Editing Protecting group (section)

{{Short description|Group of atoms introduced into a compound to prevent subsequent reactions}}
{{Use dmy dates|date=January 2024}}
[[File:Acetal-protection-example.png|thumb|400px|right|[[Ethylene glycol]] protects a [[ketone]] (as an [[acetal]]) during an [[ester]] reduction, vs. unprotected reduction to a diol]]
A '''protecting group''' or '''protective group''' is introduced into a molecule by chemical modification of a [[functional group]] to obtain [[chemoselectivity]] in a subsequent chemical reaction. It plays an important role in [[multistep synthesis|multistep organic synthesis]].<ref>{{cite book|title=Protecting Groups in Organic Synthesis|author=Theodora W. Greene |author2=Peter G. M. Wuts |publisher=J. Wiley|edition=3|year=1999|isbn=978-0-471-16019-9}}</ref>

In many preparations of delicate [[organic compound]]s, specific parts of the molecules cannot survive the required reagents or chemical environments. These parts (functional groups) must be '''protected'''. For example, [[lithium aluminium hydride]] is a highly reactive reagent that usefully reduces [[ester]]s to [[Alcohol (chemistry)|alcohol]]s. It always reacts with [[carbonyl]] groups, and cannot be discouraged by any means. When an ester must be reduced in the presence of a carbonyl, hydride attack on the carbonyl must be prevented. One way to do so converts the carbonyl into an [[acetal]], which does not react with hydrides. The acetal is then called a '''protecting group''' for the carbonyl. After the hydride step is complete, aqueous acid removes the acetal, restoring the carbonyl. This step is called '''deprotection'''.

Protecting groups are more common in small-scale laboratory work and initial development than in industrial production because they add additional steps and material costs. However, compounds with repetitive functional groups – generally, [[biomolecule]]s like [[peptide]]s, [[oligosaccharide]]s or [[nucleotide]]s – may require protecting groups to order their assembly.  Also, cheap [[Chiral auxiliary|chiral protecting groups]] may often ''shorten'' an [[enantioselective synthesis]] (e.g. [[shikimic acid]] for [[oseltamivir]]).

As a rule, the introduction of a protecting group is straightforward.  The difficulties honestly lie in their stability and in selective removal.  Apparent problems in synthesis strategies with protecting groups are rarely documented in the academic literature.<ref>Michael Schelhaas, Herbert Waldmann: "Schutzgruppenstrategien in der organischen Synthese", in: ''[[Angewandte Chemie]]'', '''1996''', ''103'', pp.&nbsp;2194; [[doi:10.1002/ange.19961081805]] (in German).</ref>