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When looking at Stetter's reaction (umgepolter aldehyde + michael akzeptor -> 1,4-dicarbonyle), I was wondering wether the occuring intermediate, after the umpolung step, is aromatic. I can imagine that the resonance structure on the right exhibits aromaticity, but does the left one do so as well due to the double bond? I assume into a set of resonance structures aromaticity cannot just "dissapear", but I can not explain how the the pi-electrons behave in that structure

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    $\begingroup$ What is "umgepolter", "michael akzeptor", "umpolung"? $\endgroup$ Commented Mar 15 at 17:19
  • $\begingroup$ there are alot of typo $\endgroup$
    – Anjankumar
    Commented Mar 20 at 7:59

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Aromaticity can indeed "disappear" between one contributing resonance structure and another when the conjugated pi-electron structure extends beyond the ring. That's because you don't have just an aromatic ring.

What happens is that with a favorable total count of pi electrons (which need not follow the 4n+2 rule when you don't have just a ring) and in some cases bond polarities, the delocalization and extra stabilization extend beyond the ring. In effect ring aromaticity is superseded by this extended delocalization. The compound in the question offers one example.

Such "extended aromatic" systems are commonplace in organic chemistry. The most often cited laboratory examples are cyclopropenone and tropone (in which a pendant oxygen is attached to a cyclopropenyl or cycloheptatrienyl ring and the polarity of the carbon-oxygen bond favors forming an aromatic-cation contributing structure); but many natural organic compounds exhibit this characteristic, including the caffeine in your coffee or tea.

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