Dicyclohexylcarbodiimide activation

From the very beginning to date dicyclohexylcarbodiimide [89] has been by far the most utilized reagent to activate N-protected amino acids, because of its convenience in use and its very suitable reactivity. Usually the carboxylic component and the lipophilic diimide in equimolar portions are admixtured to the polymer-bound amino component in a three to fivefold excess, in most cases dissolved in dichloromethane, which was found not to favor racemization and to suppress other side reactions during activation. To approach, as closely as possible, the desired complete transformation on polymer, the dissolved compounds are introduced in high concentrations so that the volume depends solely on the swelling rate of the gel phase, which has to be well suspended in the reaction liquid. 

Under those conditions, the highly reactive 0-acyl isourea in a first intermediate stage is formed, initiated with protonation of the diimide by the carboxylic component. 

The amino functions on polymer in a nucleophilic attack react with this acylating intermediate from the moment of its formation. In contrast to the situation in real solution, the reactivity of the amino groups in the gel phase is lowered, leaving time for a branched pathway of side reactions of the 0-acyl isourea. It is attacked by another carboxylic partner forming the symmetric anhydride as a second highly reactive intermediate, which acylates the amino groups on polymer. In both the direct and the indirect acylation, dicyclohexylcarbodiimide loses its condensation potency by transformation into the dicyclohexylurea, which in most of the solvents employed in the synthesis step of the Merrifield procedure is only slightly soluble (Fig. 41). 

As a further consequence of the high reactivity of the excessive 0-acyl isourea in the heterogeneous peptide synthesis mixture, a base-catalyzed intramolecular 0-N-acyl migration takes place, forming the inactive N-acyl dicyclohexylurea. At the same time, this by-product is formed by acylation of already formed still dissolved amounts of dicyclohexylurea, which can be attacked by the symmetric anhydride or the 0-acyl isourea of the carboxylic component as well. Both the N,0-acyl shift and the latter side reaction decrease the total concentration of the activated masked amino acid even N,N -diacyl derivatives of the urea also can be formed as further by-product. Fortunately, all of these acyl urea derivatives are not fixed to the polymer phase and are well soluble in dichloromethane, so that they can be washed out easily from the gel phase after the 

In a shortened synthesis program one can combine the wash-out of the urea derivative with the subsequent deprotection reaction [61]. The acidic reagents used on that step, such as trifluoroacetic acid/dichloromethane or hydrogen-chloride/acetic acid, are excellent solvents for the urea by-product. Under these circumstances, however, the repetition of the actual peptide synthesis step, which eventually enhances its completion, is no longer possible, since the next cycle of operations is already initiated with the N-terminal deprotection of the just elongated sequence. We utilized this time-saving, shortened program successfully in the synthesis of several uncomplicated decapeptide precursors of the cyclic antamanide from Boc-amino acids. 

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Use of the relatively small cyclopropane ring drastically reduces the potential for deleterious steric bulk effects and adds only a relatively small lipophilic increment to the partition coefficient of the drug. One of the clever elements of the rolicyprine synthesis itself is the reaction of d,l tranylcypromine (67) with L-5-pyrrolidone-2-carboxylic acid (derived from glutamic acid) to form a highly crystalline diastereomeric salt, thereby effecting resolution. Addition of dicyclohexylcarbodiimide activates the carboxyl group to nucleophilic attack by the primary amine thus forming the amide rolicyprine (68). 

Then another N-protected amino acid is coupled to the free amino group of the polymer-bound substrate using the dicyclohexylcarbodiimide activation or the active ester method. The N-deblocking and coupling steps are repeated until the desired sequence is formed. Finally the resin-peptide bond is split by a suitable acid cleavage reaction with HBr—AcOH, trifluoroacetic acid or HF. This results in a simultaneous N-deblocking and deprotection of most of the side-chain functionalities. 

Mild oxidation of primary and secondary alcohols, promoted by dicyclohexylcarbodiimide activation of dimethyl sulfoxide, evidently involving the alkoxysulfonium ylides, which rearrange intramolecularly to generate aldehydes and ketones, respectively ... 

The most dangerous contamination of the peptide fixed to the gel phase exists in the incorporation of the specific amino acid racemized to a small extent by the method of C-terminal activation. In several investigations it was demonstrated, however, that dicyclohexylcarbodiimid activation in dichloromethane of urethane-protected amino acids — with the exception of histidine — beyond room temperature level does not cause significant racemization [210], which is enhanced, however, in dimethylformamide as solvent. In cases of apprehended racemization, additives like N-hydroxysuccinimide [43] or - more recently explored - 1-hydroxy-benzotriazole [44, 139,140] are in use, which admixtured in two equivalents per one of the diimide, intermediately form the activated ester with a considerably diminished tendency to racemize. This technique will be described in Sect. 3.3.5, since it was shown to be a helpful method to activate even peptide fragments without racemization. 

