A Simple and Sustainable Synthetic Lab-Protocol for Obtaining Racemic Dominicalure-Aggregation Pheromone of the Grain Beetle Rhyzopertha dominica F. (Coleoptera, Bostrichidae)

The pheromones constitute a decisive component in the strategies for the conservation of stored foods. In Ecuador, several studies are developing, under eco sustainable conditions, on synthetic pheromones and their applicability in the control of species Rhyzopertha dominica, which causes severe damage in stored grains. Previously reported information details the use of expensive reagents for carrying out the synthesis of the aggregation pheromone of this species. Using propionic and isobutiric aldehydes as starting substrates by means of aldolic condensation, selective oxidation with Ag2O-methanol of monoenic aldehydes, treatment with SOCl2, and esterification with 2-pentanol, the respective components of the racemic aggregation pheromone of this species were synthesized.


Introduction
The grain beetle (Rhyzopertha dominica F. Coleoptera Bostrichidae) constitutes a devastating pest of stored grains, characterized by its cosmopolitan habits and wide geographic distribution. In Ecuador, it causes severe damage that affects the development of local food programs. This species has shown genetic variability and resistance to organophosphate insecticides [1,2] and synthetic pyrethroids, which makes its control difficult [3]. The use of repellants and natural toxic agents and essential oils [4], as well as biological control using toxins from Bacillus thuringiensis and natural parasitic predators [5,6], have been economically unsatisfactory. One of the methods of controlling the population dynamics of this harmful species considers the use of the aggregation pheromone secreted by these insects during the infestation stage. This pheromone was isolated and identified by Silverstein et al. [7] as a mixture of the S-(+)-isomers of isoamyl esters I and II ( Figure  1), being called dominicalure. These components were synthesized from glutamic acid, natural amino acids, and crotonaldehyde using the Sharpless asymmetric epoxidation. Reference [8] Cheskis and Schpiro, using the enantiomers of 2-pentanol and a co-catalytic mixture of Et2CuLi-ZnCl2, synthesized the chiral dominicalures with yields greater than 50% [9], Razkin, Gil and Gonzalez [10] synthesized the dominicalures by esterifying α, β  unsaturated acids with (S)-(+)-2-pentanol obtained by an asymmetric reduction of 3-penten-2-one. Rossi et al. [11] synthesized (S)-1-methylbutyl (E)-2-methyl-2-pentenoate, one of the components of dominicalure, from the palladium catalyzed reaction between ethyl-2-pentynoate and Bu3SnH. These synthetic efforts are characterized by the use of rare and expensive reactants, in addition to using non-classical reaction conditions, generating significant amounts of contaminants and toxic residues at laboratory scale. The objective of our communication is to report a synthetic pathway, operationally affordable, of the racemic form of both components of the pheromone of the species Rhyzopertha dominica F. (Coleoptera-Bostrichidae) with potential high attractant activity.

Materials and Methods
All reagents used were supplied by MERCK, Darmstadt, Germany, and were used without prior purification. The physicochemical characteristics of the synthesized esters are in correspondence with the data reported in the literature. Boiling points were not corrected. The synthetic processes were controlled by thin-layer chromatography using silica 60 G chromatoplates of 0.25 mm thickness, a mixture of ethyl acetate/n-hexane (Ac-OEt/Hex. 3:7 v/v) as a solvent and concentrated H2SO4 was used as a chromogenic agent with subsequent heating up to 110 °C, and iodine chambers.

IR Spectroscopy
The infrared spectra were recorded on a PHILIPS ANALYTICAL FTIR PU-9600 spectrophotometer, Germany; the samples were prepared in potassium bromide (KBr) tablets at 25 °C. Alternatively, the spectra were recorded in a JASCO-Canvas 4600, Japan system in CsBr tablets at 25 °C.

