Amines isoureas

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 3.1 EDC reacts with carboxylic acids to create an active-ester intermediate. In the presence of an amine nucleophile, an amide bond is formed with release of an isourea by-product.

Cyanogen bromide can be used to activate hydroxyl groups on particles to create reactive cyanate esters, which then can be coupled to amine-containing ligands to form an isourea bond (Figure 14.17). CNBr activation also can produce cyclic imidocarbonate groups, which are less reactive than the cyanate ester, but can form imidocarbonate bonds. The exact reactive species formed by the reaction is dependent on the structure of the hydroxylic support being activated (Kohn and Wilchek, 1982). 

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. 

That being said, it must be recognized that the evidence that the (V-acy I isourea is the precursor of the 2-alkoxy-5(4//)-oxazolone is only circumstantial because experiments starting from the former have yet to be achieved. The oxazolone could theoretically come from the symmetrical anhydride. The latter generates 2-alkoxy-5(4//)-oxazolone in the presence of tertiary amines (see Section 4.16) even dicy-clohexylcarbodiimide (DCC) was basic enough to generate 2-t< rt-butoxy-5(4 )-oxazolone from Boc-valine anhydride. However the weight of evidence points to O-acylisourea as the precursor of the 2-alkoxy-5(4//)-oxazolone. In the absence of. V-nucleophile, such as in the preparation of esters, the major precursor of product is the symmetrical anhydride.7,8... 

This section is new and only one article appeared in this area. The reaction of iV-/-butyl-iV -pyridazin-3-ylcarbodi-imide with amines, thiols, and alcohols was studied by Rakowitz and co-workers and yielded respectively novel guanidines, isothioureas, and isoureas <2002JHC695>. 

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. 

The selected example by Sim and Ganesan [19] reported the design of a 3078-member discrete thiohydantoin library, prepared without purification of intermediates, with a three-step synthesis, which is shown in Figure 7.2. Nine amino acid esters were reacted with eighteen aromatic aldehydes to produce imines, which were reduced by sodium triacetoxyborohydride. The resulting amines were treated with nineteen isothiocyanates in the presence of triethylamine (TEA), producing intermediate isoureas, which eventually cyclized to the final thiohydantoins. 

O-tert-Butyl trichloroacetimidate, prepared in 70% yield by reacting potassium rerr-butoxide with trichloroacetonitrile, reacts with carboxylic acids and alcohols in the presence of a catalytic amount of boron trifluoride etherate at room temperature in cyclohexane-dichloromethane [Scheme 6.35], 7 The method also converts alcohols to ferr-butyl ethers (see section 4.3.2). A very similar reaction that allows /erf-butylation under essentially neutral conditions on a large scale involves reaction of a carboxylic acid with 3-4 equivalents of JV,N -di-isopropyl-Orerf-butylisourea88 [Scheme 6,36].56S9 The reaction proceeds via a tertiary carbocation ion intermediate and since capture of the cation is inefficient, excess isourea is required. The presence of alcohols is tolerated but not thiols or unhindered amines. The reaction conditions are compatible with a range of acid sensitive groups such as AMrityl derivatives and cydopentylidene acetals.90... 

Although a variety of primary and secondary amines can be utilized as nitrogen nucleophiles, amina-tion of allylic substrates with ammonia has been reported to be unsuccessful. Therefore, bis(p-methoxy-phenyDmethylamine, sodium p-toluenesulfonamide or sodium azide have been utilized instead of ammonia. In the presence of a palladium catalyst, imides, such as phthalimide, react with allylic esters with the exception of geranyl and linalyl acetates. 0-Geranyl- and 0-linalyl-isourea derivatives, however, reacted with phthalimide giving the expected -allylic phthalimides in 65% and 71% yields, respectively. ... 

On the other hand, imidothiocarbonates 178 react with sulfamoyl chloride to give the sulfamide 179. Reaction with one equivalent of primary amine provides the isourea 180, which can be readily cyclized by refluxing with trimethyl orthoacetate or trimethyl orthoformate to give 177 (R = Me or H, respectively) (Scheme 65). 

Traditionally in biochemistry (e.g. for affinity chromatography), enzymes have been covalently linked to carbohydrates such as agarose and dextran (Sepharose, Sephadex). The most widely used activation method is the cyanogen bromide (CNBr) method, yielding isourea and imidocarbonate functionalities that react with amines on the enzyme to produce N-substituted carbamate linkages. 

Guanidines from amines and isoureas Protection of a-amino acid groups... 

A -substituted isourea, which is capable of protonation in the neutral and alkaline region. In the presence of primary amines and ammonia this linkage is unstable [98]. This is the primary flaw of this method when it is used for a single attachment. 

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