Protecting group

A protecting group (english protecting group - therefore often referred to as common abbreviation in the formula schemes PG) is in chemistry a substituent is introduced during a complicated, multi-step chemical synthesis in a molecule to protect a particular functional group temporarily and as an adverse reaction prevent. After carrying out the desired reaction at another site in the molecule, the protecting group is split off again. For many functional groups several possible protecting groups are known, which differ in their stability and the conditions for their elimination.

In the synthesis of specific compound classes with repeating functional groups - usually these are biomolecules such as peptides, oligosaccharides or nucleotides - have standard rates established at protecting groups. Protection groups have become an important tool in the synthesis of complex molecules.

The requirements for a protection group are quite high. This also means that they can be with a very good yield and specifically introduce a functional group and must be split off again as well under mild conditions. For both steps, the reaction conditions should be standardized. In addition, the protecting group must be stable under as many reaction conditions. If possible, the resulting reaction products should be easily separable, and, optimally, the protecting group reagent is also inexpensive. The wider the experience with a protecting group is, the better is the predictability of the reactivity of the protecting group.

History

The history of the protecting group technology is inextricably linked with the targeted use of different starting compounds for the synthesis of a target molecule. The early protective groups were based generally that the starting material was chosen so that a reactive functional group is blocked by a radical and thus was unreactive. For example, anisole were chosen instead of phenols or esters instead of free alcohol groups. Only with the emergence from the early 20th century directed synthesis of increasingly complex compounds, the protecting group technique was really important. From about 1960 began to invest significant amount of research in the chemistry of protecting groups. During this time, chemists began to synthesize more complex natural products. These are mainly the former works of the Nobel laureate Robert B. Woodward, Elias J. Corey and Albert Eschenmoser, who pioneered the synthesis of complex natural products.

Today, there are a variety of protective groups are summarized in monographs for their properties. There are alongside established protecting groups very many exotic protecting groups that have been developed only for a synthesis, or a fairly specialized field.

Requirements for a protection group

The insertion and removal of protecting groups do not represent productive reactions in a sequence of synthetic steps, their product is the desired final product of the synthesis in detail. Therefore, high demands in terms of price, yield and development expenses are provided for the reaction of protection group reactions often.

As the basic requirements for a good protective group, the following features have emerged:

  • The reagent must be commercially available and inexpensive or easy to produce
  • The protection group must be simple, specific, and introduced in high yields
  • It must be stable to the largest possible number of reaction conditions and workup and purification methods
  • You must be specific, highly selective, and cleaved in high yields. The conditions should be standardized.
  • You may not constitute a new stereocenter and no diastereotopes center
  • You should just be visible in NMR spectra and interfere as little as possible due to signal overlap

A very important aspect is the high selectivity of the fork, because it often have different functional groups are protected and deprotected independently. In the ideal case is only one of many protective groups affected by the spin-off process. The behavior of protecting groups in practice can be, especially if several different protecting groups are used in a molecule, not always correctly predict on the basis of literature. Therefore, must be made both for the insertion as well as for the splitting, considerable development work during synthesis, in some cases, despite a wealth of experience.

Orthogonality of the protecting groups

Orthogonality of the protecting groups means that at each use more protecting groups of different types, each protecting group and can split off in any order because of the different Abspaltreagenzien, without any of the other protecting groups is attacked. In the illustrated example, the protected amino acid, the benzyl ester of tyrosine can hydrogenolytically, the Fluorenylmethylenoxy group ( Fmoc) by base (such as piperidine ) and tert-butyl ether with the phenolic acids are cleaved (e.g., with trifluoroacetic acid).

A common example of this application is the Fmoc peptide synthesis, which has gained a great importance both in solution and on solid phase. The protecting groups in the solid phase synthesis have with respect to the reaction conditions such as reaction time, temperature, and reagents to be standardized so that they are performed by a machine, while yields of well over 99 % can be achieved, otherwise the separation of the resulting mixture of the reaction products is virtually impossible.

