Insecticidal Activity of Cyanohydrin and Monoterpenoid Compounds

The insecticidal activities of several cyanohydrins, cyanohydrin esters and mono- terpenoid esters (including three monoterpenoid esters of a cyanohydrin) were evaluated. Topical toxicity to Musca domestica L. adults was examined, and testing of many co m- pounds at 100 μg/fly resulted in 100% mortality. Topical LD 50 values of four compounds for M. domestica were calculated. Testing of many of the reported compounds to brine shrimp (Artemia franciscana Kellog) resulted in 100% mortality at 10 ppm, and two compounds caused 100% mortality at 1 ppm. Aquatic LC 50 values were calculated for five compounds for larvae of the yellow fever mosquito ( Aedes aegypti (L.)). Monoterpenoid esters were among the most toxic compounds tested in topical and aquatic bioassays.


Introduction
There is a growing need for effective, biodegradable pest-control compounds.Nineteen new major pesticides were introduced from 1961 to 1970, eight from 1971 to 1980, and only three from 1981 to 1985 [1].Several recent publications from our laboratory have reported insect activity in cyanohydrin [2] and monoterpenoid compounds [3][4][5].Naturally occurring cyanohydrins from flax, cassava, bam-boo, peach pits and almonds probably serve a chemical defense function in the plants to protect against insect herbivory [6].Hundreds of monoterpenoids are produced in plant essential oils, and apparently serve a defensive function as well [7].In the current work we report the synthesis and biological activities of several new simple cyanohydrins, as well as novel cyanohydrin esters and monoterpenoid esters.Three compounds were monoterpenoid esters of a potent synthetic cyanohydrin, 1-cyano-1hydroxy-2-propene (1; synonyms: CHP, 2-hydroxy-3-butenenitrile, acrolein cyanohydrin).CHP is an analog of two naturally occurring cyanohydrins in flax.These three cyanohydrins, CHP, methyl ethyl ketone cyanohydrin and dimethyl ketone cyanohydrin, have been shown to be potent insect fumigants [2].We examined the activity in topical application to adult house flies (Musca domestica L.), as well as in aquatic bioassay to brine shrimp (Artemia franciscana Kellog) and the larvae of the yellow fever mosquito (Aedes aegypti (L.)).

Results and Discussion
The cyanohydrins synthesized by the methods reported in this study and tested against the invertebrates are shown in Figure 1. Figure 2 shows the structures of cyanohydrin esters synthesized by the reported methods.Note that compounds 12, 13 and 15 are monoterpenoid esters of the cyanohydrin CHP (1).Topical testing on M. domestica showed that most of the cyanohydrins and their analogs had LD 50 values between 10 and 100 µg/fly (Table 1).Monoterpenoid esters containing the alcoholic moieties CHP (12, 13 and 15), propargyl (20 and 24) and allyl (19 and 23), all three of which are unsaturated, were the most effective.The presence of the CHP moiety was not always associated with high toxicity, and esterification of CHP with acetate, propionate or pivalate moieties produced less effective compounds than esterification of CHP with monoterpenoid moieties (compare 7-9 to 12, 13 and 15 in Table 1).More studies need to be conducted in order to determine the nature of toxicity in relation to structure.The three cyanohydrin-monoterpenoid esters were among the most toxic compounds tested, with CHP citronellate (13) being the most toxic, showing 91% mortality at 10 µg/fly.CHP decanoate (12), CHP cinnamate (15), allyl cinnamate (19) and propargyl cinnamate (20) showed appreciable mortality at 10 µg/fly as well.Topical LD 50 values on M. domestica (95% fiducial limit) were calculated by using SAS [8] and are reported in Table 2. Several compounds showed 100% mortality at 10 ppm, and two compounds, propyl citronellate (22) and allyl citronellate (23), displayed 100% mortality at 1 ppm.The most toxic compounds tested on A. franciscana were esters of cinnamic acid or citronellic acid.Esterification of CHP with monoterpenoid moieties produced more effective compounds than esterification with the smaller moieties (compare 12, 13 and 15 to 7 and 9-11 in Table 3).The cyanohydrins and cyanohydrin esters were of lower toxicity than monoterpenoid esters (17-24), except for CHP citronellate (13) and CHP cinnamate (15).
Aquatic LC 50 values (95% fiducial limit) for A. aegypti larvae were calculated by using SAS [8] and are reported in Table 4.For all tests, it appeared that esterification of CHP ( 1) with a nonmonoterpenoid moiety resulted in equal or lower activity in relation to CHP itself, and this was possibly related to moiety size and polarity.This result was also seen in the comparison of mandelonitrile (4) to mandelonitrile acetate (16) in topical LD 50 values for M. domestica.Esterification of CHP with a monoterpenoid moiety, however, resulted in equal or higher activity in relation to CHP in topical and aquatic testing.Only three CHPmonoterpenoid esters were tested in this study; therefore, any conclusions regarding comparative effectiveness due to the properties of the monoterpenoid moieties are speculative.It is possible that their hydrolysis in vivo results in two insecticidal moieties, CHP and a monoterpenoid acid.
The limited series presented here indicates that alcoholic moieties containing double or triple bonds may be more effective than saturated ones, as methyl and propyl monoterpenoid esters were less effective than CHP, allyl or propargyl esters.Monoterpenoid esters were in most cases more toxic than monoterpenoid cyanohydrins.Esterification of some monoterpenoids has been demonstrated previously to enhance insecticidal activity [4,5].
Structures of cyanohydrin esters are shown in Figure 2, and are named as follows: CHP acetate (7) Compounds 1-3 and 6 were synthesized from potassium cyanide and their corresponding aldehyde.Stoichiometric amounts of potassium cyanide, the reactant aldehyde and glacial acetic acid were required.Potassium cyanide (KCN) was added to anhydrous diethyl ether and stirred with a magnetic stir bar.The aldehyde corresponding to the desired cyanohydrin was added slowly to the reaction mixture.Glacial acetic acid was added, and the reaction proceeded until the reactant aldehyde was no longer detected by thin-layer chromatography.The ethereal reaction mixture was washed three times with a saturated aqueous NaHCO 3 solution, and the aqueous portion was back-extracted with three volumes of diethyl ether and the water layer discarded.The diethyl ether was removed by rotary evaporation, and the product purified by using column chromotography as necessary.Compounds 4 and 5 were purchased from Sigma Chemical (St.Louis, MO, USA).
Topical bioassays to M. domestica (Orlando regular strain) were conducted after Rice and Coats [4] using a 1-µl volume of acetone to deliver the chemical to the thoracic venters of house flies.The mosquito larvae (A.aegypti) and brine shrimp (A.franciscana) were tested by adding the chemical to water according to the method of Tsao et al. [9].The mosquito larvae were provided by Dr. W. A. Rowley, Medical Entomology Laboratory at Iowa State University, Ames, IA.The brine shrimp were purchased from Carolina Biological Supply (Burlington, NC).

Figure 1 .
Figure 1.Structures of cyanohydrin compounds tested in this study.
Figure3shows the structures of esters of two monoterpenoids, cinnamic acid and cironellic acid.

Table 1 .
Results of topical toxicity testing on M. domestica (percentage mortality).

Table 2 .
Topical LD 50 values to Musca domestica (expressed in µg/fly) and 95% fiducial limits.Aquatic testing with A. franciscana resulted in 100% mortality at 100 ppm for nearly all compounds tested (