Synthesis of the Minor Sex Pheromone Component of Two Brazilian Soybean Stink Bugs ( Het . : Pentatomidae ) , and an Analogue Compound

Os percevejos Euschistus heros e Piezodorus guildinii (Heteroptera: Pentatomidae) são importantes pragas da cultura da soja no Brasil. Uma característica marcante nestas espécies é o fato de que ambas se utilizam das mesmas estruturas químicas como feromônios sexuais, ou seja, os ésteres metílicos 2,6,10-trimetildodecanoato de metila (1) (minoritário) e o 2,6,10-trimetiltridecanoato de metila (2). No intuito de se verificar a exata função biológica destas estruturas e melhor entender o sistema de comunicação de cada espécie, realizou-se a síntese do componente minoritário 1 juntamente com um composto análogo, o 2,6,10-trimetiltetradecanoato de metila (3). Tais compostos serão submetidos a testes em laboratório, juntamente com o componente majoritário 2, já previamente sintetizado.


Introduction
Soybean is damaged by complexes of stink bugs around the world.In Brazil, the species of major importance in practically all soybean growing regions are Nezara viridula (L.), Piezodorus guildinii (Westwood) and Euschistus heros (F.), which attacks has led to the use of more than 4 millions litres of chemical insecticides annually 1,2 .Forming part of a system of integrated pest management, the use of pheromones can be an useful alternative tool for the control of these pests.
Although there are some 35,000 species of Heteroptera worldwide, yet sexual pheromones are fully known only for a few species 3 .The diverse nature of the group itself has hindered progress 3 .In 1994, Aldrich and his co-workers identified methyl 2,6,10-trimethyldodecanoate (1) and methyl 2,6,10-trimethyltridecanoate (2) as components of the male-produced sex pheromones of E. heros 4 .They also identified 2 as the second major component of the malespecific volatile constituents of the Central America stink bug Euschistus obscurus (Figure 1).We have recently reported that compounds 1 (minor) and 2 are also male-produced sex pheromones of the stink bug P. guildinii 5 .Although the different species liberate a similar pheromone blend, unique ratios of these compounds might be involved in species isolation, since it was previously observed on indoor bioassay that the activity of pure synthetic isomers of 2, in females E. heros, does not significantly differ from that of the synthetic racemic mixture 6 .
In order to investigate the specific biological function of these compounds and to understand the communication system mechanism of each species, this work is aimed to synthesizing the minor component 1 and an analogue structure, methyl 2,6,10-trimethyltetradecanoate (3) (Figure 1).These compounds and the previously synthesized major component 2 7 will be tested individually and in different ratios on indoor bioassay and the results will be reported in due course.

