Product of the Month: Fmoc-L-Cys(Allocam)-OH for 1-Step On-Resin Disulfide Bond Formation

Product of the Month: Fmoc-L-Cys(Allocam)-OH for 1-Step On-Resin Disulfide Bond Formation

Published on 10/09/2019

The Allocam protecting group allows for selective deprotection and disulfide bond formation on-resin in one step.
The synthesis of disulfide-bridged peptides is a well-known challenge in peptide chemistry. When performed in solution, high dilutions have to be used in order to avoid intermolecular SS-bond formation. The on-resin formation of disulfide bonds has the advantage to favor intramolecular disulfide formation. Key on-resin methods include the treatment of peptides bearing (Ph)Acm-, Trt- or Mmt-protected cysteines with excess iodine, which may be problematic if electron-rich protecting groups are present (Postma and Albericio 2014, Eur. J. Org. Chem.). Acidic cleavage of benzyl-type protecting groups such as Tmob, and subsequent oxidation of the resulting free thiols (e.g. using DMSO) also affords the corresponding disulfide bond. Another rather labor-intensive method for on-resin disulfide formation starts with the reductive cleavage of an StBu group, followed by oxidation with 2,2′-dithiobis(5-nitropyridine) (DTNP) to form RS–SNP. A single Mmt group is then cleaved under acidic conditions, and the resulting free thiol forms the desired SS-bond by microwave-assisted displacement of the SNP group (Galanis et al. 2009, Pept. Sci.).
On Resin SS-Bond Formation

Key methods for on-resin disulfide bond formation.

Allocam Catalytic Cycle The Allocam-protecting group, which is a variant of the Acm protecting group, allows for a palladium-mediated single-step approach using mild reaction conditions and readily available reagents. In a recently published synthesis of the C-carboxy analog of oxytocin (Kondasinghe et al. 2017, Org. Biomol. Chem.), complete conversion was achieved on Aminomethyl ChemMatrix® resin using 1.5 equivalents of Pd(OAc) 2 as catalyst, together with 3% NMM as well as 5% AcOH (v/v). No reduced peptide intermediates or S-allylated products were observed, and no residual palladium was detected in the final product. The reaction proceeds using the protected resin-bound peptide without loss of any acid-labile side chain protecting groups. By using Fmoc-L-Cys(Allocam)-OH, solution-phase manipulations of the peptide can be avoided, and the number of HPLC purification steps can be minimized.

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