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I was told to propose a mechanism for the following conversion:

Reaction

One obvious step is the conversion of cyclic ketal to ketone because of acidic conditions. But I am unable to understand how the three membered ring opened to give an alkene? (However, I can understand that the three membered ring is unstable)

I feel that somehow, a carbocation is being generated and E1 type elimination is taking place.

What is happening exactly? And does this type of thing take place with any three membered ring under given conditions?

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  • $\begingroup$ Break the three mbered ring to form an enol. Rearrange the carbocation and heat it to form an alkene in the mean while your enol tautomerizes to a ketone in acidic medium. Havent the solutions been released along with the paper though? $\endgroup$ – Avnish Kabaj Feb 4 at 18:22
  • $\begingroup$ @AvnishKabaj No solution was given with the paper. $\endgroup$ – Akshat Joshi Feb 5 at 1:15
  • $\begingroup$ Since you get hydrolysis of the ketal, you should list the hydronium ion over the arrow. With just a proton as a catalyst, the ring cleavage can occur with the retention of the ketal. Temporary opening of the ketal to the enol ether and protonation of the ketone causes fragmentation. The ketal can then reform. $\endgroup$ – user55119 Feb 5 at 2:26
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Possible mechanism for cyclopropyl ring opening of given compound in acid medium:

Possible mechanism for cyclopropyl ring opening

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  • $\begingroup$ Is there any name to the step in which the alkene attacks to open the cyclopropyl ring? Is it related to Neighbouring Group Participation? $\endgroup$ – Akshat Joshi Feb 5 at 1:26
  • $\begingroup$ Not really to my understanding. But, you see when the first enol formed, it was conjugated all the way to the other ketone (you can write few resonance to see how stabilizing it is). $\endgroup$ – Mathew Mahindaratne Feb 5 at 2:59

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