Carboxylic reaction with carbodiimides

The presence of the carboxylic group will cause further breakdown. The removal of this group by the reaction with carbodiimide helps stop continued breakdown of the polyester. Carbodiimides are made from isocyanates. Their reaction with the carboxylic acid generated in the hydrolysis is illustrated in Figure 2.42. 

Table 1 Reaction of 1-alkylazetidine-2-carboxylates 61 with carbodiimides 62 (Equation 15)...

The first step of the mechanism, the formation of the ion-pair intermediate, explains why the activation of a carboxylic acid is much slower in the presence of a base. Thus, DCC reacts 30-fold more slowly with acetic acid (HOAc) in the presence of triethylamine (9) (TEA) [18]. This lower activation rate in the presence of a base [A,A -diisopropylethylamine (10) (DIEA)] was corroborated when Fmoc-amino acids were activated with DIPCDI in the presence of 1-hydroxybenzotriazole (11) (HOBt) [23] as a trapping agent [24]. However, once the activated species is formed, addition of a base accelerates the coupling reaction [24,25]. When the activation of carboxylic acids with carbodiimides is carried out in a solvent of low dielectric constant such as CHCI3 or DCM, the formation of the O-acylisourea occurs instantaneously [26,27]. In the absence of a nucleophile or a base. 

Figure 14-1 Functionalization and activation of electrode materials, a) silanization of supports bearing hydroxylic groups b) reaction with cyanuric chloride c) activation with cyanogen bromide d) binding via amide bonds after activation with 1,1-carbonyldiimidazole e) activation with good leaving groups such as tresyl chloride f) formation of Schiff bases between aldehyde groups on the support and amino groups at the biomolecule g) activation of carboxylic groups with carbodiimides or h) via the acid chlorides.

Activation of the carboxylic acid moiety of the amino acid is required to be able to react with the Mx-amino group of the growing peptide chain. The first step is the reaction with an electrophile, in some cases in the presence of a base. Carbodiimide-based coupling reagents, such as DCC or DIG (A,A"-diisopropylcarbodiimide), have been used for decades (Figs. 6 and 7). Potential side reactions with carbodiimide-based reagents include the O-to-N rearrangement of the O-acylisourea intermediate and overactivation by formation of the symmetrical anhydride, which can lead to epimer-ization (Fig. 5). These side reactions can be prevented by the addition of auxiliary nucleophiles such as 1-hydroxybenzotriazole... 

The reaction of Ccf -ATPase with dicyclohexylcarbodiimide Carbodiimides readily react in aqueous solutions with protein amino, carboxyl and sulfhydryl groups slower reactions with tyrosine and serine have also been reported [369,370]. The primary reaction product of carboxyl groups with dicyclohexylcarbodiimide is dicyclohexyl-O-acyl isourea [370]. Dicyclohexyl-O-acyl isourea is susceptible to nucleophilic attack either by water or by endogenous or exogenous nucleophiles, yielding a complex series of reaction products [369-371]. 

Figure 9 A synthetic mixture of water-soluble carboxylic acids separated by anion-exchange chromatography. Column 0.3 cm x 300 cm Diaoion CA 08, 16-20 p (Mitsubishi Kasei Kogyo). Eluant 200 mM HC1. Detection reaction with Fe3-benzohy-droxamic acid-dicyclohexy carbodiimide-hydroxylamine perchlorate-triethyl amine with absorbance at 536 nm. Analytes (1) aspartate, (2) gluconate, (3) glucuronate, (4) pyroglutamate, (5) lactate, (6) acetate, (7) tartrate, (8) malate, (9) citrate, (10) succinate, (11) isocitrate, (12) w-butyrate, (13) a-ketoglutarate. (Reprinted with permission from Kasai, Y., Tanimura, T., and Tamura, Z., Anal. Chem., 49, 655, 1977. 1977 Analytical Chemistry).

Figure 1.88 The maleimide group of BMPA reacts with a thiol-containing molecule to result in a modification having a terminal carboxylate group. Amine-containing molecules then can be conjugated to the carboxylate using a carbodiimide reaction with EDC.

Molecules containing phosphate groups, such as the 5 phosphate of oligonucleotides, also may be conjugated to amine-containing molecules by using a carbodiimide-mediated reaction (Chapter 27, Section 2.1). The carbodiimide activates the phosphate to an intermediate phosphate ester similar to its reaction with carboxylates (Chapter 3, Section 1). In the presence of an amine, the ester reacts to form a stable phosphoramidate bond (Reaction 13). 

Carboxylated silica particles may be coupled with amine-containing ligands, such as proteins, using a carbodiimide reaction with EDC. A similar protocol to that previously described for coupling to carboxylate polymer particles may be used. The following protocol is based on the method of Zhao et al. (2004), which was used for immobilizing monoclonal antibodies to E. coli 0157. 

Underivatized PE in liposomal membranes contains an amine group that can participate in the carbodiimide reaction with carboxylate groups on proteins or other molecules (Dunnick et al.,... 

More recently, the fixation efficiency on cotton of Cl Reactive Red 177 (7-43) and its 4-carboxyphenylazo analogue in the presence of various carbodiimides (including 7-44 and 7.45) was investigated, as well as homogeneous reactions of selected carboxylic acids with alcohols (including acetylcellulose in acetone). The carboxylated dye reacted more effectively with cotton cellulose in the presence of cyanamide rather than dicyandiamide,... 

The carboxylic functionalities inserted onto the tubes can be used as platforms to obtain further transformations (Fig. 3.5). A commonly utilized route is the reaction of carboxylic groups with thionyl chloride or oxalyl chloride to prepare the corresponding acyl chlorides, which are useful intermediates for amidation or esterification reactions. Amides can also be prepared directly from the acids by means of standard solution chemistry conditions, using carbodiimide derivatives in the presence of the selected amine. 

Ethyl 5-aminopyrazole -carboxylates are converted into pyrazolo[3,4- / pyrimidines via condensation with Ph3P/Bt2 subsequent reaction with an arylazocyanate and further reacting two carbodiimide with amines. Yields ranged from 13 to 65% dependant on the substituent <2006MI1584, 2007BMCL2203>. A similar approach was used for synthesis of triazolo[4,5- / pyrimidine-7(6)ones from ethyl 4-aminothiazolo-5-carboxylates <2007MIxx>. 

The modification of carboxyl groups has been carried out (1) by esterification with dry methanol and HC1, (2) by esterification with aliphatic diazo compounds, (3) by the formation of adducts with carbodi-imides, or (4) by the formation of amides through activation with carbodiimides. Both complete and, apparently specific, partial modification of the 11 free carboxyl groups have been obtained. In general, the first method suffers from the denaturing medium, the second from incomplete reaction, and the third from the uncertain nature of the products. The fourth procedure is perhaps subject to the least question. There are a total of 11 free carboxyl groups in native RNase-A l (Val), 5/ (Asp), 5y(Glu). A summary of the derivatives is given in Table V. 

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