Acylation at carbon

Background Needed for this Chapter References to Clayden, Organic Chemistry Chapter 27 The Aldol Reaction Chapter 28 Acylation at Carbon. 

Chapter 27 The Aldol Reaction Chapter 28 Acylation at Carbon. 

The final product has been formed by the acylation at carbon of the enolate of an ester. This general process—acylation at carbon—is the subject of this chapter. I, 50 happened in this case that the... 

Only the simple ester can give an enolate, and the low concentration of this enolate reacts preferentially with the more electrophilic diethyl oxalate in a typical acylation at carbon. 

If we consider all three possible enolate anions, the choice is more easily made. First, the reaction that does happen. An enolate anion is formed from the ketone at the green site and acylation at carbon follows,... 

Combining the enamine with the acid chloride led to a clean acylation at carbon in 82% yield and eventually to a successful synthesis of the natural product longifolene. 

We should not leave the subject of acylation at carbon without considering a problem that affects all such reactions to some degree. It can be understood most easily if we imagine some functional group Z that is able to stabilize a carbanion, and the acylation of that car-banion with an acid chloride—something like this. 

Combining the two reagents at neutral pH gives clean specific acylation at carbon. This is very like the biological reaction as CO2 is lost during acylation. 

Attempted acylation at carbon often fails. What would be the true products of these attempted acylations, and how would you actually make the target molecules ... 

A different cyclization leads to the flavones and anthocyanidins. Reaction of the stable enol from a 1,3-diketone with the thiol ester as electrophile results in acylation at carbon in the manner of the Claisen ester condensation (Chapter 28) with loss of CoASH and the formation of a trihydroxyben-zene ring. 

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