Nucleophilic substitution allyl-based protecting groups

Trityl resins are particularly suitable for immobilization of nucleophilic substrates such as acids, alcohols, thiols, and amines. They are quite acid-sensitive and are cleavable even with acetic acid this is useful when acid-labile protecting groups are used. The stability of trityl resin can be tailored by use of substituted arene rings, as shown by chlorotrityl resin, which furnishes a more stable linker than the trityl resin itself. Steric hindrance also prohibits formation of diketopiperazines during the synthesis of peptides. Orthogonality toward allyl-based protective groups was demonstrated in the reverse solid-phase peptide synthesis of oligopeptides [30] (Scheme 6.1.4). 

Allyl carbonate esters are also useful hydroxy-protecting groups and are introduced using allyl chloroformate. A number of Pd-based catalysts for allylic deprotection have been developed.209 They are based on a catalytic cycle in which Pd° reacts by oxidative addition and activates the allylic bond to nucleophilic substitution. Various nucleophiles are effective, including dimedone,210 pentane-2,4-dione,211 and amines.212... 

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