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 (∆ Phe), (Z)-α,β-didehydrotyrosine (∆ Tyr), (Z)-α,β-didehydrotryptophan (∆ Trp), (Z)-α,β-didehydromethionine (∆ Met), (Z)-α,β-didehydroleucine (∆ Leu), and (Z/E)-α,β-didehydroisoleucine (∆ 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.

Impact of Dehydroamino Acids on the Structure and Stability of Incipient 310-Helical Peptides

“…A brief survey of conditions revealed that serviceable yields of tetrapeptide 3a could be obtained by first hydrolyzing the azlactone through treatment with NaOH and then employing PyBOP to mediate coupling with Ala-OMe. 34 Construction of variant 3b wherein the Aib-3 residue of 1 is replaced by ΔAla is summarized in Scheme 5. Hydrogenolysis of the dipeptide Cbz-Ser-Ala-OMe (16) was followed by coupling with dipeptide 13, furnishing tetrapeptide 17 in moderate yield.…”

“…Ring opening and coupling of the hindered azlactone intermediate was difficult. A brief survey of conditions revealed that serviceable yields of tetrapeptide 3a could be obtained by first hydrolyzing the azlactone through treatment with NaOH and then employing PyBOP to mediate coupling with Ala-OMe …”

Therapeutic peptides: current applications and future directions

“…Boc-SPPS uses trifluoroacetic acid solution to remove the amine protection groups and hydrogen fluoride solution to cleave the final peptide, but these processes are associated with irritating odor and toxicity. Fmoc can be removed under milder conditions, and the Fmoc-SPPS strategy is thus often preferred 120 . However, Boc-SPPS has advantages for long peptide synthesis, because trifluoroacetic acid deprotection effectively destroys the aggregation during the peptide synthesis 121 .…”

Ultrashort Peptide Self-Assembly: Front-Runners to Transport Drug and Gene Cargos

“…Non-proteinogenic α,β-didehydro-α-amino acids, with peculiar conformational properties-planar conformation due to a double bond, between Cα and Cβ carbon atoms and fixed values of t φ and ψ torsion angles, have shown their ability to form hydrogels (Vilaça et al, 2015b) and applicable in drug delivery. A procedure for cheaper, less laborious and less time taken than other methods of dehydropeptide synthesis (α,β-didehydro-α-amino acid) through oxidation of the C-terminal residue in N-Boc-protected and N-Fmocprotected peptides, both in solution and on the solid support for peptide synthesis has been reported by Wołczanski and Lisowski (Wołczanski and Lisowski, 2018). The ability to form secondary and super secondary structures and highly stable nanostructures has made α,β-didehydrophenylalanine a center of attraction for the researchers (Gupta and Chauhan, 2011).…”