Problems in the synthesis of cyclic peptides through use of the Dmab protecting group †

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Abstract
The preparation of side-chain-to-side-chain-cyclised peptides through lactam bridge formation requires orthogonal protecting groups for side-chain amino and carboxylate functionalities. Use of the 4-{N-[1-(4,4-dimethyl-2,6-dioxocyclohexylidene)-3-methylbutyl]amino}benzyl ester (Dmab) group for this role in the protection of the glutamyl side-chain resulted in the formation of unexpected side-products. During synthesis of fully protected peptide targets, Nα-pyroglutamyl chain-terminated peptides were observed. Pyroglutamyl peptides were not observed in analogous peptides synthesised using the traditional tert-butyl ester protecting group. Selective removal of the Dmab group proceeds through a two-stage procedure, hydrazinolytic cleavage of the dimedone moiety followed by 1,6-elimination of the resulting peptide-glutamyl 4-aminobenzyl ester. The latter reaction is sufficiently slow to allow isolation of the transiently stable glutamyl derived 4-aminobenzyl ester peptide. Attempted side-chain-to-side-chain cyclisation (through orthogonally protected Glu and Lys residues) of peptides prepared via Glu(ODmab) failed and led to modification of the Lys Nε-amino group when 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HBTU) was used as carboxy-activating reagent. Analogous peptides prepared utilising allyl side-chain protection for glutamyl residues were successfully cyclised using HBTU.

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