Protecting Group Strategy: Orthogonality and the Order of Removal
On a molecule with three reactive groups, protection stops being a single decision and becomes a schedule. Orthogonal sets can be removed in any order under conditions that leave the others untouched, which is what lets a long route stay flexible.
What a learner can do afterwards
- Selects an orthogonal pair of protecting groups for two hydroxyls that must be freed at different points
- Reads a deprotection condition and says which other groups in the molecule would not survive it
- Places protection and deprotection steps in a route so that no step is wasted
- Explains what a global deprotection at the end costs in yield and why it is still often chosen
1 · Read
You mask reactive groups so the rest of the route survives. Hydroxyls react with nearly every strong reagent, so you cap them as ethers or esters to shut down their acidity until the route is safe. Carbonyls get masked as acetals that shrug off bases, hydrides, and organometallics, then open again in aqueous acid.
You play the orthogonality game when two sites must free at different points. A pair counts as orthogonal only when each strip recipe was checked to leave the other mask untouched, like one removed by hydrogen gas and one by mild acid. Each mask survives the middle steps and removes cleanly at its own moment.
Take a five-step plan as given: neutral steps with brief mild acid exposures and one longer mild aqueous acid treatment. The ether was checked to survive the brief exposures and leave in the longer one; ester and amide masks stood both. Mask amine, alcohol, acid as amide, ether, ester. The longer treatment frees the alcohol. Mild aqueous base opens the ester while the amide stands; acidic workup frees the acid. Stronger heated aqueous acid last removes the amide, leaving the amine protonated.
Free the alcohol, open the ester, then unmask the amine.
You place each strip step where nothing else gets hurt, using stability tables that say which ethers survive acid, base, or oxidation. A global deprotection fells every shared mask in one wash, which saves steps but costs any group you hoped to keep. Count that trade only for groups the route can spare or restore; a group the target requires must survive or come back.
You mask early, you pair checked orthogonal strips, you order removal by stability, and you weigh global speed against yield for groups you can spare.
Rewritten readings were written for this dataset.
2 · Watch
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Where it sits
Learn first
Nothing; this is a starting point.
Optional connections
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.