Fmoc solid-phase peptide synthesis using orthogonal protecting groups

Another important use of orthogonal protecting groups is a carbohydrate chemistry. Since carbohydrates have hydroxy groups with very similar reactivity, protection or deprotection of individual hydroxyl groups must be possible for a targeted synthetic implementation. A similar case, the synthesis of nucleotides dar. This has on the one hand, the problem ( as in the case of peptide synthesis ), that it is vector molecules. On the other hand this has the problem of carbohydrate chemistry, to the sugar moiety of the ribose in the synthesis of RNA molecules.

But also in the synthesis of complex natural products or agents with many functional groups, one is dependent on the orthogonality of protective groups.

Lability or removal of protective groups

In protecting groups different reaction conditions have been established corresponding to the orthogonality principle, be cleaved under which protecting groups. One can distinguish roughly between following cleavage conditions:

  • Acid labile protecting groups
  • Base labile protecting groups
  • Fluoride labile protecting groups
  • Enzyme -labile protecting groups
  • Reduction labile protecting groups
  • Oxidation labile protecting groups
  • Protecting groups which are cleaved by heavy metal salts or complexes thereof
  • Caging Groups
  • Two -stage protective group

Acid labile protecting groups can be cleaved by the action of acids. The driving force here is often the formation of a relatively stable carbocation or an acid catalyzed equilibrium, which is on the free functional group page. Examples of acid-labile protecting groups are the tert- butyl ester, ether and carbamate forming stable cations and acetals in which the acid-catalyzed equilibrium lies in the presence of water on the side of the corresponding aldehydes or ketones.

In the base-labile protecting groups can be mechanistically distinguish between the basic hydrolysis and base-induced β - elimination. Carboxylic acid esters ( with the exception of the tert- butyl esters ) are nucleophilic attack by hydroxide ions and cleaved hydrolytically so. Amides, however, are rarely so divided, as they require very harsh conditions. An exception here is the phthaloyl group, as it is cleaved with hydrazine already under quite mild conditions. In the β -elimination reaction cascade takes place: First of a proton by the base and formed a carbanion. By a suitable leaving group, the protecting group is then cleaved to form a vinyl compound. For the latter case counts especially the Fmoc group.

With silicon fluoride ions form a very stable bond. Therefore, the silicon - protecting groups are virtually without exception cleaved by fluoride ions. Depending on the nature of the counter ion or the Abspaltreagenzes but various silicon protecting groups may be cleaved selectively in dependence on the steric hindrance of the silicon atom. The advantage of fluoride -labile protecting groups is that no other protecting group is attacked under the cleavage conditions.

Esters can often be cleaved by enzymes such as lipases. Since enzymes operating at a pH between 5 and 9 and at moderate temperatures of about 30-40 ° C, and also with regard to the carboxylic acid, yet highly selective, this method is a true seldom used, but a very attractive method for the protecting group cleavage.

Benzyl can be reductively cleaved by catalytic hydrogenation. Benzyl groups are used as ethers, esters, urethanes, carbonates or acetals and for the protection of alcohols, carboxylic acids, amines and diols used.

Few protecting groups which can be removed by oxidation, are also common. This is usually around methoxybenzyl ether. They can be cleaved with cerium (IV ) ammonium nitrate (CAN) or dichlorodicyanobenzoquinone (DDQ ) a Chinomethin.

The double bond of an allyl radical may be isomerized to the vinyl compound of platinum group elements ( such as palladium, iridium or platinum). The resulting enol ether with protected alcohols or enamines with protected amines can be hydrolyzed slightly acidic.

Photolabile protecting groups contain a chromophore which can be activated by irradiation with an appropriate wavelength and then cleaved. One example is the o- nitrobenzyl ( ONB ) listed.

A special form of protection groups represent the two-stage protection selcetion These are characterized by a high stability, as the protecting group must be converted by a chemical transformation into a leaving group first. This type of protecting groups rarely find, however, application, since an additional activation step is necessary, which increases the synthesis by one step.

