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Highly selective synthesis of conjugated dienoic and trienoic esters via alkyne elementometalation—Pd-catalyzed cross-coupling
Guangwei Wang, Swathi Mohan and Ei-ichi Negishi
Proceedings of the National Academy of Sciences of the United States of America
Vol. 108, No. 28 (July 12, 2011), pp. 11344-11349
Published by: National Academy of Sciences
Stable URL: http://www.jstor.org/stable/27978802
Page Count: 6
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All four stereoisomers (7–10) of ethyl undeca-2,4-dienoate were prepared in ≥98% isomeric purity by Pd-catalyzed alkenylation (Negishi coupling) using ethyl (E)- and (Z)-β-bromoacrylates. Although the stereoisomeric purity of the 2Z,4E-isomer (8) prepared by Suzuki coupling using conventional alkoxide and carbonate bases was ≤95%, as reported earlier, the use of CsF or nBu4NF as a promoter base has now been found to give all of 7–10 in ≥98% selectivity. Other widely known methods reveal considerable limitations. Heck alkenylation was satisfactory for the syntheses of the 2E,4E and 2E,4Z isomers of ≥98% purity, but the purity of the 2Z,4E isomer was ≤95%. Mutually complementary Horner—Wadsworth—Emmons and Still—Gennari (SG) olefinations are also of considerably limited scopes. Neither 2E,4Z nor 2Z,4Z isomer is readily prepared in ≥90% selectivity. In addition to (2Z,4E)-dienoic esters, some (2Z,4E,6E)- and (2Z,4E,6Z)-trienoic esters have been prepared in ≥98% selectivity by a newly devised Pd-catalyzed alkenylation—SG olefination tandem process. As models for conjugated higher oligoenoic esters, all eight stereoisomers for ethyl trideca-2,4,6-trienoate (23–30) have been prepared in ≥98% overall selectivity.
Proceedings of the National Academy of Sciences of the United States of America © 2011 National Academy of Sciences