Direct condensation coupling reaction

In the case of the stepwise synthesis of peptide fragments starting from an amino acid without C-terminal protection, peptide bond forming methods are limited to the mixed anhydride method,the active ester method, and the azide methodJ l Although such peptide fragments can be directly used for the segment condensation reaction, the purification of the products after each coupling reaction is not so easy. 

Other auxiliary nucleophiles can be used in combination with DCC. The most important ones include 7-aza-l,2,3-benzotriazol-l-ol (7, HOAt)f l and 3-hydroxy-l,2,3-benzotriazin-4(3//)-one (8, HODhbt) which will be described in the section concerning the fragment condensation (Section 4.3.2.2). The use of isolated 3-hydroxy-l,2,3-benzotriazin-4(3//)-ones (8, HODhbt)f l will be discussed under active esters, (Section 4.3.3.1.5). The use of HOAt in peptide chemistry has been discussed by Carpino.f It decreases the time course of coupling reactions, reduces the loss of chiral integrity, and provides a visual indication, yellow to colorless, of the reaction endpoint in solution. HOAt was shown to be a superior coupling additive in both solutiont and solid-phase synthesesf when compared with HOBt. HOAt can be used as a direct substitute for HOBt. Furthermore, HOAt like HOBt is compatible with a variety of activation methods. The performance of HOAt derivatives was compared... 

Phosphodiesteric/peptidic/glycosidic bond formation (coupling reaction) is a nucleophilic substitution reaction of a hydroxy/amino/hydroxy group at a phosphoesteric/carboxyl/ acetal group, resulting in dehydration. The coupling reaction can be conducted in two procedures activation-substitution (via stable activated intermediate) and direct condensation... 

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