Functional groups

Amines

For the amino function by far the largest variety of protecting groups is available. This has to do with the fact that amines in peptide synthesis plays a special importance, but also on their characteristics: they are for a fairly potent nucleophiles, but also relatively strong bases. These properties led to ever new protecting groups for amines have been developed.

Many protecting groups for amines based on carbamates. These can be easily inserted in the form of carboxylic acid chlorides. Your driving force in the cleavage they relate by the formation of very stable carbon dioxide molecule. Based on different residues at different carbamate cleavage options were developed. The carbamates are the most commonly used t-butyloxycarbonyl, benzyloxycarbonyl, allyloxycarbonyl, and the Fluorenylmethylenoxycarbonyl - compounds.

In addition to the carbamates are a number of other N -acyl derivatives are not as widely used as protective groups is important, but by far. These include for example the phthalimide, either accessible via a Gabriel synthesis of primary amines by the reaction with phthalic anhydride or by the structure of the amino group. The cleavage of the phthalimide is normally done by hydrazine hydrate or sodium borohydride. Trifluoroacetamides are extremely easy to hydrolyze under basic conditions, therefore, the acetamides obtained by the reaction with trifluoroacetic anhydride are sometimes used as a protecting group for amines.

In indoles, pyrrole and imidazole, ie heterocyclic compounds, find the N-sulfonyl derivatives as protecting group their application. In normal amines this protecting group is often too stable. The presentation here by sulfonation with phenylsulfonyl chloride and deprotonated heterocycle. The cleavage is carried out by basic hydrolysis. N-acyl- derivatives of primary and secondary amines are relatively readily available by the reaction of the amines with an arylsulfonyl chloride, but may be difficult, for example, under the conditions of a Birch reduction ( sodium in liquid ammonia ), or by reaction with sodium naphthalide can be split.

Among the N- alkyl derivatives, can be represented by alkylation or reductive alkylation, N-benzyl derivatives have some significance. The cleavage is carried out as the Cbz group and reductive typically by catalytic hydrogenation or by Birch reduction. N- alkyl amines have here the decisive disadvantage compared to carbamates or amides that the basic nitrogen is retained.

Alcohols

The classical protecting group for alcohols are carboxylic acid esters. Often the ester precursors are commercially available or can be easily obtained by transesterification but by reacting the alcohols with acid chlorides or anhydrides by a Schotten- Baumann reaction or. The cleavage of the esters is generally carried out by reaction with nucleophiles such as alkali metal hydroxides, alkali metal alkoxides or lithium or magnesium - organic compounds; alternatively by reduction by reaction with complex hydrides such as lithium aluminum hydride. The reactivity of the ester with respect to nucleophilic attack decreases as the steric hindrance of the carboxylic acid in the order:

The reactivity of the alcohols also decreases with increasing steric hindrance of the alcohols:

The main esters are used as protecting groups, the acetic acid esters, benzoic acid esters and the Pivalinsäureester since these are differentiated by the specified reactivities from each other can be split off.

Among the most important protecting groups of alcohols and phenols also include very well-studied and documented trisubstituted silyl ether. Here, the silicon contributes as organic residues, both alkyl and aryl groups. This type of protection group has the advantage that it is very good moderierbar introducing and particularly with respect to the spin-off. These ethers are produced either in a Williamson ether synthesis from the chlorosilane and an alcoholate ion, or by the use of activating agents such as imidazole.

For purely analytical purposes, such as a carbohydrate to make volatile and be able to detect by means of GC-MS, there are commercially available reaction kits. Silyl ethers are generally sensitive to acids and fluoride ions. The latter is usually exploited for their cleavage. However, the commercial prices of the chlorosilanes are very different depending on the substitution. The cheapest chlorosilane here is chlorotrimethylsilane (TMS -Cl), which is a by-product of the silicone preparation after Rochow and Muller. Another common source of the trimethylsilyl group is hexamethyldisilazane (HMDS). However, the trimethylsilyl ethers are also extremely sensitive to acidic hydrolysis ( for example, is already sufficient silica gel as a proton donor ) and are therefore rarely used today as a protecting group.

