A protecting group temporarily changes a reactive functional group so that another transformation can proceed selectively. It is installed, survives the desired intervening conditions, and is later removed to regenerate the functionality.
For example, converting an alcohol into a suitable protected derivative can suppress its reaction during a different bond-forming step. Protection is useful only if the installation and removal are sufficiently selective; a group stable under one set of conditions can be unsuitable under another.
Cleavage of a silicon–oxygen protecting group to reveal an alcohol. Fluoride can drive the transformation through strong silicon–fluorine bond formation.
A cyclic acetal protecting group made from an alcohol and dihydropyran. It protects the original alcohol oxygen and can be removed under suitable acidic hydrolysis conditions.
A tert-butoxycarbonyl group used to protect an amine as a carbamate. Acid-promoted cleavage releases carbon dioxide and restores the amine.
A functional group that can later be converted into a carbonyl at a mapped carbon. A secondary nitro group is a useful example when it also enables carbon–carbon bond formation before unmasking.
Use of ester protecting groups that can be selectively removed under different conditions. A methyl/tert-butyl pair can permit kinetic base hydrolysis at one end and acid-promoted alkyl cleavage at the other.
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