Editing Protecting group (section)

=== Cleavage categorization ===
Many reaction conditions have been established that will cleave protecting groups. One can roughly distinguish between the following environments:<ref>Michael Schelhaas, Herbert Waldmann: "Schutzgruppenstrategien in der organischen Synthese", in: ''[[Angewandte Chemie]]'', '''1996''', ''103'', pp.&nbsp;2195–2200; [[doi:10.1002/ange.19961081805]] (in German).</ref>

* [[Acid]]-labile protecting groups
* [[Base (chem)|Base]]-labile protecting groups
* [[Fluoride]]-labile protecting groups
* [[Enzyme]]-labile protecting groups
* [[Reduction (chem)|Reduction]]-labile protecting groups
* [[Oxidation]]-labile protecting groups
* Protecting groups cleaved by heavy metal salts or their complexes.
* [[Photochemistry|Photo]]labile protecting groups
* Double-layered protecting groups

Various groups are cleaved in acid or base conditions, but the others are more unusual.

Fluoride ions form very strong bonds to [[silicon]]; thus silicon protecting groups are almost invariably removed by fluoride ions.  Each type of counterion, i.e. cleavage reagent, can also selectively cleave different silicon protecting groups depending on [[steric hindrance]]. The advantage of fluoride-labile protecting groups is that no other protecting group is attacked by the cleavage conditions.

[[Lipase]]s and other enzymes cleave ethers at [[biological pH]] (5-9) and [[Body temperature|temperatures]] (30–40&nbsp;°C).  Because enzymes have very high substrate specificity, the method is quite rare, but extremely attractive.

Catalytic [[hydrogenation]] removes a wide variety of [[benzyl group]]s: ethers, esters, urethanes, carbonates, etc.

Only a few protecting groups can be detached oxidatively: the methoxybenzyl ethers, which oxidize to a [[quinomethide]].  They can be removed with [[ceric ammonium nitrate]] (CAN) or [[dichlorodicyanobenzoquinone]] (DDQ).
[[File:PMB_deprotection.svg|center|frameless|440x440px]]
[[File:PMB_deprotection_mechanismn.svg|center|frameless|440x440px]]
[[allyl group|Allyl compounds]] will [[Isomerization|isomerize]] to a vinyl group in the presence of [[noble metal]]s.  The residual [[enol ether]] (from a protected alcohol) or [[enamine]] (resp. amine) hydrolyzes in light acid.

Photolabile protecting groups bear a [[chromophore]], which is activated through radiation with an appropriate wavelength and so can be removed.<ref>V.N. Rajasekharan Pillai: "Photoremovable Protecting Groups in Organic Synthesis", in: ''[[Synthesis (journal)|Synthesis]]'', '''1980''', pp.&nbsp;1–26.</ref>  For examples the ''o''-nitrobenzylgroup ought be listed here.
[[File:Photodeprotection_is.svg|center|alt=Mechanism of photodeprotection of an o-nitrobenzyl ether and formation of an alcohol|frameless|440x440px]]
The rare double-layer protecting group is a protected protecting group, which exemplify high stability.