I came across this question recently,
I tried to devise a mechanism but I'm struck here, My try -
Moreover, I think my mechanism is somewhat inefficient because of formation of some byproducts, which we don't want.
In short in my mechanism,
First we convert succinic acid to its ester (by esterification using ethanol).
Then, we use this ester to react with acetaldehyde in basic medium (dilute NaOH), intramolecular magic occurs to yield an ESTER of the desired product.
Now, here's where i'm struck, how do I convert ester of the desired product into acid, Can I do Hydrolysis, but I think then the ring might get cleaved!
Moreover, I think during this reaction, some acetaldehyde may undergo self condensation to yield a byproduct, this may be true for ester of succinic acid.
Any help will be appreciated!