of these compounds, mass spectra and elemental analysis were used to determine their conformation.

Biological Activity
We studied six compounds (1a-1f) against the second instar larvae of C. quinquefasciatus. In view of the structure-activity relationships, the final compounds contained geranylacetone with various types of amines, thus exerting larvicidal and toxic effects depending on their chemical compositions.
The larvicidal activity of geranylacetone derivatives has never been studied before, yet other bioactivities of geranylacetone are scarce in the literature [23,24], and it also appears monoterpenes such as geranylacetone have no water-polluting properties. In addition, its antifeedant actives (Ichthyotoxicity) have not been reported. Figure 2 shows that the structure-activity relationship for geranylacetone and their derivatives in both larvicidal and Ichthyotoxicity activities. Compound 1f showed higher larvicidal activity than other compounds, with an LD50 of 14.1 µg/mL, which was also higher than those of the other natural products. Compounds 1f induced a mortality rate of 100% at 100 µg/mL concentration and 13.9% of mortality in the test fish, O. mossambicus with respect to the antifeedant activity. This suggests that the presence of keto and hydrazine groups may be the reason for the observed biological effects, respectively. Compound 1a shows a mortality rate of 92% with LD50 values of 42.0 µg/mL, whereas it resulted in 26.1% mortality in the case of antifeedant screening. Compound 1f induced 100% mortality at 100 µg/mL and 13.9% antifeedant activity due to the presence of acetophenone and hydrazine groups causing biological effects. Other compounds 1b, 1c, 1d, and 1e show that around 97.3 to 100% of mortality in larvicidal screening with LD50 values around 18.6 Scheme 1. Synthetic route of geranylacetone derivative (1a-1f).

Biological Activity
We studied six compounds (1a-1f) against the second instar larvae of C. quinquefasciatus. In view of the structure-activity relationships, the final compounds contained geranylacetone with various types of amines, thus exerting larvicidal and toxic effects depending on their chemical compositions.
The larvicidal activity of geranylacetone derivatives has never been studied before, yet other bioactivities of geranylacetone are scarce in the literature [23,24], and it also appears monoterpenes such as geranylacetone have no water-polluting properties. In addition, its antifeedant actives (Ichthyotoxicity) have not been reported. Figure 2 shows that the structure-activity relationship for geranylacetone and their derivatives in both larvicidal and Ichthyotoxicity activities. Compound 1f showed higher larvicidal activity than other compounds, with an LD 50 of 14.1 µg/mL, which was also higher than those of the other natural products. Compounds 1f induced a mortality rate of 100% at 100 µg/mL concentration and 13.9% of mortality in the test fish, O. mossambicus with respect to the antifeedant activity. This suggests that the presence of keto and hydrazine groups may be the reason for the observed biological effects, respectively. Compound 1a shows a mortality rate of 92% with LD 50 values of 42.0 µg/mL, whereas it resulted in 26.1% mortality in the case of antifeedant screening. Compound 1f induced 100% mortality at 100 µg/mL and 13.9% antifeedant activity due to the presence of acetophenone and hydrazine groups causing biological effects. Other compounds 1b, 1c, 1d, and 1e show that around 97.3 to 100% of mortality in larvicidal screening with LD 50 values around 18.6 to 35.6 µg/mL suggesting the presence of different types of hydrazine as the reason for the observed biological effects. On the other hand, this set of compounds exhibited some toxic behavior in the range of 23.1 to 33.4 µg/mL compared to 1a, 1f, and geranylacetone. The larvicidal screening results showed that compound 1f is significantly active. The percentages of mortality and LD 50 values are presented in Tables 1 and 2, respectively. Figure 3 indicates the GC-MS of the compound geranylacetone, which shows that it is highly pure and has a molecular ion peak at 194 m/z. In Figure 4, compound 1f Agronomy 2021, 11, 2342 6 of 9 shows high purity with a mass spectrum of molecular ion corresponding to 400.56 m/z, which is confirmatory of synthesized compounds.