0-Acylisourea derivatives

N-substituted carbodiimides can react with carboxylic acids to form highly reactive, o-acylisourea derivatives that are extremely short-lived (Reaction 11). This active species then can react with a nucleophile such as a primary amine to form an amide bond (Reaction 12)... 

An alternative mechanism [8] entails reaction of cyanamide (or dicyandiamide) with the dye phosphonate to give an O-acylisourea derivative (7.47). This is able to react directly with cellulose to form dye-fibre bonds, urea being released as the anticipated by-product (Scheme 7.31). In support of this mechanism, it is known that O-acylisourea derivatives of arylcarboxylic acids react readily with alcohols and this constitutes an efficient route for the preparation of carboxylic esters [44]. 

The use of carbodiimides in organic synthesis includes the Moffat oxidation of primary alcohols to aldehydes using a dicyclohexylcarbodiimide/DMSO adduct as reagent. Also, conversion of alcohols or phenols into hydrocarbons via hydrogenation of acylisoureas derived from the corresponding carbodiimide adducts is a useful reaction. Furthermore, aldoximes, on treatment with carbodiimides, are converted into nitriles, and numerous uses of carbodiimides as condensation agents or catalysts are known (see Chapter 13). 

In the presence of weakly acidic additives, such as Ai-hydroxysuccinimide (HOSu),P l 1,2,3-benzotriazol-l-ol (HOBt),f l 3-hydroxy-l,2,3-benzotriazin-4(3//)-one (HODhbt),b l or 7-aza-l,2,3-benzotriazol-l-ol (HOAt),f l the O N acyl migration as well as racemization/ epimerization (vide infra) are largely prevented.f d Concurrently, conversion of the highly reactive O-acylisourea derivatives and symmetrical anhydrides into the related less reactive active esters 13 derived from these additives takes place which leads to still sufficiently activated species to allow rapid in situ amide formation. 

O-acylisourea derivative. From here on two alternative pathways lead to the desired amide. In the first of these the amine component itself attacks the reactive... 

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. 

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. 

The more activated the ester, the less stable is the compound. All the esters mentioned above can be used as shelf-stable reagents except benzotriazolyl esters, which decompose too readily. In addition to their use as activated forms of the A - a I ko x y ca r bo n y I am i n o acids, the esters derived from hydroxamic acids are implicated as intermediates in coupling reactions in which the A-hydroxy compounds have been added to promote efficient coupling between an acid and a primary or secondary amine (see Section 2.10). It is pertinent to mention that the O-acylisourea generated from carbodiimides (see Section 2.02) is an activated ester but one of nature different than those alluded to above. 

The symmetrical anhydride is prepared using dicyclohexylcarbodiimide in dichloromethane, the urea and solvent are removed, and the anhydride is dissolved in dimethylformamide and added to the peptide-resin (see Section 2.5). The anhydride is a more selective acylating agent than the 0-acylisourea and, thus, gives cleaner reactions than do carbodiimides, but twice as much amino-acid derivative is required, so the method is wasteful. It avoids the acid-catalyzed cyclization of terminal glutaminyl to the pyroglutamate (see Section 6.16) and is particularly effective for acylating secondary amines (see Section 8.15). 

This compound removes a proton from the carboxylic acid, producing a cation that is readily attacked by the carboxylate nucleophile across one of the C-N double bonds - the protonated imine behaves as a good electrophile (see Section 7.7.1). The product is now an activated ester (an O-acylisourea) that can be attacked by any available nucleophile. The amino group of the second amino acid derivative provides the nucleophile, resulting in expulsion of a very stable urea as the leaving group, and production of the... 

Support-bound primary or secondary aliphatic alcohols can be acylated under conditions similar to those used in solution, provided that these conditions are compatible with the chosen linker. For instance, acids can be activated with a carbodiimide either as symmetric anhydrides or as O-acylisoureas, which quickly react with alcohols in the presence of a catalyst, such as DMAP or another base, to yield esters (Table 13.12). Further acid derivatives suitable for esterification reactions on solid phase include acyl halides and imidazolides. HOBt esters react only slowly with alcohols, but enable the selective acylation of primary alcohols in the presence of secondary alcohols (Entry 5, Table 13.12). 

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). 

In the first step ester 50 is formed from secondary alcohol 15 and acid 16 Dicyclohexylcarbodiimide (51) activates acid 16 for attack by the nucleophile. First the acid is deprotonated and then the resulting carixjxylate 52 adds to the prolonaled cart xliimide species 53, leading to an O-acylisourea, 54. This actuated ester is very reactive with respect to nucleophiles, and as a result of attack by alcohol 15 the urea derivative 55 is eliminated producing the desired ester 50. 

The O-acylisourea 667 can be trapped by a nucleophile present in the reaction, usually a hydroxylamine derivative to give a more stable reactive ester 668. Racemization can occur involving the enol form of 668. 

Reactions are carried out at room temperature in dichloromethane which is an excellent swelling agent for polystyrene supports. Dimethylformamide can be added to increase the solubility of some amino acid derivatives e.g. Boc-Arg(N02), Boc-Arg(Tos), Boc-His(Tos), Boc-Trp, and Boc-Asn(Xan), but it increases the rate of undesirable rearrangement of O-acylisourea to A-acylurea which is not reactive.P The principal limitations in using car-bodiimides are racemization, rearrangement of O-acylisourea to A-acylurea 5, and dehydration of Asn and Gin side-chain carboxamide groups. Fortunately this dehydration problem is completely avoided by the use of additives such as A-hydroxysuccinimide (HOSu) and 1,2,3-benzotriazol-l-ol (HOBt). Additives also reduce racemization and A-acyl-urea formation. Other carbodiimides have also been used in SPPS such as or N-tert-... 

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." ... 

Formation of the oxazolone occurs more readily because the carbonyl group of amides is more nucleophilic than that of urethanes [22]. Both symmetrical anhydrides and 5(4H)-oxazolone are also acylating reagents. The O-acylisourea can be trapped by a nucleophile present in the medium, predominantly hydroxylamine derivatives (R"R NOH), to give a less reactive but more stable species (8). 

Carbodiimide method, a procedure for peptide bond formation using carbodi-imides, R-N=C=N-R, such as dicyclohexyl carbodiimide (DCC), diisopropyl carbodiimide (DIC) and water-soluble carbodi-imides. The carbodiimide reacts in a one-pot procedure with the carboxylate anion of the carboxy component to form a highly reactive O-acylisourea intermediate. The former reacts immediately with the amino function of the amino component to yield the desired peptide derivative and the urea byproduct. Indeed, a more complex mechanism must be taken into consideration. Unwanted side reactions are racemization via the oxazolone mechanism and formation of the unreactive N-acylurea by base-catalyzed acyl migration from the isourea oxygen to nitrogen. The side reactions can be diminished by preparing the O-acylisourea at 0 °C... 

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