Introduction and removal of alkyl protecting groups of several common amino groups

 

The alkoxycarbonyl protecting group is the most commonly used type of amino protecting group. This article briefly introduces the protection and deprotection methods of the following common alkoxycarbonyl amino protecting groups. These common protecting groups include: benzyloxycarbonyl (Cbz), tert-butoxycarbonyl (Boc), methoxycarbonyl (Fmoc), allyloxycarbonyl (Alloc), trimethylsilylethoxycarbonyl (Teoc) , 2,2,2-Trichloroethoxycarbonyl (Troc).

 

1. Benzyloxycarbonyl (Cbz) protecting group

1.1 Introduction of benzyloxycarbonyl (Cbz)

 

N-benzyloxycarbonylamino compounds can be easily reacted with Cbz-Cl or Cbz-OSu and free amino groups under basic conditions such as triethylamine, pyridine, and sodium bicarbonate. The reactivity of Cbz-Cl is higher than that of Cbz-OSu, and the reaction is usually carried out in aprotic organic solvents such as dichloromethane. Since the nucleophilicity of amino groups is greater than that of hydroxyl groups, it is sometimes necessary to use protic solvents. In addition, Cbz-ONB (4-O2NC6H4OCOOBn) and other weakly active benzyloxycarbonyl activated esters can also be used as the introduction reagent of benzyloxycarbonyl. This reagent makes the primary amine easier to be protected than the secondary amine. Due to the lack of nucleophilicity of aniline, it can be compared with This reagent does not react.

 

Introducing a protected base instance:

 

 

1.2 Removal of benzyloxycarbonyl (Cbz)

 

There are several methods for the removal of benzyloxycarbonyl: 1) catalytic hydrogenolysis; 2) strong acid cleavage (HBr, TMSI); 3) Na/NH3 (liquid) reduction. A common and concise method in the laboratory is catalytic hydrogenolysis; when there are groups in the molecule that are sensitive to catalytic hydrogenolysis (benzyl ether, olefin, etc.) or passivate the catalyst (thioether, etc.), we need to use chemical methods such as acid Cleavage of HBr or Na/NH3 (liquid) reduction, etc.

Catalytic hydrogenolysis is the most commonly used and mildest deprotection method, which can be completed by hydrogenation at normal temperature and pressure. The hydrogen donor of the reaction can be hydrogen, cyclohexadiene, 1,4-cyclohexadiene, ammonium formate and formic acid, etc. The reaction of the latter four reagents as hydrogen donors is also called catalytic hydrogenation reaction. If the hydrogenation is performed with Pd/C in the presence of Boc2O, the liberated amines are directly transformed into Boc derivatives. Moreover, this type of reaction is often faster than that without Boc2O, mainly because the amine produced by hydrogenolysis often has a certain complexation with the noble metal catalyst, which reduces the activity of the catalyst. Reaction with Boc2O as an amide removes this effect. In addition, sometimes adding an appropriate acid during hydrogenolysis can promote the reaction for the same reason. The protonated amine can avoid the complexation with the catalyst, thereby speeding up the reaction rate.

Catalytic hydrogenation catalysts mainly use 5-10% palladium-carbon, 10-20% palladium hydroxide-carbon or palladium-polyethyleneimine, palladium-polyethyleneimine/formic acid is better than the former two for removing Cbz . In addition, when there are halogen atoms (Cl, Br, I) in the molecule, the direct use of Pd/C will generally cause dehalogenation. In this case, PdCl2 is used as the catalyst, and ethyl acetate or dichloromethane is used as the solvent. The occurrence of dehalogenation can be better avoided.

Furthermore, when HBr/HOAc deprotects the Cbz group, the decomposition yields the carbocation of the benzyl group. If there is a carbocation-capturing group (activated benzene ring, etc.) in the molecule, the corresponding by-product will be obtained.

 

 

Example of deprotection:

 

 

Catalytic Deprotection of PdCl2 in the Presence of Halides

 

To a solution ocompound 1(900 mg) in methylene chloride (16.5 ml) was added PdCl2 (30mg) and triethylamine (0.229 ml). Triethyl silane was added (2 x 0.395 ml) over2 h. The reaction mixture stirred 1 h and 2 ml of trifluoroacetic acid was added. After 30 min the reaction was basified with 2 N NaOH, extracted with methylene chloride, dried over MgSO4, filtered and concentrated. Chromatography was run with 3-5% MeOH/CH2Cl2 with 0.5% NH4OH to provide compound 2 as an oil (501mg, 74%).

