Trans-ylidation

In the first step (ij) the ylides 3 (R1 = H) can be alkylated with primary alkylhalogenide or with ethyl bromoacetate they can also be acylated with acetic anhydride in a reverse addition operation, (In spite of trans-ylidation process, the acylated phosphonium salt can be easily purified by liquid -liquid extraction with bases). Semi-stabilized ylides3(R1= 0) are not reactive enough to be alkylated or acylated in this way. 

Many phosphonium ylids can be alkylated as in (6.425), and trans ylidation reactions are also known (6.426). 

---