{"id":23434,"date":"2021-03-24T09:36:36","date_gmt":"2021-03-24T09:36:36","guid":{"rendered":"https:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=23434"},"modified":"2021-03-24T09:39:37","modified_gmt":"2021-03-24T09:39:37","slug":"deltamethrin-a-polymorphed-insecticide","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=23434","title":{"rendered":"Deltamethrin – a polymorphed insecticide."},"content":{"rendered":"
\n

Deltamethin is a pyrethroid insecticide for control of malaria which has been used for a little while. Perhaps inevitably, mosquitoes are developing resistance to it. So what could be done about countering this? Well, perhaps surprisingly, form a polymorph![1]<\/a><\/span>\u00a0These crystal structure isomers are often highly undesirable; thus\u00a0Ritonavir, which changed its polymorphic form<\/a> during manufacture to become far less active (due it has to be said to insolubility).\u00a0Now a polymorph of Deltamethin has been discovered, which when applied as a powder, increases<\/strong>\u00a0its effectiveness more than 10 times against Anopheles<\/em> mosquitoes and provides a potentially new affordable malaria control solution for countries that are loosing protection.<\/p>\n

\"\"<\/a><\/p>\n

You can view this new polymorph at DOI: 10.5517\/ccdc.csd.cc24mrxg<\/a> and 10.5517\/ccdc.csd.cc24mryh<\/a>. The two structures differ in the torsion angle around the O-C(CN) bond, resulting in differing anomeric interactions<\/a> between the ester oxygen and the C-CN group.<\/p>\n

\"\"<\/a><\/p>\n

The anomeric effect can be quantified by the dihedral angle between a donor oxygen lone pair and the accepting (in this case C-CN) bond. The position of the lone pair itself can be localised using the ELF (electron localisation function) method. For one polymorph, this has a torsion angle of 127\u00b0 and O-C and C-CN lengths of 1.455 and 1.480\u00c5. The other form has an almost orthogonal (i.e. non-interacting) torsion of 79\u00b0 with lengths of 1.461 and 1.477\u00c5. The first form exhibits some electron pair donation into the C-O bond, shortening it slightly, accompanied by transfer of C-CN bond electrons onto the N, weakening the C-N bond. A very similar effect was noted for a herbicide[2]<\/a><\/span> which resulted in very differing stabilities for two stereoisomers in water.<\/p>\n

Just such aqueous hydrolytic stability was indeed reported for Deltamethrin[3]<\/a><\/span> and its other 7 stereoisomers in water (3 stereocentres, 23<\/sup> = 8). Persistence of any chemical applied to the soil is a very important aspect of its properties; recollect that a now banned insecticide, DDT, suffered from very long soil persistence. The article[3]<\/a><\/span> looks at what happens to Deltamethrin (labelled compound 1<\/strong>) in water in the dark. As it goes, a new isomer labelled 2′<\/strong> appears and then too decays, all in a period of ~5 days. This new isomer was found to be inactive as an insecticide.<\/p>\n

\"\"<\/a><\/p>\n

So what is 2′<\/strong>? Well here is a table of all 8 stereoisomers, where 1<\/strong> is labelled as \u03b1-S,1R<\/em> cis<\/strong>\u00a0and 2′<\/strong> is labelled as \u03b1-S,1S<\/em> cis<\/strong>. Here we have only one of the three stereocentres labelled using Cahn-Ingold-Prelog (CIP) notation, the 1-R<\/em>\/S<\/em><\/strong>\u00a0carbon (the convention is to use italics for CIP). The cis<\/em> designation implies that the 3-position is also S<\/strong>\u00a0<\/em>and the cyano carbon remains\u00a0S<\/em><\/strong>. This cis <\/i>label however conflicts with the abstract to the article, which indicates that the dark water isomerisation is subject to cis\/trans<\/em> isomerisation. So we have a water-induced isomerisation which inverts the configuration of 1R<\/em>,3R<\/em><\/strong>\u00a0in molecule 1<\/strong> to apparently\u00a01S<\/em>,3S<\/em><\/strong> in molecule 2′<\/strong>?\u00a0\u00a0\"\"<\/a><\/i><\/p>\n