 

2. tert-butoxycarbonyl (Boc) protecting group

In addition to the Cbz protecting group, tert-butoxycarbonyl (Boc) is also a widely used amino protecting group in peptide synthesis. Especially in solid-phase synthesis, Boc is often used instead of Cbz for the protection of amino groups. Boc has the following advantages: it is easy to be removed by acidolysis, but it has stability when the acidity is weak; what is produced during acidolysis is that tert-butyl cation is decomposed into isobutylene, and it generally does not bring side reactions; Stable to hydrazinolysis and many nucleophiles; Boc is stable to catalytic hydrogenolysis, but much more sensitive to acids than Cbz. When Boc and Cbz exist at the same time, Cbz can be removed by catalytic hydrogenolysis, Boc remains unchanged, or Boc can be removed by acid solution without Cbz being affected, so the two can be used together well.

 

2.1 Introduction of tert-butoxycarbonyl (Boc)

Free amino groups can easily react with Boc2O in a mixed solvent of dioxane and water under basic conditions controlled by NaOH or NaHCO3 to obtain Boc-protected amines. This is one of the common ways to introduce Boc, and its advantage is that the by-products are non-interfering and easy to remove. Sometimes, for some amines with high nucleophilicity, they can be directly reacted with Boc anhydride in methanol, without other bases, and the treatment is convenient. For amino derivatives that are sensitive to water, it is better to use Boc2O/TEA/MeOH or DMF at 40-50°C. For amino groups with weaker activity, DMAP can be added to catalyze the reaction rate.

 

Introducing a protected base instance:

 

 

2.2 Removal of tert-butoxycarbonyl (Boc)

 

Boc is more sensitive to acid than Cbz, and the acid hydrolysis products are isobutene and CO2 (see formula below). In the synthesis of liquid phase peptides, TFA or 50% TFA (TFA:CH2Cl2 = 1:1, v/v) can be used to remove Boc. TBDPS and TBDMS bases are relatively stable when using dilute 10-20% TFA in the process of Boc removal. In addition, neutral conditions such as: the combination of TBSOTf/2.6-lutidine or ZnBr2/CH2Cl2 can also remove BOC very well, and make some acid-sensitive functional groups can also be retained. Although BOC is mostly removed under acidic conditions, BOC on amino groups with weaker basicity can also be removed under alkaline conditions.

When there are some functional groups in the molecule that can react with by-product tert-butyl carbocations under acidic conditions, it is necessary to add thiophenol (such as thiophenol) to remove tert-butyl carbocations, which can prevent thiol (ether , phenol) (such as methionine, tryptophan, etc.) and other electron-rich aromatic rings (indole, thiophene, pyrazole, furan polyphenol hydroxyl-substituted benzene, etc.) Other scavengers such as anisole, thioanisole, thiocresol, cresol, and dimethyl sulfide may also be used.

 

Example of deprotection:

3. Wat methoxycarbonyl (Fmoc) protecting group

A major advantage of the Fmoc protecting group is that it is extremely acid stable and in its presence the Boc and benzyl groups can be deprotected. After deprotection of the Fmoc, the amine is released as the free base. In general, Fmoc is stable to hydrogenation, but in some cases, it can be removed by H2/Pd-C in AcOH and MeOH. The company’s previous tweets specifically described the introduction and removal of the Fmoc protecting group in detail. Interested friends can refer to the previous tweets again.

 

3.1 Introduction of Wat methoxycarbonyl (Fmoc)

The amino group protected by Fmoc can be obtained by reacting Fmoc-Cl and Fmoc-OSu with amino group under weak base conditions such as pyridine or NaHCO3. (Be sure not to use a strong base such as triethylamine!). The activity of Fmoc-OSu is slightly lower than that of Fmoc-Cl, and the impurities produced by the reaction are usually less, and it is generally more preferred to use Fmoc-OSu on Fmoc.

 

Introducing a protected base instance:

3.2 Removal of Wat Methoxycarbonyl (Fmoc)

The Fmoc protecting group can generally be removed by various basic conditions such as concentrated ammonia water, piperidine, ethylenediamine, cyclohexylamine, morpholine, DBU, Bu4N+F-/DMF, etc. Tertiary amines (such as triethylamine) are less effective in removal, and more sterically hindered amines (such as DIEA) are less effective in removal.

4. Allyloxycarbonyl (Alloc) protecting group

Different from the aforementioned Cbz, Boc and Fmoc, Alloc is very stable to acids and alkalis. In its presence, Cbz, Boc and Fmoc can be selectively deprotected, while the removal of Alloc is usually in Pd( 0) in the presence of.

 

4.1 Introduction of allyloxycarbonyl (Alloc) protecting group

Usually, Alloc-Cl or Alloc-OSu reacts with amino compounds in organic solvent/Na2CO3, NaHCO3 solution or pyridine to obtain Alloc-protected amino derivatives.