Experimental
The IR spectra refer to films and were measured on a Bomem M-102 spectrometer.The 1 H-NMR spectra were recorded with TMS as an internal standard at 400 MHz on a Bruker ARX 400 spectrometer.The 13 C-NMR spectra were recorded with TMS as an internal standard at 100 MHz on a Bruker ARX 400.GC-EIMS (70 eV) analysis was carried out on a Varian Saturn 2000 GC-MS spectrometer in a split injector mode.A VA-5 capillary column (30 m x 0.25 mm x 0.25 µm) was operated at 70° C for 1 min, increased to 270° C at a rate of 7° C/min and held at this temperature for 10 min.All reagents and solvents used in the syntheses were of highest commercially available standard.Chromatographic purifications were carried out on silica gel 60, Merck, 230-400 mesh.
(3RS)-3,7-Dimethyloct-6-enyl-1-acetate (5): To a mixture of pyridine (50 cm 3 ) and citronellol 4 (15.00 g, 96.15 mmol) cooled at 0 o C under nitrogen, acetic anhydride (14.15 cm 3 , 150 mmol) was added in one portion, and the resulting mixture was stirred for 12 h.The reaction was quenched by addition of hot water (50 cm 3 ) followed by addition of ethyl ether (100 cm 3 ).The aqueous layer was extracted with ethyl ether (3 x 60 cm 3 ) and the organic layer was dried over anhydrous magnesium sulfate and concentrated in vacuo.The product was purified by flash chromatography on silica gel using a mixture of hexane/ethyl acetate (24:1 v/v), yielding citronellyl acetate 5 as a colorless oil (17.00 g, 91.40 mmol, 95%). 1 H-NMR (400 MHz, CDCl 3 ) δ 0.91 (d, 3  (3RS, 6E)-3,7-Dimethyl-8-oxo-6-octenyl-1-acetate (6): To a mixture of selenium dioxide (5.31 g, 48.0 mmol) in methylene chloride (100 cm 3 ) maintained at 5 -10°C, tert-butyl hydroperoxide (288 mmol) was added dropwise.After 20 minutes at 10 °C, citronellyl acetate 5 (17.8 g, 95.9 mmol) was added in one portion.The resulting mixture was stirred during 30 minutes at 0 °C and then the cooling bath was removed.The mixture was allowed to warm to room temperature, and the stirring of the resulting solution was continued for 2-3 h.After all starting material had been consumed (~ 4 h, detected by gas chromatography), the mixture was carefully concentrated in vacuo without heating.The residue was diluted in hexane (50 cm 3 ) and treated with 10% aqueous potassium hydroxide solution (40 cm 3 ), yielding an aqueous lemon yellowish solution.The organic layer was treated twice with 10% aqueous potassium hydroxide solution (40 cm 3 ), followed by treatment with brine (80 cm 3 ).The organic layer was dried over anhydrous magnesium sulfate and concentrated in vacuo to afford an oil.Flash chromatography over silica gel using hexane/ethyl acetate (5:1 v/v) as eluent afforded aldehyde 6 (7.6 g, 38 mmol, 40 %) and the corresponding alcohol (7.5 g, 38 %). 1 H-NMR (400 MHz, CDCl 3 ) δ 0.97 (d, 3  (3RS, 6E)-3,7-Dimethylnona-6,8-dienyl-1-acetate (7): To a mixture of the salt CH 3 PPh 3 Br (6.25 g, 17.5 mmol) in THF (100 cm 3 ), cooled at -25 o C under argon, was added dropwise a solution of n BuLi (16.9 mmol, 10.6 cm 3 , 1.6 mol L -1 in hexane).After the addition was completed, the mixture turned dark orange.After 10 min the salt was consumed and a solution of aldehyde 6 (3.0 g, 15 mmol) in THF (30 cm 3 ) was added to the mixture.After 5 minutes the cooling bath was removed and the mixture was allowed to warm to room temperature.After all starting material had been consumed (~10 min, detected by gas chromatography) the reaction mixture was quenched with saturated aqueous NaCl solution (20 cm 3 ).The aqueous layer was extracted with petroleum ether (5 x 50 cm 3 ), and the organic phase was dried over anhydrous MgSO 4 and concentrated in vacuo.The residue was purified by flash chromatography over silica gel using a mixture of hexane/ethyl acetate (10:1 v/v), yielding 1.90 g of compound 7 (57 %). 1 H-NMR (400 MHz, CDCl 3 ) δ 0.92 (d, 3  To a solution of compound 7 (1.90 g, 9.59 mmol) in methanol (10 cm 3 ) at 0 o C, was added a solution of K 2 CO 3 (0.5 mol L -1 , 19.2 cm 3 ).The mixture was stirred at room temperature until all starting material had been consumed.This solution was concentrated in vacuo and extracted with ether.The organic layer was treated with saturated NaCl solution, dried over anhydrous MgSO 4 and concentrated in vacuo.The product was purified by flash chromatography over silica gel using a mixture of hexane/ethyl acetate (1:5 v/v), yielding 1.48 g of compound 8 (99 %). 1 H-NMR (400 MHz, CDCl 3 ) δ 0.92 (d, 3  (3RS, 6E)-3,7-Dimethylnona-6,8-dienal (9): To a stirred solution of dimethyl sulfoxide (2.21 g, 28.4 mmol) in methylene chloride (8.50 cm 3 ) cooled at -60 o C under nitrogen, was added a solution of oxalyl chloride (1.80 g, 14.2 mmol) in methylene chloride (27.4 cm 3 ), and the reaction mixture was stirred for 20 minutes.Alcohol 8 (1.48 g, 9.48 mmol) dissolved in methylene chloride (67.0 cm 3 ) was added.The mixture was stirred for 1 h at -60 o C and then treated with trimethylamine (7.88 cm 3 , 57.0 mmol).The cooling bath was removed and the mixture was allowed to warm to room temperature over a period of 30 minutes.After addition of hexane (60 cm 3 ), the mixture was poured into water (25 cm 3 ) and extracted with a solution of hexane/ ethyl acetate (2:1 v/v).The organic phase was treated with saturated NaHCO 3 solution and saturated NaCl solution, dried over anhydrous MgSO 4 and concentrated in vacuo to give 1.40 g of crude 9.This unstable aldehyde was directly used for the next step without purification.

