5
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The answer for the above multiple choice problem is option (C), from which I can understand that at some point in the mechanism the molecule must've undergone decarboxylation.

Also, I think the first step should be protonation of the epoxide oxygen. (No other choice, isn't that the most nucleophilic site?)

I'm not able to understand how the ring opening takes place (guessing it is via decarboxylation, but how?)

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You are correct about protonating the epoxide of the glycidic acid 1 (by some other molecule of glycidic acid). Ring opening gives stable, tertiary cation 3 and NOT cation 6, which is destabilized by the carbonyl group. Collapse of 3 gives CO2 and the enol 4 which tautomerizes to the aldehyde 5.

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  • $\begingroup$ Cool! Thanks a lot. Another small question, could you tell me about other 'unusual' or 'unexpected' epoxide ring openings that come to your mind? I prefer calling this one unusual because it's very rare (first time for me) to see an epoxide ring being opened via decarboxylation. $\endgroup$ – arya_stark Dec 2 '17 at 14:44
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    $\begingroup$ A similar issue is the reaction of some epoxides like 2,2-dimethyl oxirane with a Grignard reagent (RMgBr). The Lewis acid Mg++ coordinates with the epoxide and causes rearrangement transferring hydride to the tertiary site yielding aldehyde 5. It then reacts with the Grignard reagent to give a secondary alcohol. This rearrangement competes with direct additions of RMgBr to the epoxide. $\endgroup$ – user55119 Dec 2 '17 at 14:56
  • $\begingroup$ @user55119 Could you please explain how exactly 3 "collapse"d? I mean, how can we predict that a carbocation is going to collapse? And that too only by decaboxylation? $\endgroup$ – Gaurang Tandon Mar 24 '18 at 1:10
  • $\begingroup$ @GaurangTandon: Think of this as an E1 elimination with the loss of CO2. I don't know how general the procedure is but look at the uses of the glycidic esters from the Darzens condensation: organic-chemistry.org/namedreactions/darzens-reaction.shtm $\endgroup$ – user55119 Mar 24 '18 at 1:26
  • $\begingroup$ To be honest, the E1 elimination does still feel "weird" to me. I have only ever seen it in Hoffmann and Saytzeff elimination of halohydrocarbons. Considering I am a high schooler, I guess there's no better way to explain this to me, is there? $\endgroup$ – Gaurang Tandon Mar 24 '18 at 1:39

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