 

Introducing a protected base instance:

4.2 Removal of allyloxycarbonyl (Alloc) protecting group

 

The Alloc protecting group has strong stability to acids and bases, and they are usually only deprotected with Pd(0), such as Pd(PPh3)4 or Pd(PPh3)2Cl2. Under Pd(0) catalysis, a π-allylpalladium intermediate is generated, which is deprotected after reaction with a nucleophile such as morpholine or 1,3-diketone. For example, Alloc derivatives can be treated with Pd(PPh3)4/Me2NTMS to obtain easily hydrolyzed TMS carbamate [Tetrahedron Lett., 1992, 33,477]. When adding Boc2O, AcCl, TsCl, or succinic anhydride, Pd(PPh3)2Cl2/Bu3SnH can convert the Alloc group into other amine derivatives. In addition, Alloc can also be removed by Pd(PPh3)4/HCOOH/TEA [J.Med. Chem., 1992, 35, 2781] or AcOH/NMO [J.Org. Chem., 1996, 61, 3983] .

 

Examples of deprotection groups:

 

To a solution of the Alloc protected ester (140.7 mg) and 1,3-dimethylbarbituric acid (228 mg) in THF (15 mL) was added tetrakis(triphenylphosphine)palladium (43.9 mg, 17 mol%), and the resulting mixture was stirred at rt for 27 h. The mixture was then poured into saturated aq. NaHCO3 and extracted four times with Et2O. The combined extract was dried (MgSO4) and concentrated in vacuo. The residue was purified by chromatography (CHCl3/MeOH, 20 : 1 to 2 : 1) to give the corresponding free amino ester as a colorless oil (79.5 mg, 65%). [ Chem. Soc. Perkin Trans. 1., 2004, 7, 949]

 

 

To a solution of 112 (0.97 g, 1.4 mmol) in CH2Cl2 (19 mL) were added dimethylamino-trimethylsilane (1.32 mL, 8.4 mol) and trimethylsilyl trifluoroacetate (1.45 mL, 8.4 mmol). The solution was stirred at 20 °C for 10 min, and then Pd(PPh3)4(97 mg, 0.084mmol) was added and stirring was continued for 2.5 h. The mixture was evaporated and the residual oil was dissolved in EtOAc (50 mL). The solution was washed with 10 % aq NaHCO3 and brine, dried, and evaporated. The residue was chromatographed (SiO2; EtOAc/hexane 1:2) to give 113 (0.67 g, 78%). [J. Med. Chem., 1992, 47(6) , 1487].

5. Trimethylsilylethoxycarbonyl (Teoc) protecting group

Trimethylsilylethoxycarbonyl (Teoc) is different from the aforementioned Cbz, Boc, Fmoc and Alloc. It is very stable to acids, most alkalis, and noble metal catalysis. In its presence, Cbz, Boc, Fmoc and Alloc can be selectively deprotected, and its deprotection is usually carried out in fluoride anion. Such as TBAF, TEAF and HF etc.

 

5.1 Introduction of trimethylsilylethoxycarbonyl (Teoc)

In general, Teoc-Cl, Teoc-OSu, Teoc-OBt, Teoc-Nt react with amino compounds in the presence of organic solvents and bases to obtain Teoc-protected amino derivatives. Nitrotriazole, a by-product produced after protection on the Sodeoka reagent (Teoc-NT), can be removed by simple filtration because it is insoluble in solvents

Introducing a protected base example:

5.2 Removal of trimethylsilylethoxycarbonyl (Teoc)

The removal of trimethylsilylethoxycarbonyl (Teoc) is mainly through β-elimination deprotection after the reaction of fluoride ion with trimethylsilane. Fluorine reagents include TBAF (tetrabutylammonium fluoride), TEAF (tetraethylammonium fluoride) or TMAF (tetramethylammonium fluoride). During the removal process, TBAF will produce a by-product of tetrabutylamine salt, which is often difficult to remove and often affects the quality of the product. At this time, TMAF or TEAF can be used instead.

Example of deprotection:

 

6. 2,2,2-Trichloroethoxycarbonyl (Troc) protecting group

6.1 Introduction of 2,2,2-trichloroethoxycarbonyl (Troc) protecting group

In general, Troc-Cl and Troc-OSu react with amino compounds in the presence of organic solvents and bases to obtain Teoc-protected amino derivatives.

6.2 Removal of 2,2,2-trichloroethoxycarbonyl (Troc) protecting group

Deprotection is usually performed under one-electron reduction conditions of zinc-acetic acid, with by-products being volatile 1,1-dichloroethylene and carbon dioxide. Under this condition, many groups such as Boc, Fmoc, Cbz, Teoc, etc. are stable.

Example of deprotection

 

There are many alkoxycarbonyl protecting groups, which will not be introduced one by one in this article. When choosing a protecting group, careful consideration must be given to all reactants, reaction conditions, and functional groups in the substrates that will be involved in the reaction being designed. Try to choose the protecting group that is the easiest to add and remove. When several protecting groups need to be removed at the same time, it is very effective to use the same protecting group to protect different functional groups. To selectively remove protecting groups, only different kinds of protecting groups can be used. In addition, selectivity to protection generation and removal rates must be considered both electronically and sterically. The protection and deprotection of amino groups is always a strategy of last resort. If a new route can be designed or the use of precursor functional groups can be used to avoid the use of protective groups, it is a better method.

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