What Is a Carboxyl Group?

Carboxyl (carboxyl) is the basic chemical group in organic chemistry. All organic acid substances containing carboxyl groups can be called carboxylic acids. It consists of one carbon atom, two oxygen atoms and one hydrogen atom. For example, acetic acid (CH3 _- COOH) and amino acids all contain carboxyl groups, and compounds in which these carboxyl groups are directly connected to a hydrocarbon group are called carboxylic acids.

[su j]

Carboxyl [su j] is

The presence of carboxyl groups can be tested with freshly prepared acetic acid or cuprous hydroxide. Phenomenon: The blue flocculent precipitate disappears and becomes a colorless solution.

HOOC--that is, --COOH is carboxyl, --OH is

On the fabric

The method of protecting the carboxyl group is mainly an esterification method, but in some cases, it can also be protected by a method such as forming an amide or a hydrazide.

Esterification protects carboxyl groups: methyl esters and ethyl esters

Substances with complex aromatic polyhydroxy carboxyl groups

Methyl esters and ethyl esters are useful as protecting groups for carboxylic acids for a range of synthetic operations. For example, the alkylation reaction and various condensation reactions in the form of an ester, followed by hydrolysis and removal of the ester group under the catalysis of an acid or a base, and occasionally the ester group can also be removed by a pyrolysis reaction. However, simple alkyl esters are not suitable as protecting groups for carboxylic acids in some cases, and the reason is often due to the final need to use a saponification reaction to remove the ester group. Therefore, in practice, ordinary methyl and ethyl derivatives are used instead. Methyl derivatives are mainly benzyl and can be removed by acid treatment or hydrogenolysis under mild conditions. Ethyl derivatives are mainly , , 2 trichloroethyl, etc.

Esterification protects carboxyl groups: tert-butyl ester

Tert-butyl ester cannot be hydrogenolyzed and is not hydrolyzed by ammonia and alkali under normal conditions, but tert-butyl can be cleaved off in the form of isobutylene under mild acidic conditions. This property makes tert-butyl particularly attractive in situations where alkali saponification is not possible, for example: for the synthesis of ketones, 2 ketoesters, , unsaturated ketones, and base-sensitive 2 keto alcohols and peptides. In the synthesis of penicillin, the tert-butyl ester can be selectively cleaved to form 2 lactam; in the synthesis of bacteromycin and in the preparation of easily reduced ketones, tert-butyl can be used to protect the carboxyl group. Tetrahydropyranic acid has acid instability similar to that of tert-butyl ester, and this protecting group is similarly used in the synthesis of malonate-type ketones and ketoesters.

Esterification protects carboxyl groups

Benzyl, substituted benzyl and benzhydryl esters

These ester protecting groups are characterized by their ability to be quickly removed by hydrogenolysis. In the synthesis of penicillin, the benzyl ester is not destroyed by mild ester hydrolysis conditions, and the benzyl ester is finally removed by hydrogenolysis; in the synthesis of glutamine and asparagine, and in the synthesis of L2 glutamic acid and L2 aspartic acid ester In the preparation, the properties of benzyl esters are typically shown. Bowman and Ames used benzyl esters in the alkylation or acylation of active esters (with 2 active hydrogen). This method has excellently completed the synthesis of fatty acids, ketones, diketones, and 2 ketones. When the benzyl group having a substituent on the aromatic ring or methine group is removed with an acidic reagent, its sensitivity can be greatly changed. In the synthesis of ester peptides, Stewevr takes advantage of the ease of catalytic removal of methylene benzyl ester, and uses it to replace t-butyl ester. Benzyl esters and p-nitrobenzyl esters can also be used as protecting groups for carboxyl groups. A typical example is their application in the synthesis of acylated derivatives of amino groups. In the synthesis of phenyl ester and phenolic acid, diphenylmethyl has similar effects, but the solvation of diphenylmethyl in the presence of acid is too fast, so it is not easy to be a carboxy protecting group under acidic conditions. In short, this type of ester is a valuable protecting group, and its preparation can be prepared by classical methods and the aforementioned reactions.

Protect carboxyl groups with amides and hydrazides

In a limited range, people use amides and hydrazides to protect carboxyl groups, which complements the lack of protection of esters from the perspective of their release methods. Amides and hydrazides are stable to mild alkaline hydrolysis conditions for deesterification, but esters are also stable to nitrosyl esters that can effectively deamidate amides and oxidants for cracking hydrazide, which can be complementary.

The classic method for preparing amides and hydrazides is to use esters or acid chlorides to react with amines or hydrazines, respectively.

Got. The hydrazide has been used in the synthesis of antibiotics and peptides. In the synthesis of peptides, they can be converted into azides by nitrous acid, making the condensation reaction easy to occur.

Protection of esters

The protection of esters and lactones can be regarded as indirect protection of carboxyl groups, and the esters must have 2 active hydrogen, otherwise the reaction is complicated. After the introduction of the protective group, the ester can be stable under many conditions, such as HOAc / H2O / THF (25 ° C, 1h), KOH / MeOH (25 ° C, 12h), LiAlH4 / Et2O (25 ° C, 3h), CH3Li / Et2O (25 ° C, 2h) and so on. Protective groups can be degreased with mercury salts or boron trifluoride.

In summary, although there are not many methods for protecting carboxyl groups, there are many types of esters as protecting groups, and each has its own characteristics. ^[2]