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I have recently come across a paper from here.I have tried to workout the mechanism for the following reaction presented in the paper.

enter image description here

I was able to workout mechanism till acetal formation.It is a ring expansion from six to seven. However,the next step is a ring reduction from seven to five upon heating.

Can someone please explain what is actually happening during ring reduction, and why it happens?

enter image description here

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It's in scheme 3 of the paper you cited! The 7-membered acetal undergoes ring-opening to the α,β-unsaturated oxocarbenium ion, then conjugate addition to give the 5-membered tetrahydrofuran. A bit of arrow pushing and addition of solvent gives the other THF product

Not sure the legality of posting screenshots of papers, so I redrew the scheme:

enter image description here

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  • $\begingroup$ @ PCK I hit a pay wall and could not open this paper. $\endgroup$ – Chakravarthy Kalyan Jun 10 at 12:37
  • $\begingroup$ +1 for your prompt reply . $\endgroup$ – Chakravarthy Kalyan Jun 10 at 14:16
  • $\begingroup$ @ PCK it is obvious, after protonation ,C-O bond breaks,followed by 1,4 addition, just great. $\endgroup$ – Chakravarthy Kalyan Jun 10 at 14:23

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