Another class of protecting groups for alcohols are the alkyl ethers. Again, there are multiple and orthogonal ways to divide the ether. Aliphatic methoxyether be cleaved only with difficulty and under severe conditions, so they are generally used only come with phenols.

1,2- diol

A particular class of alcohol in the protective group chemistry are the 1,2- diols ( glycols ) represents the position of two neighboring hydroxyl groups can be exploited, for example to sugars in that it protects both hydroxyl groups from one another depending on the acetal. Commonly used are the benzylidene, isopropylidene and Cyclohexylidene or cyclopentylidene acetals here.

The preparation of the acetals is usually by displacing the equilibrium of a mixture of the glycol with the carbonyl component by removing the water of reaction, or by transacetalization with a simple acetal and removal of the resulting alcohol from the reaction mixture.

Especially in sugar chemistry the different position of the hydroxyl groups is exploited to each other to protect these stereochemical dependence in certain selective here. React (among the other possible combinations ), the two hydroxy groups adjacent to one another preferably forming the most stable conformation.

Acetals can be cleaved in principle in aqueous acidic solvents again. A special case here represents the benzylidene protecting group which can be cleaved reductively. This is accomplished either by catalytic hydrogenation or by the hydride donor diisobutylaluminum hydride ( DIBAL ). However, cleavage by DIBAL deprotected only one alcohol group as the benzyl on the second and sterically gehinderteren hydroxy group remains as a benzyl ether.

Carbonyl

Carbonyl groups are mainly threatened by nucleophilic attacks such as Grignard reagents or hydride ions. Aldehydes can also be oxidized to carboxylic acids yet. However, adverse reactions, which can be prevented by a suitable protecting group by acid-and base- catalyzed reactions of the carbonyl group such as aldol reactions.

The most common protecting groups for carbonyl groups are acetals and especially cyclic acetals with diols. In addition, cyclic acetals with 1,2- hydroxythiols or Dithioglycolen be used - the so-called O, S or S, S- acetals.

Principle for acetals as protecting group for carbonyl compounds are the same as for diols 1,2- acetals as protecting group. Both the preparation and the cleavage are naturally identical. However, in the process of acetals transacetalization plays as a protecting group a subordinate role, and they are generally prepared from glycols by elimination of water. More modern variants of use also glycols in which the hydroxyl hydrogen atoms have been replaced by a trimethylsilyl group. Normally find simple glycols such as ethylene glycol or 1,3-propanediol as diols for use acetals.

Acetals can be cleaved under acidic aqueous conditions. Here are the acids used are the mineral acids. Cosolvent is often acetone is used as solubilizer. As a non- acidic Abspaltmethode with a palladium (II ) chloride - acetonitrile complex, in acetone, or with iron ( III) chloride on silica gel in chloroform drawn to be worked.

Cyclic acetals are much more stable to acid hydrolysis than acyclic acetal. Therefore almost exclusively acyclic acetals are used when a very mild Anspaltung is necessary or when two different protected carbonyl groups with respect to its release must be differentiated.

Acetals find, however, in addition to their sole function as a protecting group as a chiral auxiliary reagent additionally application. Acetals such as glycols, for example, chiral derivatives of tartaric acid can be opened asymmetrically with high selectivity. This allows the development of new chiral centers.

In addition to the O, O- acetals also the S, O and S, S- acetals have one, albeit lesser, importance as a carbonyl protecting group. Thiols, one must use to produce these acetals, have a very unpleasant smell and are toxic, what the application is very restricted. Thioacetals and mixed S, O - acetals, as compared with the pure O, O- acetals, very much more stable to acid hydrolysis. This allows the selective cleavage of these protected sulfur in the presence of carbonyl groups.

The preparation of the S, S- acetals is usually similar to the O, O- acetals by acid catalysis from the dithiols and carbonyl. Because of the great stability of thioacetals, the equilibrium is on the side of the acetals. It must in contrast to the O, O- acetals no reaction water are removed in order to shift the equilibrium.