Changes in stereochemistry in Deltamethrin are likely to be the result of forming planar enols and so one might judge the hydrolytic stability by the ease of forming such an enol. Just such a process was the topic of a post<\/a> on the undesired epimerisation of thalidomide in water. Time for calculations (FAIR data doi: 10.14469\/hpc\/8020<\/a>\u00a0) at the B3LYP+GD3+BJ\/Def2-TZVPP\/SCRF=water computational level. Firstly, the free energy of the enol form on the cyclopropane by relocation of the proton at C-1 is 32.0 kcal\/mol higher. However, forming the enol by relocating the proton on the carbon bearing the CN group to the nitrogen is only 20.0 kcal\/mol higher in energy. With a small further barrier for the TS for proton removal expected, this latter enol is within an accessible energy range for room temperature reaction, whereas the previous enol is not. So the hypothesis is that 2′<\/strong> is actually the stereoisomer labelled 2<\/strong> in Table 1, ie R,R,R a<\/em>nd not S,S,S. <\/em>The free energy of this\u00a0R,R,R <\/em>diastereoisomer is calculated 1.1 kcal\/mol higher than 1<\/strong> itself, which would represent about 13% of this isomer at equilibrium.<\/p>\n

I have shown here how uncertainty caused by how to reconcile the stereochemistry of eg \u00a0Figure 4 and Table 1[4]<\/a><\/span> in modern terms can perhaps be lessened by performing “reality check” calculations on possible multiple interpretations to reduce them to the most probable. We also find that a relatively old and much used bioactive compound can have surprises lurking around the corner, in this case by a simple recrystallisation that results in a new form being discovered without having to do any other chemistry. \u00a0Still, how much longer will pyrethroid insecticides be used?<\/p>\n

