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