Results and Discussion
A brief retrosynthetic analysis is depicted for compounds 1 and 3 (Scheme 1).(±) -Citronellol 4 serves as starting material to the synthesis of the key and common intermediate for both target molecule, aldehyde 6.From here, we have mainly utilized two Wittig reaction to build the skeleton of each molecule.The first one was employed to generate compounds 9 and 9a, which have the correct numbers of carbon atoms at the A portions of the molecules, and the second Wittig reaction afforded the B portions of alcohol 11 and 11a, the direct precursors of the desired molecules.
The transformation of (±)-citronellol 4 to the aldehyde 6 was performed according to Scheme 2. The first step involves the acetylation of 4 with Ac 2 O-Py 9 resulting in the respective acetate 5 in 95 % yield.This was oxidised by SeO 2 / t BuOOH to afford a mixture of the aldehyde 6 (40 % yield) and the corresponding allylic alcohol (38 % yield) 10,11 .After purification by flash chromatography, the pure aldehyde 6 was subjected to the Wittig reaction with (triphenylphosphonium)methanide (CH 2 =PPh 3 ) or (triphenylphosphonium)propanide (CH 3 CH 2 CH=PPh 3 ) to give compounds 7 (57 % yield) or 7a (65 % yield), respectively.These new acetates intermediates were straightforward transformed into methyl 2,6,10-trimethyldodecanoate transformed in to the alcohol 10, by employing the second Wittig reaction of our synthetic plan, with ylide 12. Compound 10 and the respective analogue 10a, derived from acetate 7a, could not be adequately purified.The GC and TLC analyses of the crude residue obtained in this reaction shows a number of products, which are certainly related to the mixture of stereoisomers (resulting from the methyl branches and the double bonds).Thus, crude 10 was hydrogenated over Pd/C furnishing the alcohol 11 in 75 % yield over two steps.Finally, the routine Jones oxidation of 11 afforded the respective carboxylic acid, which was treated with CH 2 N 2 to give the desired pheromone 1 in 75 % yield 13 as a mixture of stereoisomers.Similarly, acetate 7a was converted into 3.
We have previously described the first synthesis of (2R, 6S, 10S) and (2S, 6S, 10S) isomers of methyl 2,6,10trimethyldodecanoate (1) 13 , and recently we developed a new synthesis of a mixture of stereoisomers of pheromone 2 7 .A stereoisomeric mixture of methyl 2,6,10trimethyltridecanoate (2) was firstly synthesized by Mori and Murata 14 , which later have described the synthesis of all of the eight possible isomers of pheromone 2 15 .We have now achieved a new approach to the synthesis of these stink bugs pheromones.A positive feature of our synthesis is the fact that the starting material 4 and the bromide utilized to prepare ylide 12 (3-bromo-2-methyl-1-propanol), are commercially available in both enantiomeric forms.So, a number of stereoisomers of the final compounds 1 and 3 (even for 2) could be obtained.
However, we intend in to use the racemic compounds to carry out the bioassays, since these stink bugs are responding to the racemic mixture at the same level as to the pure isomers, as already mentioned 6 .This characteristic can be an economically important issue to define the usefulness of synthetic pheromones as an alternative method in pest management control for P. guildinii and E. heros.

Figure 1 .
Figure 1.Pheromones and analogue compound of stink bugs.