[Neur0toxic]

Hi, I'm in my first year of organic chemistry study and have just encountered the michael addition. I performed a synthesis of dimedone using mesityl oxide with diethyl malonate, and am now trying to get my head around the mechanism for this reaction. I understand that the base (EtO^-) removes a proton from the alpha carbon of the diethyl malonate, where the enolate ion then attacks the beta carbon of the mesityl oxide. The alpha carbon is the protonated by the solvent and then potential ring formation to form the dimedone. The problem I am having is that I don't understand how the ester groups are removed from the structure before or after the ring formation. I have searched through my textbooks and through google, and all I have gathered is that when using ester groups, if the same group is used as a solvent eg. EtO^- in my case, a nucleophilic reaction can occur between them? (for structures, see below)

If someone could please structure my thoughts on this or lead me in the right direction, it would be very appreciated.



Dimedone: http://upload.wikimedia.org/wikipedia/commons/4/4d/Dimedone.png
Diethyl malonate: http://upload.wikimedia.org/wikipedia/commons/6/69/Diethyl-malonate.png
Mesityl oxide:http://www.sigmaaldrich.com/thumb/structureimages/00/mfcd00008900.gif

[Arctic-Nation]

For a first-year student your problem can indeed be rather daunting, as it requires the combination of several reaction types.

Your first step, Michael addition between enolate and unsaturated ketone is correct. For the second step, I guess you have seen the wiki page on mesityl oxide, which shows what happens next. In your case, the enol attacks one of the ester groups, after which ethoxide is expelled. This gives you the six-membered ring and the second ketone.

The final step, removal of the remaining ester function, doesn't happen in situ (I think, I'm a bit rusty on this), but if you know what a beta-keto-acid is and what it does, you should be able to work it out.

As for the use of ethoxide as base, this is just to eliminate any transesterification reactions. Strong non-nucleophilic bases (tertiary amines and such) can also be used, depending on the exact nature of the reaction. What to use when is something you'll learn as you go along.