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I am having trouble understanding how the protecting group (ethylene glycol) is applied to the alkene next to the ether of highlighted step above. I thought maybe TsOH protonates the alkene as seen in the mechanism below so that it forms a carbonyl but it doesn’t seem right as a carbocation should be found on the tertiary carbon. I have no idea if I am going about this correctly, any suggestions? enter image description here

  • 2
    $\begingroup$ Tosic acid just protonates to form an oxocarbenium. Go straight from 1 to 3 without intermediate 2 (that's just a resonance for 3). From here, you should add one of the alcohols of the ethylene glycol into the activated carbonyl. Proton transfer to the methoxy and eliminate. Then add the other end. Regenerate acid. $\endgroup$
    – Zhe
    Commented Jun 7, 2017 at 3:21

1 Answer 1


Steps (Source: from comment made by Zhe)

  1. Tosic acid protonates alkene to form oxocarbenium through resonance.

  2. One of the alcohols from ethylene glycol attacks the activated carbonyl and toluenesulfonate deprotonates the oxygen.

  3. Methoxy group is protonated by the regenerated torsic acid.
  4. Methanol is eliminated via SN2 with the other hydroxyl group of the ethylene glycol.
  5. Tosic acid is regenerated by deprotonating the 1,3-dioxolane group.enter image description here

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