S, O - acetals are by a factor of 10,000 hydrolyzed faster than the corresponding S, S- acetals. They are prepared in analogy to those of the thioalcohols. Her cleavage is carried out under comparable conditions and mainly by mercury (II ) compounds in aqueous acetonitrile.

To aldehydes in a temporary protection of the carbonyl group in the presence of ketones is described as hemiaminal anion. Here is exploited that aldehydes have a much higher carbonyl activity as ketones and that many addition reactions are reversible.

Carboxy

The main protective groups for carboxyl groups are esters of various alcohols. In addition, are also ortho- esters, and oxazolines in use, however, of minor importance. For the production of carboxylic acid esters, there are basically different methods:

  • Direct esterification of carboxylic acid and alcohol component. Because of the unfavorable equilibrium in the reaction between alcohols and carboxylic acids, the balance must be done either by removing the water of reaction, or by working with large excesses of alcohol. To this end, however, the alcohol must be very inexpensive. This reaction is acid catalyzed ( acid, p-toluenesulfonic acid or acidic ion exchangers are the most commonly used esterification catalysts ).
  • The reaction of acid anhydrides or acid chlorides with alcohols in the presence of auxiliary bases. Suitable auxiliary bases found here frequently pyridine, diisopropylethylamine or triethylamine application. This reaction may be catalyzed by 4-N, N-dimethylaminopyridine, which increases the reaction rate compared to pure pyridine by a factor of 104. Compared to the direct esterification of these methods carried out under very mild conditions.
  • The reaction of carboxylic acid salts with alkyl halides is another method for the preparation of carboxylic acid esters.
  • The reaction of carboxylic acids with isobutene is a gentle method for the production of tert-butyl esters. Here, isobutene is reacted with the carboxylic acid in the presence of a strong acid such as sulfuric acid.
  • The reaction of carboxylic acids with diazoalkanes is a very gentle and quantitative method to produce esters. However, it is most often used due to the poor accessibility of complex diazoalkanes for the preparation of methyl esters and benzhydril.

In addition to these classical methods of esterification more and more modern methods have been developed for specific reactions.

  • The activation of the carboxylic acid with dicyclohexylcarbodiimide and reacting the resulting O- acylisourea with the alcohol component in the presence of 4-N, N- dimethylaminopyridine ( Steglich esterification ).
  • The activation of the carboxylic acid by forming a mixed anhydride with 2,4,6- trichlorobenzoic acid by reacting the carboxylic acid with benzoyl chloride in the presence of 4-N, N-dimethylaminopyridine and triethylamine. The mixed anhydride is prepared in situ and immediately further reacted with the alcohol component ( Yamaguchi esterification ).
  • Activating the Alkoholkomponete by reaction under Mitsunobu conditions using triphenylphosphine and diethyl azodicarboxylate and then reacting in situ with the carboxylic acid ( Mitsunobu esterification).

Suitable alcohol components of different groups can serve. Are very common here, however, the methyl, the tert-butyl, benzyl and allyl. In addition, a number of protecting groups will be added to that derived from the ether protecting groups of the hydroxyl groups. However, the specific cleavage conditions are often very similar. Basically each ester in the presence of hydroxide ions can be hydrolyzed in aqueous- alcoholic solution. However, in substrates sensitive to commonly used lithium hydroxide in tetrahydrofuran and in the presence of methanol. For the Hydolysetendenz naturally, the same rules apply as for the esters as alcohol protecting group.

Alkenes

Alkenes are rare and must be protected by a protective group. They are usually affected only by electrophilic attacks, isomerization, and in the catalytic hydrogenation of unwanted side reactions. Basically one knows for alkenes two protection groups:

  • The temporary halogenation with bromine to trans-1 ,2- dibromoalkyl connection: The regeneration of the alkene takes place under restoration by the conformation of elemental zinc or with titanocene dichloride
  • The contactors wetting by a Diels -Alder reaction: The reaction of an alkene with a diene leads to a cyclic alkene, which is similar but vulnerable to electrophilic attacks as the original alkene. Cleavage of serving as a protective group diene takes place thermally, since it is in a Diels -Alder reaction is a reversible or equilibrium reaction.

