Synthesis of novel cyclopeptides containing heterocyclic skeletons

General Installation of (4H)-Imidazolone cis-Amide Bioisosteres Along the Peptide Backbone

Imidazolones represent an important class of heterocycles present in a wide range of pharmaceuticals, metabolites, and bioactive natural products and serve as the active chromophore in green fluorescent protein. Recently, imidazolones have received attention for their ability to act as a nonaromatic amide bond bioisotere which improves pharmacological properties. Herein, we present a tandem amidine installation and cyclization with an adjacent ester to yield (4H)-imidazolone products. Using amino acid building blocks, we can access the first examples of α-chiral imidazolones that have been previously inaccessible. Additionally, our method is amenable to on-resin installation which can be seamlessly integrated into existing solid-phase peptide synthesis protocols. Finally, we show that peptide imidazolones are potent cis-amide bond surrogates that preorganize linear peptides for head-to-tail macrocyclization. This work represents the first general approach to the backbone and side-chain insertion of imidazolone bioisosteres at various positions in linear and cyclic peptides.

Thioimidates provide general access to thioamide, amidine, and imidazolone peptide-bond isosteres

Thioamides, amidines, and heterocycles are three classes of modifications that can act as peptide-bond isosteres to alter the peptide backbone. Thioimidate protecting groups can address many of the problematic synthetic issues surrounding installation of these groups. Historically, amidines have received little attention in peptides due to limitations in methods to access them. The first robust and general procedure for the introduction of amidines into peptide backbones exploits the utility of thioimidate protecting groups as a means to side-step reactivity that ultimately renders existing methods unsuitable for the installation of amidines along the main-chain of peptides. Further, amidines formed on-resin can be reacted to form (4H)-imidazolone heteorcycles which have recently been shown to act as cis-amide isosteres. General methods for heterocyclic installation capable of geometrically restricting peptide conformation are also under-developed. This work is significant because it describes a generally applicable and divergent approach to access unexplored peptide designs and architectures.

Trimethylaluminum-mediated one-pot peptide elongation

Efficient and straightforward peptide bond formation of N-, and C-terminal unprotected amino acids was successfully achieved by using trimethylaluminum. The coupling reaction was accomplished by pre-reaction of N-, and C-terminal unprotected amino acids and trimethylaluminum to form a five-membered ring that smoothly reacted with nucleophilic amino acid esters. This simple and highly efficient reaction system allows one-pot tripeptide synthesis without the need for expensive coupling reagents. Furthermore, peptide bond formation can be effectively achieved even for amino acids with bulky substituents at the side chain to afford the corresponding tripeptides in high yields in a one-pot manner. In addition, the reaction can be applied for further peptide elongation by the subsequent addition of amino acids and trimethylaluminum. We anticipate that this cost-effective, straightforward, and efficient protocol will be useful for the synthesis of a wide variety of peptides.

Impact of the Histidine-Triazole and Tryptophan-Pyrene Exchange in the WHW Peptide: Cu(II) Binding, DNA/RNA Interactions and Bioactivity

In three novel peptidoids based on the tryptophan—histidine—tryptophan (WHW) peptide, the central histidine was replaced by Ala-(triazole), and two derivatives also had one tryptophan replaced with pyrene-alkyls of different lengths and flexibility. Pyrene analogues show strong fluorescence at 480–500 nm, attributed to intramolecular exciplex formation with tryptophan. All three peptidoids bind Cu2+ cation in water with strong affinity, with Trp- Ala-(triazole)-Trp binding comparably to the parent WHW, and the pyrene analogues even stronger, demonstrating that replacement of histidine with triazole in peptides does not hamper Cu2+ coordination. The studied peptidoids strongly bind to ds-DNA and ds-RNA, whereby their complexes with Cu2+ exhibit distinctively different interactions in comparison to metal-free analogues, particularly in the stabilization of ds-DNA against thermal denaturation. The pyrene peptidoids efficiently enter living cells with no apparent cytotoxic effect, whereby their red-shifted emission compared to the parent pyrene allows intracellular confocal microscopy imaging, showing accumulation in cytoplasmic organelles. However, irradiation with 350 nm light resulted in evident antiproliferative effect on cells treated with micromolar concentrations of the pyrene analogues, presumably attributed to pyrene-induced production of singlet oxygen and consecutive cellular damage.

Fluorescent Analogues of FRH Peptide: Cu(II) Binding and Interactions with ds-DNA/RNA

Four novel peptidoids, derived from the Phe-Arg-His (FRH) peptide motif, were prepared by replacing the histidine heterocycle with triazole and consequent triazole-fluorophore (coumarin) extension and also replacing arginine with less voluminous lysine. So the constructed Phe-Lys-Ala(triazole) (FKA(triazole)) peptidoids bind Cu2+ cations in water with a strong, nanomolar affinity comparable to the parent FRH and its known analogs, demonstrating that triazole can coordinate copper similarly as histidine. Moreover, even short KA(triazole)coumarin showed submicromolar affinity to Cu2+. Only FKA(triazole)coumarin with free amino groups and its shorter analog KA(triazole)coumarin showed strong induced CD spectra upon Cu2+ cation binding. Thus, KA(triazole)coumarin can be considered as the shortest peptidoid sequence with highly sensitive fluorescent and chiral CD response for Cu2+ cation, encouraging further studies with other metal cations. The FKA(triazole) coumarin peptidoids show biorelevant, 10 µM affinity to ds-DNA and ds-RNA, binding within DNA/RNA grooves. Intriguingly, only peptidoid complexes with Cu2+ strongly stabilize ds-DNA and ds-RNA against thermal denaturation, suggesting significant interactions of Cu2+ cation within the DNA/RNA binding site.

