p-cresol as a reversible acylium ion scavenger in solid-phase peptide synthesis

In this work we have defined the nature of the p-cresol and p-thiocresol adducts generated from acylium ions during HF cleavage, following contemporary Boc/benzyl solid-phase peptide synthesis. Contrary to the results in previous reports, we found that both p-cresol and p-thiocresol predominantly form. aryl esters under typical cleavage conditions. Initially we investigated a number of small peptides containing either a single glutamate residue or a C-terminal long-chain amino acid which allowed us to unambiguously characterize the scavenged side products. Whereas, the p-cresol esters are stable at 0 degrees C they rearrange irreversibly at higher temperatures (5-20 degrees C) to form aryl ketones. By contrast, p-thiocresol esters do not undergo a Fries rearrangement but readily undergo further additions of p-thiocresol to form ketenebisthioacetals and trithio ortho esters, even at low temperatures. Importantly, we found by LC/MS and FT-ICR MS analysis that peptides containing p-cresol esters at glutamyl side chains are susceptible to amidation and fragmentation reactions at these sites during standard mild base workup procedures. The significance of these side reactions was further demonstrated in the synthesis of neutrophil immobilization factor, a 26-residue peptide, containing four glutamic acid residues. The side reactions were largely avoided by mild hydrogen peroxide-catalyzed hydrolysis which converted the p-cresol adducts to the free carboxylic acids in near quantitative yield. The choice of p-cresol as a reversible acylium ion scavenger when coupled with the simple workup conditions described is broadly applicable to Boc/benzyl peptide synthesis and will significantly enhance the quality of peptides produced

Synthesis of alpha-aspartyl-containing cyclic peptides

alpha-Aspartyl-containing cyclic pentapeptides were synthesised in high yields using a strategy that maintained fluorenylmethyl protection on the aspartic acid side chain during chain assembly, resin cleavage and cyclisation of the linear precursors. Tetra-n-butylammonium fluoride treatment of the fluorenylmethyl-protected cyclic peptides catalysed imide formation, whereas piperidine-induced deprotection resulted in good yields of the target cyclic peptides

Synthesis of Difficult Cyclic Peptides by Inclusion of a Novel Photolabile Auxiliary in a Ring Contraction Strategy

Cyclic peptides comprise a large and important class of biologically active molecules. They are generally synthesized through amide bond-forming reactions of the C- and N- termini under high dilution conditions. Yields of such processes are highly dependent on the size of the ring being formed and on the particular amino acids of the linear precursor, giving rise to the well-known sequence-dependent effect of cyclization. To overcome this problem, we have developed a peptide cyclization strategy that proceeds through a ring closure/ring contraction process. The linear peptide Ala-Phe-Leu-Pro-Ala, which does not generate monocyclic product under conventional cyclization conditions, was used as a model to probe a range of auxiliaries. This has led to the development of a new photolabile peptide cyclization auxiliary. The 6-nitro-2-hydroxybenzyl group is readily and quantitatively introduced at the N-terminus via a reductive alkylation. Cyclization of the auxiliary-peptide initially proceeds through a cyclic nitrophenyl ester that preorganizes the peptide for lactamization. As the C- and N- termini are in close proximity, lactamization is achieved via an intramolecular O-N acyl transfer step to produce the N-substituted target cycle. The auxiliary is then removed by mild photolysis to produce the target cyclic peptide, cyclo-[Ala-Phe-Leu-Pro-Ala], in good yield. This strategy should find further useful applications in the assembly of libraries of small cyclic peptides