The Horner-Wadsworth-Emmons modification of the Wittig reaction is a widely used method for the preparation of α, β-unsaturated esters. The phosphonate anions are strongly nucleophilic and react readily with carbonyl compounds under mild conditions to form olefins and water-soluble phosphate esters in good yields. Since this reaction preferentially gives more stable E-olefins in general, extensive efforts have been devoted to the stereoselective construction of
Z -α, β-unsaturated esters. Several attempts have been made by the choice of cation, temperature, and solvent, but they are with a limited success. On the other hand, the special phosphonate reagents including (CF
3CH
2O)
2P (O) CH
2CO
2Me and (ArO)
2P (O) CH
2CO
2R gave highly selective routes to a wide range of Z-α, β-unsaturated esters. This review describes these efforts. The mechanism of the HWE reaction is also described briefly.
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