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DOI: 10.1055/sos-SD-032-00627

Milata, V.; Rádl, S.; Voltrová, S.Science of Synthesis, (200832677.

A simple, general, and stereoselective method for the alkylidenation of esters, developed by Takai and co-workers, gives Z-enol ethers 256 selectively by using a reagent prepared from a 1,1-dibromoalkane, zinc dust, titanium(IV) chloride and N,N,N,N-tetramethylethylenediamine in tetrahydrofuran (Scheme 119).[‌229‌,‌230‌] The reaction is catalyzed by trace amounts of lead(II), which are usually present in various amounts in commercial zinc, and the addition of lead(II) chloride is recomended. Diiodoalkanes can also be used, but the yields are lower.[‌229‌,‌230‌] Steric repulsion by substituents R1, R2, and R3 affects the Z/E ratio of the products. A high Z-selectivity is obtained in the case of bulky R1 and R3 substituents or in the case of a small R2 substituent. Esters with α-branched R1 substituents give the Z-isomers as the sole products; tert-butyl esters thus give modest amounts of the Z-enol ether, whereas 2-methylpropanoates give the Z-enol ether exclusively.[‌229‌,‌230‌]

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