Unnatural synthons electrophilic

Allyl (7) and propargyl (22) haiides can act as latent unnatural synthons (24) and (2S) respectively by electrophilic addition to the double bond in (21) or hydration of the triple bond in (23). If the nucleophile is an enolate ion, a... 

You might think you could escape this problem by choosing the alternative disconnection 8, but this is not so. We have more choice here we can use the a3 synthon 7 with natural polarity, in real life an enone, but then we shall have to use the acyl anion equivalents 6 that we met in chapter 23. Reversing the polarity gives us the naturally polarised electrophile, an a1 synthon 9 represented by an acylating agent and the homoenolate, or d3 synthon, 10 with unnatural polarity. 

And we know what the reagent was (A). This is a d synthon. Aldehydes are naturally electrophilic at C3 (by conjugate addition) so to make a reagent with unnatural polarity umpolung , p. 798), the aldehyde must be protected. 

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