The Schlosser Modification
The major limitation of the traditional Wittig reaction is that the reaction goes mainly via the erythro betaine intermediate, which leads to the Z-alkene. However Schlosser & Christmann found that the erythro betaine can be converted to the threo betaine using phenyllithium at low temperature (forming a betaine) followed by HCl. Upon workup this leads to the E-alkene product as shown.
E. J. Corey and Hisashi Yamamoto found that the utility can be extended to a stereoselective synthesis of allylic alcohols, by reaction of the betaine ylid with a second aldehyde. For example:
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