How can I synthesize an (E)-configured enone using the Wittig reaction?
The Wittig reaction allows for the construction of alkenes from a phosphorus ylide and an aldehyde (or ketone). Shown in the retrosynthetic sense, keeping in mind that there are two possible Wittig disconnections:



Applying this idea to (E)-5-methylhex-3-en-2-one gives the following possibilities:



Which option is chosen often comes down to which gives the most readily accessible starting materials. However, in this case, the stereochemical outcome is critical, and that is controlled by the nature of the ylide used. Looking now in the forward direction, the top option uses an unstabilized ylide, which tend to give the cis-alkene as the major product. The bottom option uses a stabilized ylide, which changes the stereochemical outcome to favor the trans-alkene.



In practice, many chemists would opt for the Horner–Wadsworth–Emmons variant, which gives the trans-alkene as the major product and avoids the production of triphenylphosphine oxide, a byproduct that can be difficult to remove.