Side trityl-based linkers

Of those in common usage, the 2-chlorotrityl 28 [74] and 4-carboxytrityl 29 [75] linkers give the least stabilized cations and are suitable for immobilization of carboxylic acids. They are ideal for Fmoc/tBu-based SPPS as their use avoids many of the side reactions that occur with standard benzyl-based linkers. Firstly, race-mization does not occur during loading of the resin with the C-terminal residue [79], as is the case with esterification to hydroxy-functionalized resins. Secondly, the bulky trityl cation does not cause alkylation side reactions with nucleophilic amino-acid side-chains. Thirdly, cysteine does not undergo racemization [80, 81]... 

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