Protecting group
From Freepedia
A Protecting group or protective group is introduced into a molecule by chemical modification of a functional group in order to obtain chemoselectivity in a subsequent chemical reaction. It plays an important role in multistep organic synthesis.
In many preparations of delicate organic compounds, some specific parts of their molecules cannot survive the required highly reactive reagents and harsh chemical environments (extreme example: a boiling acid bath). Then, these parts, or groups, must be protected. For example, lithium aluminum hydride is a highly reactive but useful reagent capable of reducing esters to alcohols. It will always react with carbonyl groups, and this cannot be discouraged by any means. When a reduction of an ester is required in the presence of a carbonyl, the attack of the hydride on the carbonyl has to be prevented. For example, the carbonyl is converted into an acetal, which does not react with hydrides. The acetal is then called a protecting group for the carbonyl. After the step involving the hydride is complete, the acetal is removed (by reacting it with an aqueous acid), giving back the original carbonyl. This step is called deprotection.
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Common protecting groups
Alcohol protecting groups
- Tetrahydropyranyl ether (THP) - Removed by acid.
- Methoxymethyl ether (MOM) - Removed by acid.
- β-Methoxyethoxymethyl ether (MEM) - Removed by acid.
- p-Methoxybenzyl ether (PMB) [1] - Removed by acid or hydrogenolysis.
- Methylthiomethyl ether - Removed by acid.
- Silyl ether (most popular ones include trimethylsilyl (TMS), tert-butyldimethylsilyl (TBDMS), and triisopropylsilyl (TIPS) ethers) - Removed by acid or fluoride ion.
Amine protecting groups
- Carbobenzyloxy group (Cbz) - Removed by hydrogenolysis
- tert-Butyloxycarbonyl (BOC) (Common in solid phase peptide synthesis) - Removed by acid.
- 9-Fluorenylmethyloxycarbonyl (FMOC) (Common in solid phase peptide synthesis) - Removed by base.
Carbonyl protecting groups
- Acetal - Removed by acid.
