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I am trying to understand the stereochemistry of iodolactonisation of this cyclohexene. I can't work out if the iodonium ion would form on top or the bottom.

Reaction scheme

There is $\ce{CH2COO-}$ and $\ce{Me}$ pointing up, and $\ce{CH2OTBS}$ and $\ce{Me}$ pointing down. My guess would be iodine attacks from the top because there is a bulky $\ce{TBS}$ group pointing down as well as a methyl, however there is a semi-big $\ce{CH2COO-}$ group pointing up right next to the double bond. Would that have more of an effect as it is nearer to the double bond?

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Here, it does not matter which face is preferred. The formation of the iodonium ion is reversible and it can be formed on both faces. However, the iodonium ion can only be attacked by the carboxylate if it is formed on the opposite face from the carboxylate, which is the bottom face in your diagram. Attack on the top face is said to be unproductive because it cannot lead to iodolactonisation.

mechanism of iodolactonisation

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