enter image description here

Okay so the first step is the use of Grignard. I know that Grignard does a 1,2 addition(generally) and the attack happens directly on the carbonyl,and I think HClO4 basically does protonation.Not sure about the role of NaOH. Also the final answer given is 3,5 dimethylphenol. The final product is devoid of the oxygen which is present in the ring. How should I approach this question?

  • $\begingroup$ I think you are correct that the HClO4 is merely a protonation reagent, don't know why that was chosen. The NaOH, I think, is to do an intramolecular aldol cyclisation. To get the product 3,5-dimethylphenol the Grignard cannot be adding 1,2. Try drawing tautomers of the starting material and see if Grignard addition to one of those makes a reasonable pathway. $\endgroup$ – Waylander Apr 5 '20 at 7:51
  • $\begingroup$ @Waylander I tried but i dont see an intramolecular aldol possible as the compund itself is a ring,i dont see how anintramol aldol can occur(its already cylised). A little more help would be nice. Thanks a lot! $\endgroup$ – booma vijay Apr 5 '20 at 10:14
  • $\begingroup$ The ring has to open. The MeMgCl attacks one of the carbons bonded to the oxygen in the ring giving an unsaturated dicarbonyl. The NaOH forms the anion which does the intramolecular aldol $\endgroup$ – Waylander Apr 5 '20 at 10:37
  • $\begingroup$ Yup! Got it after seeing your answer! Just one doubt. How did u know that the attack had to happen on a tautomer? Basicaaly u have done a 1,4 attack right? thx! $\endgroup$ – booma vijay Apr 5 '20 at 11:16
  • $\begingroup$ My feeling is that showing the tautomer makes it easier to understand why the attack is at that point. As you correctly commented, unmodified Grignards will usually attack 1,2. This is a special case. $\endgroup$ – Waylander Apr 5 '20 at 11:35

Consider the resonance structures of the pyranone. MeMgCl attacks the structure shown second. The perchloric acid workup hydrolyses the enol ether to give the diketone shown bottom right. NaOH promotes the internal aldol cyclisation to give 3,5-dimethylphenol. enter image description here

  • $\begingroup$ They're resonance structures, not tautomers - not that it detracts from the other steps $\endgroup$ – orthocresol Apr 5 '20 at 11:43
  • $\begingroup$ You are correct, I have amended my answer $\endgroup$ – Waylander Apr 5 '20 at 12:08

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