N-Hydroxysuccinimide-Derived Polymers

The cross-linked polymer 27 (cross-linker not represented) containing the HOSu residue for anchored DCC-promoted active ester formation was also obtained by copolymerization of N-acetoxymaleimide with styrene and divinylbenzene, followed by hydrolysis [102]. In addition, supported HOSu-active esters have been prepared by a method consisting of the synthesis of N-hydroxymaleimide esters of N-protected amino acids and subsequent copolymerization of these esters with styrene, divinylbenzene and 4-chlorophenylmaleimide [103]. 

Active esters obtained from 1-hydroxybenzotriazole (HOBt) are considered more reactive than those from other common active ester-forming agents. Thus, polymeric HOBt active esters appear to be about 100 times more reactive than the corresponding polymeric nitrophenol esters. However, this value rises to 8000 for their soluble analogues [90], thus showing the lowering of reactivity when active esters are solid-supported. 

Polystyrene-bound 1-hydroxybenzotriazole (20) [41] (Section 7.3) was originally prepared by Friedel-Crafts alkylation of macroporous polystyrene or copolysty-rene-2% divinylbenzene using 3-nitro-4-chlorobenzyl bromide or 3-nitro-4-chloro-benzyl alcohols in the presence of aluminium trichloride. The so-obtained 

Other examples of the use of the polymeric esters from P-HOBt (20) for the preparation of peptides after reaction with amino group-free amino acids or pep- 

A polymeric HOBt (132), which is soluble in dichloromethane, DMF or water and bears a poly (ethylene glycol) (PEG) chain (average molecular wt 4000), has been prepared from PEG-NH2 and l-hydroxybenzotriazole-5-carboxylic acid using DCC as coupling agent [135]. The corresponding active esters were prepared by reaction with an excess of carboxylic acid anhydrides and used for the synthesis of the model tetrapeptide Cbz-Leu-Ala-Gly-Val-OtBu [136]. 

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