Alkynes

For alkynes are also recognizes two types of protecting groups. With terminal alkynes, it is sometimes necessary to mask the acidic hydrogen atom. This is normally done by deprotonation ( by means of strong bases, such as butyllithium or methylmagnesium bromide in tetrahydrofuran / dimethylsulfoxide) followed by reaction with chlorotrimethylsilane to the terminal TMS -protected alkyne. They are cleaved off by hydrolysis - with potassium carbonate in methanol - or by fluoride ions, such as by means of tetrabutylammonium fluoride.

To protect the triple bond itself, sometimes a complex of the alkyne compound with dicobalt octacarbonyl is used. The cleavage of the cobalt occurs through oxidation.

Applications

Protecting groups are used in many areas of synthetic organic chemistry. This concerns both the laboratory -synthesized large-scale syntheses of complex agents. When a functional group is found to be disruptive or may be attacked undesirable place the protective group technique its application. Almost at every synthesis of a complex target molecule are protecting groups used. Since both the insertion and the removal of the protecting groups in addition to the effort also has a yield loss of, it is desirable to do without protecting groups, but this is often difficult to achieve.

In the automated synthesis of nucleotides, peptides and the protecting group is an integral part of the chemical synthesis approach. From sugar chemistry are protecting groups due to the very similar hydroxyl groups in the sugar molecules also not indispensable.

An important example of industrial application of the protective group technique, the synthesis of ascorbic acid (vitamin C) Reichstein.

To prevent oxidation of the secondary alcohols by potassium permanganate, this can be protected by acetylation and deprotected again with acetone to give the carboxylic acid by the oxidation of the primary hydroxy group.

A very spectacular example of natural product synthesis to the use of protecting groups is the total synthesis of palytoxin carboxylic acid by the working group to Yoshito Kishi from the year 1994. This had to be protected 42 Functional Groups (39 hydroxy groups, a diol, an amino group and a carboxylic acid group). This was done by eight different protecting groups ( a methyl ester, the acetate groups of five, 20 TBDMS-E ether, p- methoxybenzyl ether nine, four benzoates, methyl hemiacetal, an acetal with acetone, and a SEM - ester).

The introduction or modification of a protection group influenced, sometimes, the reactivity of the whole molecule. An example is shown a section of the synthesis of an analogue of mitomycin by Danishefsky.

The change of the protecting group from a methyl to a MOM ether prevented here the opening of the epoxide to give the aldehyde.

An important application of protective group chemistry can be found in the automated synthesis of peptides and nucleosides. In peptide synthesis by automated synthesizer, the orthogonality of the Fmoc group ( basic cleavage ), the tert -butyl group (acid cleavage ), and protecting groups for various functional groups in the side chain of the amino acids is used. In automated nucleotide synthesis of DNA and RNA sequences protecting groups are used on the one hand the blocking of functional groups as well as redox chemicals, it is made to the protected atoms. The phosphorus is protected and oxidized to phosphate during the Kupplungszyklusses.

In general, the introduction of a protecting group is not a problem. The difficulties lie more in their stability and the selective cleavage. Field problems in synthesis strategies with protective groups are rarely documented in the literature.

Atom economy

Syntheses using protecting groups generally have a low atom economy. Sometimes the detour the use of protecting groups must be taken to eliminate undesirable competing reactions and to achieve the desired selectivity of a synthesis. In the synthesis of complex structures protecting group strategies are often indispensable.

As an example of a protective group strategy compared to a protective -group-free synthesis, the synthesis of Hapalindole U were compared. During the synthesis of Hideaki Muratake of 1990 tosyl used as a protecting group, was omitted in the synthesis of Phil S. Baran from the year 2007 to each protection group. The number of synthetic steps was significantly reduced.

Hapalindole U Baran 2007 protecting group free synthesis

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