[gpli]

Hi folks,

I am having a hard time figuring out the mechanism of the following reaction. Can anybody help me here? Great thanks.

[OrganicDan96]

could you show an attempt before we help you?

[kriggy]

Its quite insterseting problem, probably involves some C-C bond cleavege which feels unlikely under the conditions

[gpli]

could you show an attempt before we help you?

I only know the mechanism of the last decarboxylation step. It is an acid-assisted 6-member ring process.

[rolnor]

Can it be something in the scheme thats wrong? Maybe it should be diethylmalonate in the first step?

[hollytara]

Make the enolate between the two carbonyls

This attacks the anhydride to start....

[rolnor]

And then hollytara, what happens next?

[hollytara]

When you make an imide from a cyclic anhydride, you take an amine and make the amic acid - one half of ahydride is amide, the other half is carboxylic acid.  This is treated with acetic anhydride to re-cyclize to the imide - acetic anhydride acts to "dehydrate" and remove the elements of water from the amic acid. 

This is the same, but we are replacing the amine nucleophile with a carbon nucleophile that is based on enolate chemistry. 

[rolnor]

But you need to loose a acetyl group in the first step?

[clarkstill]

Presumably at some point (maybe the final step) there's a retro-Claisen condensation to lose the acetyl? That should be a favourable reaction, since the leaving group is an anion stabilized by the other three carbonyl groups. Not sure what the nucleophile would be, maybe acetate?

[rolnor]

Yes, it has to be something exotic....

[pgk]

Please, take a look at the attached file, below:

[rolnor]

Thanks pgk, that really was something exotic, nice!

[hollytara]

Had time to review old papers.... 

If this follows from the imidization reactions: 

Make enolate, this attacks anhydride.  Result is first product shown by pgk in attached file.

The acetic anhydride reacts with the carboxylate by anhydride exchange: the new carboxylate formed in the first product is now a mixed anhydride with an acetate group, and the other piece of acetic anhydride exits as the acetate ion. 

An enolate is formed again between the three carbonyls - this displaces the acetate of the mixed anhydride to re-form the 5 membered ring. 

So now the product is a 1,3-cyclopentanedione with a 2-acetyl group and a 2-carboxyethyl group. 

This does the retro-Claisen as suggested by clarkstill. 

The second reaction is acid hydrolysis to the acid, which (like acetoacetic acid) readily decarboxylates. 

[clarkstill]

Please, take a look at the attached file, below:

Presumably you meant to show nucleophilic enolates rather than hydride attacks in your mechanism? Regardless, step 2 is not plausible.