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I started to do the problem by coordinating H+ with the lone pair of oxygen then after that I can't proceed .please send the solution by show the detailed mechanism.


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    $\begingroup$ Are you sure the carbonyl will be protonated when there's an imine group around? $\endgroup$ – William R. Ebenezer Apr 28 at 17:47
  • $\begingroup$ I think answer is (C), which is impossible to achieve under given conditions. $\endgroup$ – Mathew Mahindaratne Apr 28 at 20:37

There are two equally important path for the given reaction: (i) Via protonation of imine $\ce{N}$; and (ii) Via protonation of keto $\ce{O}$.

(i) Protonation of imine $\ce{N}$ would end up with 1,4-addition to give structure (A), which rearranges to structure (B) via keto-enol tautomerism.

(ii) It is also possible that Protonation can be undergone through keto $\ce{O}$, which would end up with 1,4-addition followed by keto-enol tautomerism to give structure (D). See the depicted arrow-pushing mechanism below:

possible products

To get structure (C), it need to have 1,6-addition, which is impossible. Therefore, the answer is (C).

  • $\begingroup$ Did you mean, therefore the answer is (B)? $\endgroup$ – user55119 Apr 30 at 1:47
  • $\begingroup$ @user55119: I meant answer is (C). Asking is "which is X cannot be?"! $\endgroup$ – Mathew Mahindaratne Apr 30 at 15:42

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