Acyl isourea

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]. 

In a related study by the same authors, the effect of microwave irradiation on car-bodiimide-mediated esterifications on a solid support was investigated, employing benzoic acid [29]. The carboxylic acid was activated using N,N -diisopropylcarbodi-imide (DIC) through the O-acyl isourea or the symmetrical anhydride protocol (Scheme 7.9). The isourea protocol was carried out in a dichloromethane/N,N-di-methylformamide mixture in sealed vessels, whereas the anhydride reactions were carried out in l-methyl-2-pyrrolidinone (NMP) at atmospheric pressure. 

Table VIII lists some commonly used activating groups. That most widely in use is dicyclohexylcarbodiimide (DCC), often in conjunction with additives such as A -hydroxysuccinimide (HONSu) or HOBt. These convert the O-acyl isourea intermediate 12 into the N-acyl derivative 13 (Scheme 14), which is less prone to racemization under the experimental conditions. But it must be emphasized that all such chemical methods involve some racemization of asymmetric centers, and the trick is to reduce this to an absolute minimum.

Another in situ procedure for activating carboxylic acids utilizes earbodiimides, such as dicyclohexylcarbodiimide (DCC). DCC (19) plays an important role in peptide synthesis. Addition of a carboxylic acid to the C-N double bond leads to the activated species, an acyl isourea 20, which upon attack by a nucleophile (and alcohol or an amine) releases the corresponding ester or amide along with 21 (for the mechanism, see Chapter 5). However, in the conversion of 5 to 7 the DCC procedure gives poor results. 

All other carboxylic acid derivatives in Table 6.1, in which the leaving group is bound to the carboxyl carbon through an O atom, are increasingly better acylating agents than carboxylic acid alkyl esters (entry 3) in the order carboxylic acid phenyl ester (entry 4) < acyl isourea (entry 7) < mixed carboxylic acid/carbonic acid anhydride (entry 8) < carboxylic acid anhydride (entry 9) < mixed carboxylic acid anhydride (entry 10). 

Poor nucleophiles react with acyl isoureas B so slowly that the latter start to decompose. In some sense they acylate themselves. The N atom designated with the positional number 3 intramolecularly substitutes the O-bound leaving group that is attached to the carboxyl carbon Cl. A four-membered cyclic tetrahedral intermediate is formed. When the Cl -Ol bond in this intermediate opens up, the N-acyl urea E is produced. Because compound E is an amide derivative it is no longer an acylating agent (cf. Section 6.2). 

In its original form the DCC procedure for peptide synthesis has one basic disadvantage, which you have already learned about as side reaction B —> E in Figure 6.15 When acyl isoureas are exposed to a poor nucleophile, they rearrange to form unreactive A-acyl ureas. These mismatched reactivities characterize the situation encountered in the peptide syntheses in Figures 6.31 and 6.32. The example given in Figure 6.31 uses 0,0-di-fert-butylserine as a... 

B, so-called O-acyl isoureas. To a certain extent, these constitute diaza analogs of the mixed anhydrides B of Figure 6.14. As can therefore be expected, O-acyl isoureas react with good nucleophiles with the same regioselectivity as their oxygen analogs at the carboxyl carbon of the carboxylic acid moiety. 

This mostly minor side reaction takes up major properties if the amino component is hindered or if it is a weak nucleophile. Also, the 0-acyl-isourea derivatives are probably fairly strong bases and this might be the cause of racemization, perhaps through enolization, often observed when DCC was used for the coupling of peptide segments. 

In 0-acyl-isoureas, the intermediates formed in the addition of carboxylic acids to carbodiimides, the N = C group provides powerful activation which leads to coupling ... 

It seems to be reasonable to attribute some basic character to O-acyl-isoureas and therefore general base catalysis... 

The presence of a second nucleophile in the reaction mixture reduces the concentration of the O-acylisourea and thereby the extent of racemization. Also, HOBt, a weak acid, prevents proton abstraction from the chiral carbon atom and thus contributes to the conservation of chiral purity in a second manner as well. Last, but not least the availability of the auxiliary nucleophile (HOBt) efficiently shortens the lifetime of the overactivated O-acyl-isourea intermediate and thus diminishes the extent of O N acyl-migration leading to... 

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