The solution to this reaction sequence given was,

enter image description here

What I don't understand is the last step. How do we know which double bond will react to form an epoxide? My guess was that the double bond adjacent to the methyl group should react as it has a more double bond character than the other one which is in resonance with the lone pair of oxygen.

Any help would be appreciated.

  • 3
    $\begingroup$ mCPBA is electrophilic, so electron-rich alkenes react first -- related: chemistry.stackexchange.com/q/116760/16683 $\endgroup$
    – orthocresol
    Mar 27 '20 at 5:02
  • 1
    $\begingroup$ The epoxide shown may well be unstable in the presence of the by-product m-chlorobenzoic acid and rearrange to an alpha-methoxycyclohexenone. $\endgroup$
    – user55119
    Mar 27 '20 at 15:17
  • $\begingroup$ @Tony: how do you explain the first step? $\endgroup$ Mar 27 '20 at 18:26

When two or more double bond are present then the more electron rich double bond will epoxidise. In above example the double bond containing the methyl group has electron donating inductive effect while the double bond containing the methoxy group will have an electron donating resonance effect. The resonance effect is dominant over the inductive effect. Therefore the double bond containing the methoxy group is more electron rich. Therefore epoxidation occur on it.


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