Chiral total syntheses of all six insecticidal natural pyrethrins (three pyrethrin I and three pyrethrin II compounds) contained in the chrysanthemum (pyrethrum) flower were performed. Three common alcohol components [(S)-cinerolone, (S)-jasmololone, and (S)-pyrethrolone] were synthesized: (i) straightforward Sonogashira-type cross-couplings using available (S)-4-hydroxy-3-methyl-2-(2-propynyl)cyclopent-2-en-1-ones (the prallethrin alcohol) for (S)-cinerolone (overall 52% yield, 98% ee) and (S)-pyrethrolone (overall 54% yield, 98% ee) and (ii) traditional decarboxylative-aldol condensation and lipase-catalyzed optical resolution for (S)-jasmololone (overall 16% yield, 96% ee). Two counter acid segments [(1R,3R)-chrysanthemic acid (A) and (1R,3R)-second chrysanthemic acid precursor (B)] were prepared: (i) C(1) epimerization of ethyl (±)-chrysanthemates and optical resolution using (S)-naphthylethylamine to afford A (96% ee) and (ii) concise derivatization of A to B (96% ee). All six pyrethrin esters (cinerin I/II, jasmolin I/II, and pyrethrin I/II) were successfully synthesized utilizing an accessible esterification reagent (TsCl/N-methylimidazole). To investigate the stereostructure–activity relationship, all four chiral stereoisomers of cinerin I were synthesized. Three alternative syntheses of (±)-jasmololone were investigated (methods utilizing Piancatelli rearrangement, furan transformation, and 1-nitropropene transformation). Insecticidal activity assay (KD50 and IC50) against the common mosquito (Culex pipiens pallens) revealed that (i) pyrethrin I > pyrethrin II, (ii) pyrethrin I (II) > cinerin I (II) ≫ jasmolin I (II), and (iii) “natural” cinerin I ≫ three “unnatural” cinerin I compounds (apparent chiral discrimination).
In this paper, an accessible chiral pool synthesis of benzyl (R)-2-(acetylthio)propanoate (acetylthiolactate), which is less odorous than the methyl or ethyl analogue, was performed through a clean SN2 displacement reaction using available AcSK with tris[2-(2-methoxyethoxy)]ethylamine (TDA-1), starting from commercially available benzyl (S)-lactate in 76%, 94% ee (2 steps). Deprotection of the acetyl group using N,N-dimethylethylenediamine afforded benzyl (R)-2-sulfanylpropanoate in 93% yield with 90% ee. These two sulfur-containing benzyl esters were sufficiently odorless to be purified by column chromatography. Direct HPLC analysis was applied to determine the enantiomeric excess without thiazolidin-4-one derivatizations. A complementary debenzylation of benzyl (R)-2-(acetylthio)propanoate was also performed using HBr/AcOH to afford (R)-2-(acetylthio)propanoic acid without critical racemization in 92% yield with 92% ee.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.