Reversible Dieckmann reaction

Reverse Dieckmann reaction. 1,3-Diketones of type (1), prepared as shown, can be converted into 11-membered lactones (2) by an intramolecular reverse Dieckmann reaction. ... 

The reverse Claisen reaction is common, especially with cyclic P-ketoesters, such as one gets from the Dieckmann reaction (see Section 10.8). If one only wants to hydrolyse the ester, it thus becomes necessary to use the rather less effective acid-catalysed hydrolysis method (see Section 7.9.2). 

The reversibility of the process means that cyclic 2-alkoxycarbonyl ketones can be cleaved by alkoxide in the retro-Dieckmann reaction, a process of synthetic significance (see Section 3.6.9.1). More recently, nonequilibrium conditions have been employed in the reaction and this may lead to a change in regioselectivity (see Section 3.6.3.3.3). 

The two-stage reaction involves a reverse Dieckmann condensation followed by a forward Dieckmaim using a different enolate. The original enolate cannot lead back to the starting... 

Figure I 1.2 Example of a disconnection or transformation of the structure on the left, affording the synthon on the right. The reverse of this transform (induced by base) is the Dieckmann condensation reaction.

The equilibrium character of the reactions described in this chapter means that the reverse processes also occur. The retro-Claisen, retro-Dieckmann and retro-Thorpe-Ziegler reactions are all known and can be synthetically useful. In some cases the substrate may have been formed by the same or similar forward reaction, whereas in others the substrate has to be formed by another process since the equilibrium lies completely on the side of the retro-reaction product. As in the case of the forward reaction, tandem reaction sequences can be constructed, often with the same types of preceding reaction as were described for the forward sequence. 

There are few mechanistic studies for the Dieckmann condensation. " The reaction can simply be described as an intramolecular Claisen condensation and hence entirely reversible. The summation of the consensus for its mechanism is as follows the reaction is initiated by anion formation through the abstraction of the most acidic proton on the a-carbon to one of the di-... 

There are several variations of the Claisen condensation. The intramolecular variation is called the Dieckmann condensation. A crossed Claisen condensation is possible between two different esters, but this reaction can lead to multiple products. Claisen condensations can operate in the forward direction or in the reverse direction, and these two processes lead to the construction and deconstruction of fatty acids. 

Enolates also add to esters.The biologically important Claisen condensation is the standard reaction of this kind. Variations of the Claisen condensation include the Dieckmann condensation and the reverse Claisen condensation. 

Claisen and Dieckmann condensations are reversible. The driving force is the deprotonation of the alpha hydrogen in the product between the two carbonyls. This deprotonation prevents the reaction from reversing, pulling the product out of the equilbrium. 

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