- Highly efficient one-pot double-Wittig approach to unsymmetrical (1Z,4Z,7Z)-homoconjugated trienes
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We describe a novel one-pot double-Wittig approach towards unsymmetrically substituted skipped trienes using the symmetrical (Z)-hex-3- ene-1,6-bis(triphenylphosphonium iodide) (2) as key reagent. Double alkenylation of the corresponding bis(ylide) 3 with sequentially added aldehydes gives (Z)-1,4,7-homoconjugated trienes in good yields. Dissymmetrization of the bis(ylide) 3 is feasible, since it displays enhanced reactivity compared to the monoylide resulting from the first olefination. Symmetrical products from statistical coupling of the bis(ylide) 3 can be drastically suppressed by slow release of the first aldehyde component through in situ thermal decomposition of an intermediate aluminate complex, generated by reduction of a methyl ester with DIBAL-H. The novel strategy is successfully applied to the one-pot synthesis of functionalized and isotopically labelled polyunsaturated fatty acids as well as to the synthesis of the geometrid moth pheromone (3Z,6Z,9Z)-nonadeca-1,3,6,9-tetraene (6a). The dissymmetrization strategy was also found to be suitable for the synthesis of homoconjugated dienes from 1,3-propylbis(triphenylphosphonium bromide).
- Pohnert, Georg,Boland, Wilhelm
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p. 1821 - 1826
(2007/10/03)
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- Beyond the cyanine limit: Peierls distortion and symmetry collapse in a polymethine dye
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Theory predicts that cyanine dyes and related linear systems undergo symmetry collapse and bond localization at long chain lengths. Beyond this 'cyanine limit', the properties of these systems do not extrapolate from their shorter counterparts. To test this prediction, dipyridocyanines have been synthesized and shown to undergo such symmetry collapse with chain lengths as short as 13.
- Tolbert, Laren M.,Zhao, Xiaodong
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p. 3253 - 3258
(2007/10/03)
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Coupling reactions of lithium compounds, (Z)-carbonyl olefination of phosphonium ylides and 'crossed' Wittig olefination of bisylides are the key reaction steps in the synthesis principle for nonconjugated, bisolefinic Lepidoptera pheromones and structural analogs. Double unsaturated (E,Z)- and (E,E)-1-vinyl halides are converted into the corresponding (E,Z)- and (E,E)-1-vinyl lithium compounds and coupled selectively to (E,Z)- and (E,E)-alkadienols, alkadienals and alkadienyl acetates with different carbon chain length, geometry and relative positions of double bonds. 'Crossed' Wittig reactions of 1,ω-alkylidene bisylides with two different alkanals gives rise to the formation of corresponding (Z,Z)-dienic sex attractants and derivatives. A monounsaturated (E)-1-vinyl iodide is the synthon for the preparation of an (E)-alkenyl bromide which is converted to an (E)-alkenyl phosphonium salt and (Z)-selectively olefinated to (Z,E)-isomers of moth sex pheromones.
- Bestmann,Zeibig,Vostrowsky
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p. 1039 - 1047
(2007/10/02)
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