Although dicyclohexylcarbodiimide activation to date is still the widely used coupling method on polymer phase, we searched for an alternate to circumvent all the problems combined with diimide side reactions and the formation of the scarcely soluble dicyclohexylurea. For this purpose we reinvestigated the direct synthesis of synunetric anhydrides from N-protected amino acids with phosgene [142-145] (Fig. 43). 

With the dicyclohexylcarbodiimide (DCQ reagent racemization is more pronounced in polar solvents such as DMF than in CHjCl2, for example. An efficient method for reduction of racemization in coupling with DCC is to use additives such as N-hydroxysuccinimide or l-hydroxybenzotriazole. A possible explanation for this effect of nucleophilic additives is that they compete with the amino component for the acyl group to form active esters, which in turn reaa without racemization. There are some other condensation agents (e.g. 2-ethyl-7-hydroxybenz[d]isoxazolium and l-ethoxycarbonyl-2-ethoxy-l,2-dihydroquinoline) that have been found not to lead to significant racemization. They have, however, not been widely tested in peptide synthesis. 

The most intensively studied oxidizing system is that developed by Pfitzner and Moflatt in which the oxidation is carried out at room temperature in the presence of dicyclohexylcarbodiimide (DCC) and a weak acid such as pyridinium trifluoroacetate or phosphoric acid. The DCC activates the DMSO which in turn reacts with the carbinol to give an oxysulfonium intermediate. This breaks down under mild base catalysis to give the desired ketone and dimethyl sulfide. 

Schemes are available, however, that start from the free carboxylic acid, plus an activator . Dicyclohexylcarbodiimide, DCC, has been extensively employed as a promoter in esterification reactions, and in protein chemistry for peptide bond formation [187]. Although the reagent is toxic, and a stoichiometric concentration or more is necessary, this procedure is very useful, especially when a new derivative is targeted. The reaction usually proceeds at room temperature, is not subject to steric hindrance, and the conditions are mild, so that several types of functional groups can be employed, including acid-sensitive unsaturated acyl groups. In combination with 4-pyrrolidinonepyridine, this reagent has been employed for the preparation of long-chain fatty esters of cellulose from carboxylic acids, as depicted in Fig. 5 [166,185,188] ...

The Ca transport and Ca -stimulated ATPase activity of sarcoplasmic reticulum is inhibited by 10-30nmol dicyclohexylcarbodiimide per mg protein in a Ca free medium [372]. A23187 enhanced the sensitivity of the enzyme to DCCD, while Ca or Sr at micromolar concentrations prevented the inhibition. Since Ca -loaded vesicles retained their sensitivity to DCCD in a Ca -free medium, the reactivity of the enzyme with DCCD is controlled by the occupancy of the high-affinity Ca sites on the cytoplasmic surface of the membrane. 

Further evidence that carboxyl groups are important for transport activity was provided by Igarashi and Aronson [22], Friedrich et al. [23], and Kinsella et al. [24] using the carboxyl group-specific reagent, A,A -dicyclohexylcarbodiimide (DCCD). DCCD irreversibly inactivated the brush border Na /H exchanger in rabbit and... 

A great deal of work has been done on the in situ activation of carboxylic acids toward nucleophilic substitution by amines. This type of reaction is fundamental for synthesis of polypeptides (see also Section 13.3.1). Dicyclohexylcarbodiimide... 

All of the compounds in the previous sections containing C-O bonds to be hydrogenolyzed easily were activated at the carbon atom. However, O-alkyl-N,N -dicyclohexylisoureas (11), which can be prepared by the reaction of alcohols with dicyclohexylcarbodiimide, can also be hydrogenolyzed but they are activated on the oxygen atom (Scheme 4.61).273... 

Similar Suzuki couplings have been performed by Hu and coworkers utilizing a poly(dicyclohexylcarbodiimide)/palladium nanoparticle composite [152]. This PDHC-Pd catalyst showed remarkable activity and stability under microwave irradiation. Near quantitative conversion (95% isolated yield) was obtained after 40 min of microwave heating of a mixture of iodobenzene with phenylboronic acid in dioxane. Re-using the immobilized catalyzed showed no significant loss of efficiency, as the fifth cycle still furnished a 90% isolated yield of the desired biphenyl. 

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.

The condensation of DMDP with carbon disulfide/dicyclohexylcarbodiimide (DCC) afforded the indolizine derivative—potentially biological active thionocarbamate on carbohydrate scaffold, fused from the nitrogen side. 

JE Zimmerman, GW Callahan. The effect of active ester components on racemization in the synthesis of peptides by the dicyclohexylcarbodiimide method. J Am Chem Soc 89, 7151, 1967. 

W Konig, R Geiger. A new method for the synthesis of peptides activation of the carboxyl group with dicyclohexylcarbodiimide and 1-hydroxybenzotriazoles. Chem Ber 103, 788, 1970. 

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