[Fzang]

I'm given a 13C spectrum with 10 peaks for an ester which has undergone intramolecular Claisen condensation.

This condensation could have happened in two ways, where one of them kicks off EtOH as a leaving group, while the other one keeps all 10 carbons.



Now, my problem (if I've done above right), is why the 13C shows 2 peaks around 200. One of them is the carbonyl obviously, but I have no idea what other carbon could near 200 ppm? Help please.

[Doc Oc]

That mechanism isn't right.

1) That's not a Claisen
2) 4-membered rings are VERY unfavored.

Go study up on what a Claisen is and see if you can figure out what's happening here.  I don't think it matters, but if it helps you do some searching, intramolecular Claisens are known as Dieckmann cyclizations.

[Fzang]

Simply following the mechanism, when the nucleophilic deprotonated alpha-carbon attacks the electrophilic carbonyl carbon at the other end.

http://en.wikipedia.org/wiki/Claisen_condensation

Call me deluded but I can't how else the structure in question could undergo "intramolecular condensation", without losing any carbons.

Also, I drew this. Am I even more wrong than before? (Except that step 2/3 needs tidying)

[Doc Oc]

You're not counting the solvent peaks, are you?  If there's 3 peaks at 77 ppm those are from the CDCl₃, not your product.

[Fzang]

Nope. I'm specifically asked to explain the reason for 10 peaks.

[Doc Oc]

And are there 3 equal size peaks at 77 ppm?

[Fzang]

13C


ppm are 203.66; 191.40; 103.10; 57.26; 54.13; 46.31; 32.76; 30.92; 28.32; 28.24

Some are very close.

[Doc Oc]

I just drew the product and I think I figured out what's going on, this is a tricky question.

I won't give you the answer, but carbons ARE lost in the Claisen.  You see multiple peaks arising from something else (related to pKa).

[Fzang]

Okay, different approach.



6-membered ring, and it throws off EtOH. But the spectrum confuses me even more. This structure is symmetrical. Where do all these signals come from?

[Doc Oc]

Yes, that's true.  Look at that alpha carbon between the two carbonyl carbons.  The protons are very acidic.  What could happen?

[Fzang]

One of the protons will dissociate, thus forming a resonance stabilized enolate.

In the text it says that the compounds are treated with NaOEt, then EtOH and finally H+

What would you use the nucleophile EtOH for? Acetals? That would eliminate at least one carbonyl, which doesn't correspond to the 200 ppm peak then.

So lost.

[Doc Oc]

You're on the right track.  That proton is so acidic the compound actually more stable as an enol tautomer rather than the diketone.  So now can you rationalize the C-NMR?

http://en.wikipedia.org/wiki/Diketone

[Fzang]

So is this giving 10 tops because it interconverts between keto/enol tautomer, which essentially gives another two kinds of carbon atoms which are there "briefly"?

[Doc Oc]

Exactly right.  See?  Tricky.

[Fzang]

Damn.