References<\/h2>\n
  1. J. Yang, B. Erriah, C.T. Hu, E. Reiter, X. Zhu, V. L\u00f3pez-Mej\u00edas, I.P. Carmona-Sep\u00falveda, M.D. Ward, and B. Kahr, \"A deltamethrin crystal polymorph for more effective malaria control\", Proceedings of the National Academy of Sciences<\/i>, vol. 117, pp. 26633-26638, 2020. https:\/\/doi.org\/10.1073\/pnas.2013390117<\/a>\n\n<\/li>\n
  2. P. Camilleri, D. Munro, K. Weaver, D.J. Williams, H.S. Rzepa, and A.M.Z. Slawin, \"Isoxazolinyldioxepins. Part 1. Structure\u2013reactivity studies of the hydrolysis of oxazolinyldioxepin derivatives\", J. Chem. Soc., Perkin Trans. 2<\/i>, pp. 1265-1269, 1989. https:\/\/doi.org\/10.1039\/p29890001265<\/a>\n\n<\/li>\n
  3. R.J. Maguire, \"Chemical and photochemical isomerization of deltamethrin\", Journal of Agricultural and Food Chemistry<\/i>, vol. 38, pp. 1613-1617, 1990. https:\/\/doi.org\/10.1021\/jf00097a039<\/a>\n\n<\/li>\n
  4. https:\/\/doi.org\/<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    Deltamethin is a pyrethroid insecticide for control of malaria which has been used for a little while. Perhaps inevitably, mosquitoes are developing resistance to it. So what could be done about countering this? 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At the Beilstein conference, Ian Bruno mentioned another type; \u00a0tautomeric polymorphism, where a compound can crystallise in two forms differing in the position of acidic protons. Here I explore\u2026","rel":"","context":"In "Chemical IT"","block_context":{"text":"Chemical IT","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=2"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2017\/06\/109-1024x537.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":17333,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=17333","url_meta":{"origin":23434,"position":1},"title":"Ritonavir: a look at a famous example of conformational polymorphism.","author":"Henry Rzepa","date":"January 2, 2017","format":false,"excerpt":"Here is an inside peek at another one of Derek Lowe's 250 milestones in chemistry, the polymorphism of Ritonavir.\u00a0The story in a nutshell concerns one of a pharma company's worst nightmares; a drug which has been successfully brought to market unexpectedly\u00a0\"changes\" after a few years on market to a less\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":23973,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=23973","url_meta":{"origin":23434,"position":2},"title":"Two record breakers for the anomeric effect; one real, the other not.","author":"Henry Rzepa","date":"July 1, 2021","format":false,"excerpt":"The classic anomeric effect operates across a carbon atom attached to oxygens. One (of the two) lone pairs on the oxygen can donate into the \u03c3* orbital of the C-O of the other oxygen (e.g. the red arrows) tending to weaken that bond whilst strengthening the donor oxygen C-O bond.\u2026","rel":"","context":"In "crystal_structure_mining"","block_context":{"text":"crystal_structure_mining","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1745"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2021\/07\/dist-vs-dist-1024x747.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":12224,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12224","url_meta":{"origin":23434,"position":3},"title":"What is the best way of folding a straight chain alkane?","author":"Henry Rzepa","date":"April 6, 2014","format":false,"excerpt":"In the previous post, I showed how modelling of unbranched alkenes depended on dispersion forces. When these are included, a bent (single-hairpin) form of C58H118 becomes lower in free energy than the fully extended linear form. Here I try to optimise these dispersion forces by adding further folds to see\u2026","rel":"","context":"In "General"","block_context":{"text":"General","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":13618,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=13618","url_meta":{"origin":23434,"position":4},"title":"Ionizing ultra-strong acids with water molecules.","author":"Henry Rzepa","date":"March 15, 2015","format":false,"excerpt":"My previous posts have covered the ionization by a small number of discrete water molecules of the series of halogen acids, ranging from HI (the strongest,\u00a0pKa -10) via HF (weaker, pKa 3.1) to the pseudo-halogen HCN (the weakest, pKa 9.2). Here I try out some even stronger acids to see\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":16902,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16902","url_meta":{"origin":23434,"position":5},"title":"\u03c3 or \u03c0 nucleophilic reactivity of imines? A mechanistic twist emerges.","author":"Henry Rzepa","date":"September 28, 2016","format":false,"excerpt":"The story so far. Imines react with a peracid to form either a nitrone (\u03c3-nucleophile) or an oxaziridine (\u03c0-nucleophile). The balance between the two is on an experimental\u00a0knife-edge, being strongly influenced by substituents on the imine. Modelling these reactions using the \"normal\" mechanism for peracid oxidation did not reproduce this\u2026","rel":"","context":"In "reaction mechanism"","block_context":{"text":"reaction mechanism","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1086"},"img":{"alt_text":"6ts-irc1","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2016\/09\/6TS-IRC1.gif?resize=350%2C200","width":350,"height":200},"classes":[]}],"jetpack_likes_enabled":false,"authors":[{"term_id":2661,"user_id":1,"is_guest":0,"slug":"admin","display_name":"Henry Rzepa","avatar_url":"https:\/\/secure.gravatar.com\/avatar\/897b6740f7f599bca7942cdf7d7914af5988937ae0e3869ab09aebb87f26a731?s=96&d=blank&r=g","author_category":"1","first_name":"Henry","last_name":"Rzepa","user_url":"https:\/\/orcid.org\/0000-0002-8635-8390","job_title":"","description":"Henry Rzepa is Emeritus Professor of Computational Chemistry at Imperial College London."}],"_links":{"self":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/23434","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=23434"}],"version-history":[{"count":23,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/23434\/revisions"}],"predecessor-version":[{"id":23462,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/23434\/revisions\/23462"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=23434"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=23434"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=23434"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=23434"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}