Polyol ester insecticides and method of synthesis

Details Polyol ester insecticides and method of synthesis Polyol ester insecticides and method of synthesis

2000-02-18

2002-07-16

Levy, Neil S. (Department: 1617)

Drug, bio-affecting and body treating compositions

Preparations characterized by special physical form

Biocides; animal or insect repellents or attractants

C424S406000, C514S025000, C514S053000, C514S546000, C514S549000

Reexamination Certificate

active

06419941

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to methods of synthesis of polyol ester insecticides. More particularly, this invention concerns a method of synthesis of sugar esters which ensures that the resulting ester's chemical structure has insecticidal activity.
BACKGROUND OF THE INVENTION
Sucrose octanoate has proven to be a useful insecticide compound. Varieties of sucrose esters are contained in the natural wax of leaves. Discussions of these esters may be found, for example, in Neal, J. W. Jr. et al, J. Econ. Entomol. 87, 1600-1607(1994); Puterka, G. J., et al, J. Econ. Entomol. 88, 615-619(1995), and Lui, T. X. et al, J. Econ. Entomol. 89, 1233-1239 (1996). Sucrose octanoate is contained in the mixture of sucrose esters made when coconut fatty acids are used to make sucrose esters. The sucrose esters are readily biodegradable and hydrolyze to readily metabolizable sucrose and fatty acid. Sucrose esters can be made by the methods disclosed in U.S. Pat. No. 5,756,716, William A. Farone and Robert Serfass, “Method for Production of Sugar Esters”, May 26, 1998. Other methods for making these compounds are also known and referenced in this patent.
The efficient production of sucrose octanoate involves several steps, including an esterification, a transesterification and then a purification step. It would be extremely useful to have compounds with similar insecticidal activity, similar environmental acceptability, made from similar natural products, that could be synthesized in fewer steps. Unfortunately there is no means of predicting the chemical structures that will have insecticidal activity. There is no general agreement as to exactly how the sugar ester compounds obtain their insecticidal activity.
One hypothesis is that the compounds like sucrose laurate or sucrose octanoate act as surfactants to dewax the insect's protective coating. The insect then either dehydrates or is readily attacked by microbes. This hypothesis is supported by the observation that the compounds are “contact” insecticides. Since the sucrose esters are constituents of plant leaves, there is another hypothesis that the compounds somehow interfere with the metabolism of the insect to prevent them from eating the tissue that the esters protect. This hypothesis requires ingestion of the material by the insect and cannot be ruled out since “contact” can also result in ingestion.
It is also known that the short chain sucrose esters that are effective as insecticides have certain properties that seem to enhance that activity. Chortyk and co-workers at the United States Department of Agriculture [see Chortyk, O. T., Pomonis, J. G., and Johnson, A. W., J. Agric. Food Chem., 44, 1551-1557 (1996)] concluded that the sucrose esters with fatty acid chain lengths below 12 were more effective especially when there were 2 or 3 side chains on the sucrose. The fact that there are eight hydroxyl groups that can be esterified in sucrose means that, in principal, one can make 8 sucrose monoester, 28 diester and 56 triester isomers. It is unpredictable if all esters of one type (e.g. monoesters, diesters, etc.) are equally effective. Molecular orbital calculations performed in the inventors' laboratory suggest that not all esters are equally likely to be produced during synthesis.
SUMMARY OF THE INVENTION
In one aspect the present invention relates to a new environmentally friendly method of synthesis of polyol esters. The inventors found that the synthesis method is important in defining the distribution of isomers in complex molecules with the subsequent result that one must either specify the exact nature of the isomers involved and/or the method of synthesis as a mean of selecting the best insecticides.
More particularly another aspect of this instant invention is the use of these esters as safe effective insecticides. The inventors found the surprising and unexpected result that octanoic acid (C8) sorbitol esters are more effective as insecticides, and that the decanoic acid (C10) acid esters are the most effective for xylitol.
Also there was an unexpected finding that for sucrose octanoate the monoesters were more effective as insecticides than the diesters and triesters. This finding is in contradiction to the finding of Chortyk who concluded that sucrose esters with fatty acid chain lengths below 12 were more effective especially when there were 2 or 3 side chains on the sucrose. Additionally Chortyk synthesized his esters through the use of a multi-step process using acid chlorides which generated hazardous by-products. His method of synthesis generated primarily diesters and triesters while the method of U.S. Pat. No. 5,756,716 optimally can generate a high percentage of monoester.
DETAILED DESCRIPTION OF THE INVENTION
The method of preparation of the polyol esters, in particular sorbitol and xylitol, of this invention is best explained in terms of 7 steps. One of the objects of the preparation method is have an environmentally acceptable synthesis that produces no toxic by-products. Another object is to develop a method that allows the entire range of esters to be prepared using essentially the same procedure thus allowing mixed esters to be produced or the same production facility to be used to make esters that could be targeted against specific insects. Without limiting the scope of this invention as expressed by the claims which follow, the synthesis steps will be discussed briefly.
The process is basically as follows:
1. The desired organic acid (e.g. octanoic, deconoic, but not limited to these) is charged to the reactor at a temperature sufficiently high to keep it in liquid form.
2. The polyol (e.g. either xylitol or sorbitol) is added in an amount that would allow the production of the monoester stoichiometrically plus an additional 10% to drive the reaction essentially to completion.
3. An esterification catalyst is added. Any usual catalyst can be used such as sulfuric acid or phosphoric acid. Phosphoric acid is the preferred embodiment in this case since neutralization at the completion of the reaction provides a phosphate salt that can either be left in the product (since phosphorous is an essential plant nutrient and phosphates are a known method of providing phosphorus) or removed by filtration if desired (whereupon the salt can be sold separately for fertilizer use).
4. The reactor is held at a temperature sufficiently high along with a pressure sufficiently low to allow water to be removed as the esterification reaction proceeds. For most of the esters a temperature around 150° C. and atmospheric pressure was used.
5. The reaction is allowed to proceed until the remaining organic acid reaches a low equilibrium value. This point can be determined very simply by monitoring the free acid content of the reaction mixture and comparing differing reaction times (see Example 1 and 2). When the free organic acid is reduced no further the reaction is essentially completed. The equilibrium value in weight percent depends on the molecular weight of the organic acid and the structure of the isomers formed. Once determined for a particular organic acid and polyol combination it can be used as a measure of reaction completion.
6. At the completion of the reaction (approximately 18-30 hours for the esters synthesized for the insecticidal studies) the solution is neutralized with an amount of base that is sufficient to neutralize all of the mineral acid used as a catalyst plus bring the solution to a desired pH for subsequent use. If calcium hydroxide is used as the base, calcium phosphate can be filtered out of the product. Other bases could be used depending on the desired nature of the final product. This procedure was followed to allow for a product of good water solubility with little or no residual fine solid particles.
7. The product (filtrate from Step 6) is analyzed and is ready for use.
This procedure of this present invention is deliberately made deceptively simple. Due to the fact that the insecticide nature as well as other properties of these materials change

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