Oxido Ylids

As an example, the synthesis of oct-2-ene by the normal Wittig procedure gives an 80 20 Z E mixture. By the Schlosser modification, set out in detail below, virtually pure (99 1) E-oct-2-ene is formed in good yield in a one-pot process. Notice the low temperature used when the aldehyde is added. This is necessary to avoid completion of the Wittig. After epimerisation with a second molecule of PhLi, the oxido ylid 107 is acidified to give the alcohol syn-109 that eliminates quickly with a potassium base.19... 

Another synthesis of CJH eminating from the Corey labs involved a modification of the Schlosser modification of the Wittig Reaction. Whereas Wittig reactions between non-stablilized phosphoranes and aldehydes usually provide Z-olefins, Schlosser reported a variation of this reaction that provided -olefins. His group showed that when non-stabilized phosphoranes are reacted with aldehydes in the presence of lithium halides (as opposed to the salt-free conditions that favor Z-olefins), followed by deprotonation of the intermediate adduct, and reprotonation of the resulting j3-oxido ylid, T-olefins are obtained. Corey and Yamamoto reported a variation of this reaction in which the intermediate j3-oxido ylid was treated with formaldehyde, resulting in formation of trisubstituted allylic alcohols. For example, 102 reacted sequentially with aldehyde 103 and formaldehyde to provide 105. This material was then carried on to CJH intermediate 106, and the CJH derivative where R=H. 

Wittig synthesis with As-ylids Synthesis of rraw -ethylene derivs. or trans Oxido compds. from aldehydes and arsonium salts... 

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