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A Quick Recap on Protecting Groups

Three Steps Required: “Hide” the incompatible group Do what needs to be done elsewhere in the molecule Remove the protecting group and return to the original group. A Quick Recap on Protecting Groups. Two protecting groups for alcohols:.

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A Quick Recap on Protecting Groups

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  1. Three Steps Required: • “Hide” the incompatible group • Do what needs to be done elsewhere in the molecule • Remove the protecting group and return to the original group A Quick Recap on Protecting Groups

  2. Two protecting groups for alcohols: • Trimethylsilyl ether formation using trimethylsilyl chloride (TMS-Cl) and triethylamine (NEt3) • Easily removed by aqueous acid (H3O+) or F-

  3. Two protecting groups for alcohols: • Dihydropyran (DHP) and an acid catalyst (H+) will react to form a tetrahydropyran protecting group (THP) • Easily removed by aqueous acid (H3O+)

  4. For more than one alcohol… … remember that there is an order in which alcohols are protected, beginning with primary alcohols (least sterically hindered), then secondary alcohols (more sterically hindered) and finally tertiary alcohols

  5. In most cases, when working with a single alcohol that needs to be protected, the two groups are INTERCHANGEABLE so it doesn’t matter which ones you use…

  6. Sometimes you will need to manipulate two alcohols… if you need to protect them both so they are both out of the way, use the same one…

  7. Remember the order for protecting alcohols is always going to show reaction on the 1°, then 2° and then 3°: Thus:

  8. Those problems that require you to protect the less hindered alcohol first are easier than those where the less hindered alcohol is the one you NEED for the chemistry… Example: Quick and simple to protect the primary alcohol so the secondary can be oxidized, right?

  9. Right!

  10. When the the primary alcohol is the one you NEED for the chemistry, you will have to actually protect it FIRST and then use a different protecting group on the secondary alcohol… and deprotect the first alcohol so you can do your chemistry on it… • Be sure to use two protecting groups that can be taken off with different reagents

  11. For alcohols, you have TWO protecting groups to use - one that comes off with aqueous acid and one that comes off with aqueous acid OR fluoride anion… • Just be sure not to START with the one that has to be removed using the SAME reagent as the second one…

  12. Using TMS-Cl, NEt3 first and then DHP, H+ allows you to remove the TMS group easily with F-, thus allowing the THP to stay in place:

  13. Using DHP, H+ first and then TMS-Cl, NEt3 won’t allow you to remove the THP group easily without also removing the TMS group:

  14. And hopefully you recognize that if you put the same protecting group on both alcohols, you cannot selectively REMOVE one over the other:

  15. This concludes our review of alcohol protecting groups. Hopefully it helped eliminate any questions that may have been left in your mind after lecture.

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