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?
Steps (Source: from comment made by Zhe)
Tosic acid protonates alkene to form oxocarbenium through resonance.
One of the alcohols from ethylene glycol attacks the activated carbonyl and toluenesulfonate deprotonates the oxygen.
- Methoxy group is protonated by the regenerated torsic acid.
- Methanol is eliminated via SN2 with the other hydroxyl group of the ethylene glycol.
- Tosic acid is regenerated by deprotonating the 1,3-dioxolane group.