[pecan66]

Dear all,

I have come across a particular question which I believe involves an aldol condensation but I'm not able to rationalise how and why the product is formed.

The reaction is below:
            

The reactants are:
1. NaOMe
2.
3. NaOH
4. H⁺, Heat

My attempt involved a formation of the enolate and then a direct addition onto the aldehyde. I then used an E1cb reaction to lose the OH but then become stuck as there are so many carbonyls and possible routes that the reaction can take.

Help would be much appreciated,

Thanks.

[Dan]

Funny, I actually did this reaction in the lab quite recently.

You're basically half way there. There are a number of plausible pathways differing in the order of steps. Let's go with what you have so far (a Knoevenagel condensation).

Think about that intermediate and remember that intramolecular reactions are generally faster than intermolecular reactions...

[pecan66]

Ok so after the E1cb reaction has occurred I end up with this intermediate:



Now this intermediate contains an alpha-beta unsaturated carbonyl which is electrophilic. However only direct addition (intramolecular aldol reaction) could give a 5 membered ring but there are still not enough carbon atoms to form the product?

[Dan]

Go ahead and make a 5-membered ring.

Re: Not enough carbons, the stoichiometry is 2:1 ketodiester:glyoxal.