A general method for preparation of N-Boc-protected or N-Fmoc-protected α,β-didehydropeptide building blocks and their use in the solid-phase peptide synthesis

Abstract

N-(tert-butyloxycarbonyl) or N-(9-fluorenylmethoxycarbonyl) dipeptides with C-terminal (Z)-α,β-didehydrophenylalanine (∆^Z Phe), (Z)-α,β-didehydrotyrosine (∆^Z Tyr), (Z)-α,β-didehydrotryptophan (∆^Z Trp), (Z)-α,β-didehydromethionine (∆^Z Met), (Z)-α,β-didehydroleucine (∆^Z Leu), and (Z/E)-α,β-didehydroisoleucine (∆^Z/E Ile) were synthesised from their saturated analogues via oxidation of intermediate 2,5-disubstituted-oxazol-5-(4H)-ones (also known as azlactones) with pyridinium tribromide followed by opening of the produced unsaturated oxazol-5-(4H)-one derivatives in organic-aqueous solution with a catalytic amount of trifluoroacetic acid or by a basic hydrolysis. In all cases, a very strong preference for Z isomers of α,β-didehydro-α-amino acid residues was observed except of the ΔIle, which was obtained as the equimolar mixture of Z and E isomers. Reasons for the (Z)-stereoselectivity and the increased stability of the aromatic α,β-didehydro-α-amino acid residue oxazol-5-(4H)-ones over the corresponding aliphatic ones are also discussed. It is the first use of such a procedure to synthesise peptides with the C-terminal unsaturated residues and a peptide with 2 consecutive ΔPhe residues. This approach is very effective especially in the synthesis of peptides with aliphatic α,β-didehydro-α-amino acid residues that are difficult to obtain by other methods. It allowed the first synthesis of the ∆Met residue. It is also more cost-effective and less laborious than other synthesis protocols. The dipeptide building blocks obtained were used in the solid-phase synthesis of model peptides on a polystyrene-based solid support. Peptides containing aromatic α,β-didehydro-α-amino acid residues were obtained with PyBOP or TBTU as a coupling agent with good yields and purities. In the case of aliphatic α,β-didehydro-α-amino acid residues, a good efficiency was achieved only with DPPA as a coupling agent.

Keywords: Amino acid oxidation; Dehydroisoleucine; Dehydroleucine; Dehydromethionine; Dehydropeptide; Dehydropeptide building blocks; Dehydrophenylalanine; Dehydrotyrosine; Peptide synthesis; SPPS; α,β-Didehydro-α-amino acids.