[Mimic]

Both Horner-Wadsworth-Emmons and Still-Gennari reaction belongs to Wittig reaction. While Horner-Wadsworth-Emmons gives trans(E) olefine, Still-Gennari gives cis(Z) olefine. Starting posphonare is the same: what changes is the presence, in Still-Gennari reaction, of the 18-crown ether, who catch the base cation (e.g. Na⁺ from NaOH), so there is no formation of the six-term cycle, where the cation acts as a bridge between the two oxygen atoms





In what way does the six-term cycle promote the trans(E) isomer formation?

Thanks in advance

[Babcock_Hall]

What are your thoughts?

[Mimic]

It could be in the formation of the phosphorus-oxygen bond (the one indicated with the trasversal arrow) make sure that R² group positions itself in an antiperiplanar manner with respect to the ester group