Copper(I)-Catalyzed Asymmetric Synthesis of Unnatural α-Amino Acid Derivatives and Related Peptides Containing γ-(aza)Aryls

Chiral α-amino acids are indispensable compounds in organic chemistry, biochemistry, and medicinal chemistry. Herein, by means of copper(I)-catalyzed asymmetric conjugate addition of derivatives of glycine, serine, cysteine, and β-amino-alanine to electron-deficient vinyl(aza)arenes, an array of novel unnatural chiral α-amino acid derivatives bearing a γ-(aza)aryl is prepared in moderate to high yields with high enantioselectivity. Various azaarenes, such as pyrimidine, 1,3,5-triazine, pyridine, pyridine-N-oxide, quinoline, quinoxaline, purine, benzo[d]imidazole, benzothiazole, and 1,2,4-oxadiazole, are well tolerated. Moreover, the electrophiles are nicely extended to (Z)/(E) mixtures of electron-deficient butadienylpyridine and benzene, which are transformed to the corresponding chiral α-amino acid derivatives in high (E)/(Z) ratio and high enantioselectivity. More importantly, the present methodology is successfully applied in the catalytic asymmetric functionalization of Schiff bases derived from peptides, which finally afforded a new chiral tripeptide bearing two electron-deficient azaaryls and one electron-deficient aryl in high total yield with high diastereo- and excellent enantioselectivities.

Toward the Synthesis of a Heterocyclic Analogue of Natural Cyclooligopeptide with Improved Bio-Properties

AIMS

The present investigation is targeted toward the synthesis of a novel analogue of a natural peptide of marine origin. <P> Background: Marine sponges are enriched with bioactive secondary metabolites especially circular peptides. Heterocycles are established organic compounds with potential biological value. Taking into consideration the bio-properties of heterocycles and marine sponge-derived natural peptides, an effort was made for the synthesis of a heterocyclic analogue of a natural cyclopeptide. <P> Objective: A heterocyclic analogue of a sponge-derived proline-containing cyclic peptide, rolloamide A, was synthesized by interaction of Boc-protected L-histidinyl-L-prolyl-L-valine and L-prolyl-L-leucyl-L-prolyl-L-isoleucine methyl ester and compared with synthetic rolloamide A with bioactivity against bacteria, fungi, and earthworms. <P> Methods: The synthesis of cycloheptapeptide was accomplished employing the liquid phase method. The larger peptide segment was prepared by interaction of Boc-protected L-prolyl-L-leucine with L-prolyl-L-isoleucine methyl ester. Similarly, the tripeptide unit was synthesized from Boc-protected L-histidinyl-L-proline with L-valine ester. The linear heptapeptide segment (7) was cyclized by utilizing pentafluorophenyl (pfp) ester, and the structure was elucidated by elemental and spectral (IR, 1H/13C NMR, MS) analysis. The peptide was also screened for diverse bioactivities such as antibacterial, antifungal, and potential against earthworms and cytotoxicity. <P> Results: The novel cyclooligopeptide was synthesized with 84% yield by making use of carbodiimides. The synthesized cyclopeptide exhibited significant cytotoxicity against two cell lines. In addition, promising antifungal and antihelmintic properties were observed for newly synthesized heterocyclic peptide derivative (8) against dermatophytes and three earthworm species at 6 µg/mL and 2 mg/mL, respectively. <P> Conclusion: Solution-phase technique employing carbodiimide chemistry established to be promising for synthesizing the cycloheptapeptide derivative (8), and C5H5N was proved a better base for heptapeptide circling, when compared to N-methylmorpholine and triethylamine.

Site-selective modification of peptide backbones

Exciting developments in the site-selective modification of peptide backbones are allowing an outstanding fine-tuning of peptide conformation, folding ability, and physico-chemical and biological properties.

Diverse N-Substituted Azole-Containing Amino Acids as Building Blocks for Cyclopeptides

The preparation of 16 oxazole- or thiazole-containing amino esters bearing a wide array of N-substitution is reported. These were accessed in 40-92% yield via an AgClO4-promoted substitution reaction between a primary amine and a chloromethyl-functionalized thiazole or oxazole. These new synthetic building blocks will be useful for the preparation of new cyclopeptide analogues bearing heterocyclic backbone modifications. Four macrocyclic N-substituted oligoamides that include thiazole or oxazole heterocycles were obtained, following cyclooligomerization reactions of azole-modified N-substituted amino acids.