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DOI: 10.1055/sos-SD-047-00004

Schobert, R.; Hölzel, C.; Barnickel, B.Science of Synthesis, (20104754.

The Wittig reaction is inherently stereoselective. Reactive, unstabilized ylides favor the formation of Z-alkenes, while stabilized ylides normally give rise to E-alkenes. However, selective formation of configurationally pure trisubstituted alkenes or at least of mixtures with one greatly predominating isomer is only observed for reactions with either phosphorus reagent or carbonyl component bearing bulky, significantly different substituents at the ylidic or carbonyl carbon atom, respectively. In addition, the diastereomers should be amenable to separation in order for the reaction to be of practical value. Exceptions are alkenations with symmetrically substituted ylides or ketones, which always yield a sole product alkene.