Fmoc-amino acids , anchoring

With the C-terminal residue introduced as part of the BAL anchor and the penultimate residue incorporated successfully by the optimized acylation conditions just described, further stepwise chain elongation by addition of Fmoc-amino acids generally proceeded normally by any of a variety of peptide synthesis protocols. 

During the first decade when solid-phase synthesis was executed using Fmoc/tBu chemistry, the first Fmoc-amino acid was anchored to the support by reaction of the symmetrical anhydride with the hydroxymethylphenyl group of the linker or support. Because this is an esterification reaction that does not occur readily, 4-dimethylaminopyridine was employed as catalyst. The basic catalyst caused up to 6% enantiomerization of the activated residue (see Section 4.19). Diminution of the amount of catalyst to one-tenth of an equivalent (Figure 5.21, A) reduced the isomerization substantially but did not suppress it completely. As a consequence, the products synthesized during that decade were usually contaminated with a small amount of the epimer. In addition, the basic catalyst was responsible for a second side reaction namely, the premature removal of Fmoc protector, which led to loading of some dimer of the first residue. Nothing could be done about the situation,... 

FIGURE 5.21 Methods for anchoring an Fmoc-amino acid to the hydroxymethyl group of a linker-resin. (A) 4-Dimethylaminopyridine-catalyzed acylation by the symmetrical anhydride.19 (B) Acylation by a mixed anhydride obtained from 2,6-dichlorobenzoyl chloride.39 (C) Acylation by the acid fluoride.50 (D) Dicyclohexylcarbodiimide-mediated acylation in the presence of 1-hydroxybenzotriazole.52... 

A Grandas, X Jorba, E Giralt, E Pedroso. Anchoring of Fmoc-amino acids to hydroxymethyl resins. Int J Pept Prot Res 33, 386, 1987. 

K Barlos, O Chatzi, D Gatos, G Stavropoulus. 2-Chlorotrityl chloride resin. Studies on anchoring of Fmoc-amino acids and peptide cleavage. Int J Pept Prot Res 37,513,... 

K Akaji, H Tanaka, H Itoh, J Imai, Y Fujiwara, T Kimura, Y Kiso. Fluoren-9-y lmethy loxycarbony 1 (Fmoc) amino acid chloride as an efficient reagent for anchoring Fmoc amino acid to 4-alkoxybenzyl resin. Chem Pharm Bull (Jpn) 38, 3471, 1990. 

Barlos, K., Chatzi, O., Gatos, D., and Stavropoulos, G. (1991) 2-Chlorotrityl chloride resin—Studies on Anchoring of Fmoc-Amino Acids and Peptide Cleavage. Int. J. Pept. Protein Res. 37,513-520. 

The allylic HYCRAM derivative was subsequently modified by insertion of a standard amino acid between the aminomethyl resin and the hydroxy butenoic acid moiety. Using this allylic anchor, the resin-linked, glycosylated HIV peptide T-derivative 164 was synthesized by application of Fmoc amino protection and sidechain protection with lert-butyl groups. The lac-tosamine peptide T (165) could be released from the resin by application of the palladium(0)-catalyzed allyl-transfer reaction to V-methyl aniline as the allyl acceptor. 

FIGURE 5.17 Resins and linkers for synthesis of peptides using Fmoc/tBu chemistry. The linkers are secured to supports by reaction with aminomethyl resins. A protected amino acid is anchored to the support as an ester by reaction with a hydroxyl or chloro group (italicized). The alkoxy and phenyl substituents render the benzyl esters sensitive to the cleavage reagents. 

Commonly used resins in Fmoc/tBu strategy for the synthesis of C-terminal peptide acid are reported in Fig. 5 (11-14). Anchoring reactions must be performed in an anhydrous medium and amino acids containing water should be dried before use. 

The use of phenacyl handles 15 is only compatible with the Boc/Bzl strategy since the peptide-resin anchor is not stable to the nucleophilic conditions used in the Fmoc/tBu strategy. Besides the formation of piperazine-2,5-diones, another side reaction can take place on incorporation of the second residue. Attack by the free amino group of the first amino acid on the carbonyl group of the phenacyl resin leads to a cyclic Schiff base.P Both types of side reaction can be minimized using the in situ neutralization protocol. Photolysis is carried out as for nitrobenzyl resins. 

When it comes to peptides several approaches can be envisioned. The first to be published has been related using the Fmoc strategy in both direct and reverse synthesis [141], Anchoring strategy relied on the esterification of a benzhydril functionalized linker using Fmoc protected amino acids. Supported protected amino acids were obtained as a mixture of chloride and bromide salts which confers... 

PAC/HMPP supports (see Section V) with disuccinimidyl carbonate (DSC) and used to anchor amino acids as carbamates [106]. After further chain elongations using the Fmoc strategy, lactamization reactions were conducted on resin and the resulting cyclic peptides cleaved by treatment with TFA in the presence of cation scavengers. 

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