Isoureas carbodiimides

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]. 

Carbodiimides are used to mediate the formation of amide or phosphoramidate linkages between a carboxylate and an amine or a phosphate and an amine, respectively (Hoare and Koshland, 1966 Chu et al., 1986 Ghosh et al., 1990). Regardless of the type of carbodiimide, the reaction proceeds by the formation of an intermediate o-acylisourea that is highly reactive and short-lived in aqueous environments. The attack of an amine nucleophile on the carbonyl group of this ester results in the loss an isourea derivative and formation of an amide bond (see Reactions 11 and 12). The major competing reaction in water is hydrolysis. 

Figure 3.4 The water-soluble carbodiimide CMC reacts with carboxylates to form an active-ester intermediate. In the presence of amine-containing molecules, amide bond formation can take place with release of an isourea by-product.

The reactions involved in an EDC-mediated conjugation are discussed in Chapter 3, Section 1.1 (Note EDC is l-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride MW 191.7 and is sometimes referred to as EDAC). The carbodiimide first reacts with available carboxylic groups on either the carrier or hapten to form a highly reactive o-acylisourea intermediate. The activated carboxylic group then can react with a primary amine to form an amide bond, with release of the EDC mediator as a soluble isourea derivative. The reaction is quite efficient with no more than 2 hours required for it to go to completion and form a conjugated immunogen. 

In an extension beyond hetaryl onium salt promoted hemiacetal activation, Ishido and coworkers have reported the carbodiimide activation of hemiacetals [141]. In the method (Scheme 3.13), the hemiacetal donor 1 is treated with a carbodiimide electrophile 83 and copper(I) chloride to provide glycosyl isourea intermediate 85. Highly susceptible to hydrolysis, the isourea 85 was not isolated but could be detected by 13C NMR and IR spectroscopy [142,143], Accordingly, the reaction between intermediate 85 and the glycosyl acceptor (NuH) provides glycoside product 3, along with urea by-product 84. 

A typical procedure calls for reaction of the hemiacetal donor with dicydohexyl carbodiimide and copper(I) chloride (0.1 equiv) at 80 °C, followed by an addition of the acceptor and continued heating. As an early demonstration of this protocol, oc-riboside 86 was prepared in moderate yield but with exclusive stereoselectivity [141]. Further measures were required for the glycosylation of monosaccharide acceptors, such as addition of p-toluenesulfonic add (0.1 equiv) to promote the formation of disaccharide 87 [144]. The method was more suitably applied to the synthesis of O-acyl glycopeptides, as evidenced by the formation of 88 in 60% yield [143,144]. Various peptides with non-nudeophilic side chains were found to be amenable to this stereoselective reaction. The [3-selectivity was suggested to arise from a preponderance of the a-isourea intermediate 85 in the activation step. 

These problems were solved by Miller,[104 who developed a direct isourea-mediated 3-elimination process to prepare DHA derivatives. The reaction involves treatment of a Ser (or Thr) peptide 30 with a carbodiimide (1.1 equivalents) in the presence of copper(I) chloride as a catalyst to give a AAla (or AAbu) derivative 31, respectively, in good yield (Scheme 11). 

In a related study by the same authors, the effect of microwave irradiation on carbodiimide-mediated esterifications on solid support has been investigated employing benzoic acid28. Activation of the carboxylic acid was carried out using diisopropy-lcarbodiimide (DIC) via the O-acyl isourea or the symmetrical anhydride protocol, respectively (Scheme 7.8). 

The combination of carboxyl activation by a carbodiimide and catalysis by DMAP provides a useful method for in situ activation of carboxylic acids for reaction with alcohols.10 The reaction proceeds at room temperature. Carbodiimides are widely applied in the synthesis of polypeptides from amino acids. The proposed mechanism for this esterification reaction involves activation of the acid via isourea 28 followed by reaction with another acid molecule to form anhydride... 

Thermolysis of S-methyl-N-alkyl or aryl-N -cyanoisothiourea34 (R=Ph, Et, cyclohexyl) also generates the corresponding carbodiimides 35, which were trapped with methanol to give the isourea derivatives 36. ... 

The addncts derived from carbodiimides and nncleophiles also undergo thermal elimination reactions to regenerate the carbodiimide, and they are therefore of limited preparative value. For example, the elimination of hydrogen chloride from carbonimidoyl dichlorides is used in the synthesis of arenesulfonylcarbodiimides (see Section 9.2.2). Of course, isoureas and isothioureas also undergo elimination reactions. When the elimination of isothioureas is conducted in the presence of heavy metal ions, the insoluble metal mercaptides are precipitated to facilitate the in situ generation of the carbodiimides. 

In the reaction of hydroxyalkynes 457 with carbodiimides, oxazolidines 458 are isolated, resulting from cyclization of the initially formed isoureas. ... 

Ketoximes 470 and aldoximes react similarly to alcohols with carbodiimides to give isourea derivatives 471. The reaction can be conducted in the presence of NaOH, hydrofluoric acid," or copper salts." ... 

Cyclic hydroxyimides 482 react with carbodiimides to form the expected isourea derivative, which undergoes further reaction with the starting material to give the isolated aminoacid derivative 483. 

Hemiacetals can be activated for glycosylations by reaction with carbodiimides via glycosyl isourea intermediates and different aryl glycosides were prepared this way [432]. 

Triphenyllead(IV) methoxide forms a similar stable adduct with di-l-naphthyl-carbodiimide and catalyzes the addition of methanol to the carbodiimide. Two of the four Ti-0 bonds in titanium(IV) tetraisopropoxide add to di-p-tolylcarbodiimide to give the bis(isourea) ether . 

RCO2H, R OH, DCC/DMAP, EtjO, 25°C, l-24h, 70-95% yield. This method is suitable for a large variety of hindered and unhindered acids and alcohols. The use of Sc(OTf)3 as a cocatalyst improves the esterification of 3° alcohols. Carboxylic acids that can form ketenes with DCC react preferentially with aliphatic alcohols in the presence of phenols whereas those that do not show the opposite selectivity. In some sterically congested situations the 0-acyl urea will migrate to an unreactive A-acyl urea in competition with esterification. Carbodi-imide I was developed to make the urea by-product water soluble and thus easily washed out. Isoureas are prepared from a carbodiimide and an alcohol which upon reaction with a carboxylic acid give esters in excellent yield. A polymer supported version of this process has been developed. This process has been reviewed. Note that DCC is a potent skin irritant in some individuals. 

Carbodiimides are traditionally one of the most frequently employed dehydrating reagents for the creation of the ester or amide bond, despite known problems related to difficulties in removing the corresponding generated urea or unreactive acylurea by-products [2]. Therefore, the use of polymer-supported carbodiimides is quite appropriate for the easy elimination of these by-products from the reaction medium by simple filtration, as they will remain anchored to the solid phase. In addition, if an ester or amide formation is intended an excess of acid can be used, as the remainder will also stay anchored as the corresponding isourea. 

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