[Heory]

Right!
Go on, Ulfsaar. I believe that you can solve this problem.

[Ulfsaar]

ammmm
i just think a [3,3] rearrangement ……

[Heory]

ammmm
i just think a [3,3] rearrangement ……

imine is not stable under acidic condition.

[azmanam]

Had to break out my new "Strategic Applications"[1] book for this one.  Darzens followed by Eshweiler-Clark Cyclization[2].

[1] http://www.chemistry-blog.com/2009/10/12/book-review-strategic-applications-of-named-reactions-in-organic-synthesis/
[2] http://dx.doi.org/10.1021/jo01018a011
    http://dx.doi.org/10.1021/jo01348a006

[Heory]

Closer now. You meant that imine was destroyed after its formation. But when pH=3, imine cann't be formed, or its formation was overwhelmed by another named reaction. You had a right idea, but wrong way. Look for another way for the following process. Bonus: name it.(not Eshweiler-Clark)

[Heory]

Come on! Just have a try. You needn't post your answer only when you are quite certain that you are right.

[blind]

Something like this?

[Heory]

Something like this?

No. actually imine was not the intermidate of the reaction. After Darzen reaction, there's another named reaction, which coundn't be seem from your picture.

[Heory]

about azmanam’s latest answer:
The right idea was the nuleophilic attack on the aldehyde's carbonyl carbon, but the product was far from the right one. An important functional group was ignored in the reaction immediately after Darzen reaction.

we still have a long way to go to finish step 1 (pH=3), let alone come to the end of the whole reaction, and stereochemistry should be considerd. We have just completed about 1/5 of the whole work.

BTW, 48%HBr was just used to increase acidity of the reaction system. Br- didn't react.

[azmanam]

so something happens with the hemiaminal?  some intervention of the p-methoxyaryl group?  Surely CO₂ doesn't come back and react?

[Heory]

after Darzen reaction, only 3 functional groups existed and you always chose the wrong 2 groups of them to react at first. Choose another 2 groups to react first(a named reaction).

Sorry for my poor Enlish. If I'm not descriptive enough, please let me know.

[azmanam]

Ok.  Here we go:

At the point where we have the amine and the aldehyde (those should be the two dominant structures after the Darzens rxn)

Possible Nucleophiles (in what I feel are order of strength):
N atom of amine
O atom of water
benzyllic carbon of aldehyde due to enol character
the carbon atoms ortho and para to the OMe group on the benzene ring
distal carbon of alkene in amine (the monosubstitued side)
O atom of aldehyde

Possible Electrophiles (also in order of strength, imho):
H of H₃O⁺
carbonyl carbon of aldehyde
internal carbon atom of alkene of amine (the disubstituted side)
the carbon bearing the OMe(H⁺) group for a possible ipso substitution

One of those from each list have to react, right?  Is it the alkene and the aldehyde?

[Heory]

Yes! Name it and go on.

[azmanam]

Prins followed by hydride shift followed by 5-exo cyclization

[Heory]

yes, prins.
the hydride didn't shift in presence of WATER (large amount). BTW, why didn't you choose the methyl group to shift? In a former problem, when I treated the pinacol reaction with a hydide shift, you said only the alkyl group could meet the orbital overlap requirement.
after prins reaction, N was the right nuleophile but the epoxy group was a wrong electrophile.