Carbodiimide-activated acids

Reduction of uronic acids Uronic acids (UA) were converted to the corresponding neutral sugars (NS) by carbodiimide activation of the carboxyl groups followed by a reduction with NaBD4 according to the method of Kim and Carpita [5]. In order to achieve a complete reduction of the uronic acids the procedure was repeated once. 

Figure 15 Conjugation of a carboxylic add and an amine using the carbodiimide method. The carbodiimide activates the carboxyhc acid to speed up the reaction to the amine. Carbodiimides can be used with nonpolar or polar solvents, including water. Undesirable urea complexes may form as by-products. Details of the reaction are given in Table 3...

Like the carbodiimide method, the mixed anhydride method results in an amide complex (Table 5, Figure 17). The acid-containing hapten is dissolved in a dry, inert, dipolar, aprotic solvent such as p-dioxane, and isobutyl chloroformate is added with an amine catalyst. The activated mixed anhydride is chemically stable and can be isolated and characterized. The aqueous protein solution is added to the activated acid and the pH is maintained at around 8.5. A low temperature (around 10 °C) is necessary during the reaction to minimize side reactions. 

Biotin-hydrazide also may be used to couple with carboxylate-containing molecules. Hydrazidcs can be coupled with carboxylic acid groups by using the carbodiimide reaction (Chapter 3, Section 1.1). The carbodiimide activates a carboxylate to an o-acylisourea intermediate. Biotin-hydrazide can react with this intermediate via nucleophilic addition to form a stable covalent bond. 

The esterification of support-bound carboxylic acids has not been investigated as thoroughly as the esterification of support-bound alcohols. Resin-bound activated acid derivatives that are well suited to the preparation of esters include O-acylisoureas (formed from acids and carbodiimides), acyl halides [23,226-228], and mixed anhydrides (Table 13.15). A-Acylurea formation does not compete with esterifications as efficiently as it does with the formation of amides from support-bound acids. Esters can also be prepared from carboxylic acids on insoluble supports by acid-catalyzed esterification [152,229]. Alternatively, support-bound carboxylic acids can be esteri-fied by O-alkylation, either with primary or secondary aliphatic alcohols under Mitsu-nobu conditions or with reactive alkyl halides or sulfonates (Table 13.15). 

If amino acids with unprotected hydroxyl groups are incorporated, it is better to use carbodiimide activation (such as DIC/HOBt-activation) instead of more reactive procedures to avoid acylation of the hydroxyl-groups. 

Interestingly, reaction of hyaluronic acid with EDC in the presence of primary amines affords only theN-acyl urea derivatives, as proven by C- and N-labeling experiments. Water soluble carbodiimides are also used in the depolymerization of polyuronides via reduction of their carbodiimide activated carboxyl groups. ... 

Monodispersed chitosan conjugated Fe304 nanoparticles are fabricated from chitosan via carbodiimide activation. The nanoparticles are useful for the removal of acid dyes. ... 

Figure 10 Schematic representation of the most common reactions for labeling an amine (a) reaction with isothiocyanate to give a thiourea (b) reaction with a Succinimidil ester to give an amide (c) reaction with a sulfonyl chloride to give a sulfonamide (d) reaction with an aldehyde to give an imine (Schiff s base) and (e) reaction with a carbodiimide-activated carboxylic acid to give an amide.

Carbodiimides, in particular dicyclohexylcarbodiimide, have been applied in many syntheses where dehydration had to be performed under mild conditions. It is therefore no surprise that this reagent was also introduced for the synthesis of acid anhydrides from carboxylic acids." " In order to avoid N-acylation the reactions are carried out at low temperature. First 0-acylisoureas are formed, which then react further with free acid to the acid anhydride (equation 34). The reaction has been exploited in particular for the preparation of peptides." )V-alkoxycarbonyl-protected amino acids can be transformed in high yield to the corresponding anhydrides, which themselves are activated acid derivatives and may be converted to peptides. As in many other examples polymer-bound carbodiimides may prove superior sometimes, as the isolation of the products is facilitated. Easy preparation of acid anhydrides is possible in this way." ... 

Because the preferred y carboxyl is more sterically accessible, random carbodiimide activation of folic acid usually yields a 70 30 ratio of y carboxyl a carboxyl-derivatized protein. 

Figure 4.19 Carboxylic acid reactive probes. Stabilization of an unstable 0-acylisourea intermediate by use of A/-hydroxysulphosuccinimide during the carbodiimide-activated combination of a carboxylic acid with a primary amine. Reaction normally occurs by steps A and B. With N-hydroxysulphosuccinimide the reaction now takes place by steps A, C and D.

Terminal carboxyl groups in monolayers can also be activated by treatment with carbodiimides such as dicyclohexylcarbodiimide (DCC) or l-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) 59). Alternatively, conversion to a mixed anhydride can be effected by reaction of a carboxyl-terminated film with ethyl chloroformate 49). Exposure of the surface to gaseous SOCI2 has been reported to produce carboxyl chloride groups 48,60). These activated acid derivatives then react smoothly with alcohols or amines to form esters or amides (Scheme 2). 

The most relevant methodologies of reaction for the formation of esters, which are also used in the context of PVMRs for esterification, are hereafter proposed. For the purpose of this work, information about Lewis acid-activators, esterification base-activators, carbodiimide activators, the Mitsunobu reaction, induced activation of the condensation reaction using template compounds, and other reaction methodologies between carboxylic acids and alcohols used in ester synthesis will not be proposed. For this information, the reader is referred to the literature (Otera, 2003). 

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