Dicyclohexylcarbodiimide , ester-linked

Fig. 13. Synthesis of (3-linked asparagines glycosides (a) dicyclohexylcarbodiimide (DCC) and l-hydroxybenzotriazole(HOBt) coupling (98), (b) pentafluorophenyl (OPfp) ester of aspartic acid derivative coupling (97).

To circumvent this uncertainty without use of the caesium salt procedure, hydroxymethyl [87] or aminomethyl functions on the polymer can be reacted with carboxylic partners in an esterification reaction [207] or peptide bond formation with the aid of condensing agents like carbonylbisimidazole [88], dicyclohexylcarbodiimide [89] or others. For this purpose in the author s laboratory the use of symmetric anhydrides [3] of the N-protected amino acids to be attached to the support was found to be most effective [90], especially in the formation of activated esters on the gel phase with phenolic hydroxyl functions [38]. By this procedure, on 0.5% cross-linked polystyrenes, load levels up to 1.5 millimoles/g of the support are reached. 

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