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Fmoc-Asn(Trt)-OH prevents the possibility of the amide side chain from undergoing dehydration side reactions during activation, especially with carbodiimide reagents. Fmoc-Asn(Trt)-OH dissolves readily in all standard peptide synthesis reagents, while Fmoc-Asn-OH is only somewhat soluble in NMP or DMF. The trityl group is removed with TFA. This reaction is usually complete within 1 hour at room temperature, but is slower when the Asn(Trt) residue in at the N-terminal of the peptide. In these cases, the deprotection reaction should be extended to 2 hours.
Fmoc-Asp(OtBu)-OH is the standard Fmoc-protected derivative of aspartic acid used in peptide synthesis. The t-butyl ester of the side chain is readily hydrolyzed in the same conditions used to cleave peptides from Wang resin or Rink amide resin. The t-butyl cations formed in the hydrolysis of the esters can produce byproducts by reattaching to nucleophilic residues in the peptide, so nucleophilic scavengers such as thiophenol and anisole should be added to the cleavage mixture when Fmoc-Asp(OtBu)-OH is utilized.