Carbodiimide coupling procedure

H Hagenmeier, H Frank. Increased coupling yields in solid phase peptide synthesis with a modified carbodiimide coupling procedure. Hoppe-Seyler s Z Physiol Chem 353, 1973, 1972. 

Symmetrical anhydrides are stable to washing by dilute add or aqueous sodium hy-drogencarbonate at room temperature and, consequently, can readily be obtained pure using water-soluble carbodiimide.b l Furthermore, when pure, most of the crystaUine products are stable at room temperature. The symmetrical anhydride method was reported by Chen et al.f to give superior yields of peptides as compared to the normal carbodiimide coupling procedure on a sohd phase and to be superior to the mixed anhydride method in solution. 

Schiltz and Reinbolt, 1975) combined the diisothiocyanate and carbodiimide procedures to permit sequencing of peptides that lack lysine. The peptide is first coupled by its NH2-terminus to aminopolystyrene using p-phenylene diisothiocyanate. With the peptide bound to the resin, the COOH-terminal is then coupled using a carbodiimide. This procedure is reported to be useful for larger peptides that do not couple efficiently with the carbodiimide alone (Schiltz, 1975). 

In 2010, Bao and Shen reported a novel and efficient procedure for the synthesis of benzoxazole and benzimidazole derivatives. The one-pot cascade process is based on Cul under ligand-free conditions. A variety of carbodiimides coupled with o-halophenols or o-haloanilines to give the products in moderate to excellent yields under the mild conditions (Scheme 2.134). 

PGIP, purified fi om P.vulgaris hypocotyls [11], was immobilized to the sensor ch via amine coupling. A continuous flow of HBS buffer (5 pl/min) was mantained over the sensor surface. The carboxylated dextran matrix of the sensor surface was first activated by a 6-min injection of a mixture of N-hydroxy-succinimide and N-ethyl-N - (3-diethylaminopropyl) carbodiimide, followed by a 7-min injection of PGIP (lOng/pl in 10 mM acetate, pH 5.0). Hie immobilization procedure was con leted by a 7-min injection of 1 M ethanolamine hydrochloride to block the remaining ester groups. 

The presence of unwanted carboxylate groups in macromolecules may be easily blocked by the use of a small amine-containing molecule coupled via the carbodiimide procedure (Chapter 3, Section 1). 

Amino acid derivatives can be examined for enantiomeric purity by the same procedures after removal of the protecting groups. Another approach is to couple them directly with another derivative to give protected dipeptides whose diastereomeric forms are usually easy to separate by HPLC (see Section 4.11). An A-protected amino acid is coupled with an amino acid ester, and vice versa. Use of soluble carbodiimide as reagent (see Section 1.16), followed by aqueous washes, gives clean HPLC profiles. It is understood that the derivative that serves as reagent must have been demonstrated to be enantiomerically pure.43 84-89... 

A procedure widely used for coupling ligands to supports containing spacer groups is the carbodiimide method.52,53 Many different carbohydrate derivatives have been coupled to supports in this way. A variety of diimides has been employed in these couplings, and these include dicyclohexylcarbodiimide (DCC), 3-(3-dimethylaminopro-... 

The carbodiimide method has been employed in several syntheses of depsipeptides. However, direct application of DCC for the formation of the ester bond between the amino acid and hydroxy acid components under the usual conditions of amide coupling affords the desired depsipeptides in acceptable yields only in the case of unhindered co-hydroxy units [54] or an active hydroxy group, such as in TV-benzoyl-u-hydroxyglycine benzyl ester. For example, Ravdel et al.[55 have performed the esterification of various benzyloxycarbonyl- and phthalylamino acids with /V-benzoyl-a-hydroxyglycine benzyl ester with DCC in 50-65% yield. On the other hand, Shemyakin et all21 failed to obtain the expected depsipeptide products on condensation of bulky benzyloxycarbonyl- or phthalylvaline with a-hydroxy-isovaleric acid benzyl ester. The main product was acylurea in the first case and phthalylvaline anhydride in the second. Thus, the classical carbodiimide procedure could not be applied in practical depsipeptide preparation. 

---