Protecting group
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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 both a reduction of an ester and a carbonyl is required in the end product, 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.
Common protecting groups
- for an alcohol
- tetrahydropyranyl ether or THP group
- methoxymethyl group or MOM group or beta-methoxyethoxymethyl group or MEM group
- p-methoxybenzyl (PMB) group [1] (http://www.chem.brown.edu/faculty/basu/PMB.pdf)
- silyl ether
- for a carbonyl group
- an Acetal
- for an amine group
- carbobenzyloxy group