NMR Spectroscopy
NMR spectra were recorded on a BRUKER AC-250 instrument, Germany, at 25 °C. The protonic chemical (δ) shifts are given in ppm, using tetramethylsilane as internal reference (TMS, δ = 0.0) and as a solvent CDCl3. The chemical shifts (δ) for 13 C refers to the central peak of the CDCl3 solvent at 77.03 ppm.
2,4-Dimethylpent-2E-enal (IIlb). Over 15 min, a solution of 5.05 g (50 mmoles) of diisopropylamine in 6 mL of dry THF was added (by dropping) at −20 °C (Ar) to 50 mL of a 1 N solution of n-butyllithium (50 mmoles) in hexane. The reaction mixture was stirred at ~25 °C for 30 min and then at −15 °C for 20 min. A solution of 5.65 g (50 mmoles) of the azomethine (II) in 10 mL of THF and, after 40 min, at −20 °C (saline bath: NH4SCN-NaCl 133/33 g in 150 g ice), a solution of 3.6 (50 mmoles) of isobutyraldehyde in 5 mL of THF were added to it. The reaction mixture was heated to 25 °C over 1 h, and after 15 min, 100 mL of 20% H2SO4 was added to the reaction mixture at 0 °C. The resulting emulsion was stirred vigorously (800 rpm) for 40 min and the aqueous layer was separated off and carefully extracted with ether. The combined extract was washed with saturated aqueous NaCI solution and dried with MgSO4, the solvent was evaporated off at 150 mm Hg, and the residue was distilled. This gave 2. 2-Methylpent-2E-enoyl Chloride (IVa). In one portion, a solution of 0.98 g (10 mmoles) of the aldehyde (IIIa) in 20 mL of MeOH was added to a solution of 3.6 g (20 mmoles) of AgNO3 in 15 mL of H2O, and this was followed, with stirring over 40 min, by 42 mL of a 1 N aqueous solution of NaOH (42 mmoles). After 3 h, the precipitate was filtered off and was washed with hot water and with ether, and this ether was used to extract the aqueous filter, which was then acidified with 20 mL of 10% HCI and was carefully extracted with ether. The combined extract was dried with MgSO4, the solvent was evaporated off in a vacuum, and the residue was treated with 2.4 g (20 mmoles) of SOCl2. The mixture obtained was heated at 60 °C for 1 h, and then the excess of SOCl2 was evaporated in vacuum, and the residue was distilled. This gave 1.13 g (85%) of the acid chloride l-Methylbutyl 2-Methylpent-2E-enoate (Ia). Over 15 min, a solution of 0.8 g (6 mmoles) of the acid chloride (IVa) in 2 mL of ether was added to a stirred solution of 0.62 g (7 mmoles) of sec-amyl alcohol in 4 mL of pyridine. After 2 h, the reaction mixture was diluted with 10 mL of ether, washed free from pyridine with 5% HCI, washed additionally with a saturated aqueous solution of NaCI, and dried with MgSO4. After the solvent had been driven off in vacuo and the residue had been distilled 1.05 g (94%) of the ester (la) was obtained with bp 59-60 °C (3 mm Hg). nD 20

Results and General Discussion
The synthetic sequence is shown in Figure 2. The E-configuration of the unsaturated carbon skeleton of the acidic fragment was obtained from the aldol condensation of the propanal and isobutanal aldehydes. The unsaturated aldehydes were oxidized in a quantitative way with Ag2O in their respective acids, which, in situ, were transformed into their chloro-anhydrides by SOCl2. The esterification reaction with 2-pentanol was carried out in a classical way and allowed to obtain the pheromone components of aggregation of Rhyzopertha dominica.

Conclusions
The racemic pheromone, Dominicalure, was obtained from the species Rhyzopertha dominica F. using as starting substrates commercially available aldehydes and their aldol condensation. The synthetic methodology does not generate contaminants and does not need special laboratory conditions nor the use of complex purification techniques, allowing the E-isomer of the components with yields greater than 98% to be obtained. The process is simple and is developed in three synthetic steps, where one of the sequences (oxidation-formation of chloro-anhydrides) is achieved in situ, minimizing the operational risk and the loss of derivatives. This procedure does not require sophisticated technical equipment nor excessive energy costs, constituting a feasible protocol under laboratory conditions.  Data Availability Statement: The study didn't report any data.