Synthesis of α, β-unsaturated γ-amino esters with unprecedented high (E)-stereoselectivity and their conformational analysis in peptides

Design of Chiral β-Double Helices from γ-Peptide Foldamers

Chirality is ubiquitous in nature, and homochirality is manifested in many biomolecules. Although β-double helices are rare in peptides and proteins, they consist of alternating L- and D-amino acids. No peptide double helices with homochiral amino acids have been observed. Here, we report chiral β-double helices constructed from γ-peptides consisting of alternating achiral (E)-α,β-unsaturated 4,4-dimethyl γ-amino acids and chiral (E)-α,β-unsaturated γ-amino acids in both single crystals and in solution. The two independent strands of the same peptide intertwine to form a β-double helix structure, and it is stabilized by inter-strand hydrogen bonds. The peptides with chiral (E)-α,β-unsaturated γ-amino acids derived from α-L-amino acids adopt a (P)-β-double helix, whereas peptides consisting of (E)-α,β-unsaturated γ-amino acids derived from α-D-amino acids adopt an (M)-β-double helix conformation. The circular dichroism (CD) signature of the (P) and (M)-β-double helices and the stability of these peptides at higher temperatures were examined. Furthermore, ion transport studies suggested that these peptides transport ions across membranes. Even though the structural analogy suggests that these new β-double helices are structurally different from those of the α-peptide β-double helices, they retain ion transport activity. The results reported here may open new avenues in the design of functional foldamers.

Stereoselective synthesis of backbone extended π-conjugated amino esters

Utilization of the Wittig reaction to synthesize conjugative multiple double bonds is rare. We examined the utility of the Wittig reaction to construct conjugative two and three carbon-carbon double bonds on the N-protected amino acid backbone. The ethyl esters of N-Boc amino acids with multiple carbon-carbon double bonds in the backbone were isolated in excellent yields with exceptional E-selectivity of the double bonds. The allylic alcohols of α,β-unsaturated γ-amino esters were selectively synthesized from the DIBAL-H and BF₃·OEt₂. The allylic alcohols were transformed into aldehydes using IBX oxidation. Using this protocol, we synthesized ethyl esters of N-Boc-(E,E)-α,β,γ,δ-unsaturated ε-amino acids with various side-chain functionalities and ethyl esters of N-Boc-(E,E,E)-α,β,γ,δ,ε,ζ-unsaturated η-amino acids with excellent yields. We speculated the exceptional E-selectivity is probably due to the stabilization of the planar transition state of the Wittig reaction with the double bond p-orbitals. No racemization was observed in the synthesis of amino acids. The reported process may serve as an excellent route to synthesize multiple conjugative carbon-carbon double bonds.

α,β-Unsaturated γ-Peptide Foldamers

Despite their concomitant emergence in the 1990s, γ-peptide foldamers have not developed as fast as β-peptide foldamers and to date, only a few γ-oligomer structures have been reported, and with sparse applications. Among these examples, sequences containing α,β-unsaturated γ-amino acids have recently drawn attention since the Z/E configurations of the double bond provide opposite planar restrictions leading to divergent conformational behaviors, from helix to extended structures. In this Review, we give a comprehensive overview of the developments of γ-peptide foldamers containing α,β-unsaturated γ-amino acids with examples of applications for health and catalysis, as well as materials science.

Direct Transformation of N-Protected α,β-Unsaturated γ-Amino Amides into γ-Lactams through a Base-Mediated Molecular Rearrangement

Here, we are reporting a single-step transformation of N-protected α,β-unsaturated γ-amino amides into 5,5-disubstituted γ-lactams through a base-mediated new molecular rearrangement. In contrast to the known N- to C(O) cyclization of saturated γ-amino acids into corresponding γ-lactams, the new rearrangement involves the cyclization between N-terminal C^γ- to C-terminal amide N. The cyclization process was initiated by the migration of double bond from α,β → β,γ position. The enamine-imine tautomerization of the new β,γ-double bond and subsequent 5-exo-trig cyclization of terminal amide leads to the formation of N-protected 5,5-disubstituted γ-lactam. The structures of various γ-lactams obtained from the rearrangement were studied in single crystals. Overall, the results reported here demonstrate the facile and single-step transformation of N-protected α,β-unsaturated γ-amino amides into γ-lactams and provided an excellent opportunity to construct small-molecule peptidomimetics.

Artificial β-Double Helices from Achiral γ-Peptides

Double helices are not common in polypeptides and proteins except in the peptide antibiotic gramicidin A and analogous l,d-peptides. In contrast to natural polypeptides, remarkable β-double-helical structures from achiral γ-peptides built from α,β-unsaturated γ-amino acids have been observed. The crystal structures suggest that they adopted parallel β-double helical structures and these structures are stabilized by the interstrand backbone amide H-bonds. Furthermore, both NMR spectroscopy and fluorescence studies support the existence of double-helical conformations in solution. Although a variety of folded architectures featuring distinct H-bonds have been discovered from the β- and γ-peptide foldamers, this is the first report to show that achiral γ-peptides can spontaneously intertwine into β-double helical structures.

Retro-Curcuminoids as Mimics of Dehydrozingerone and Curcumin: Synthesis, NMR, X-ray, and Cytotoxic Activity

Curcumin and its derivatives have been extensively studied for their remarkable medicinal properties, and their chemical synthesis has been an important step in the optimization of well-controlled laboratory production. A family of new compounds that mimic the structure of curcumin and curcuminoids, here named retro-curcuminoids (7-14), was synthesized and characterized using 1D ¹H- and 13C-NMR, IR, and mass spectrometry; the X-ray structure of 7, 8, 9, 10, 12, 13, and 14 are reported here for the first time. The main structural feature of these compounds is the reverse linkage of the two aromatic moieties, where the acid chloride moiety is linked to the phenolic group while preserving α, β-unsaturated ketone functionality. The cytotoxic screening of 7, 8, 9, and 10 at 50 and 10 µg/mL was carried out with human cancer cell lines K562, MCF-7, and SKLU-1. Lipid peroxidation on rat brain was also tested for compounds 7 and 10. Compounds 7, 8, and 10 showed relevant cytotoxic activity against these cancer cell lines, and 10 showed a protective effect against lipid peroxidation. The molecular resemblance to curcuminoids and analogs with ortho substituents suggests a potential source of useful bioactive compounds.

Structural features and molecular aggregations of designed triple-stranded β-sheets in single crystals

Design, synthesis, single-crystal conformations and molecular aggregations of hybrid triple-stranded β-sheets, and their structural analogy with protein structures are reported.

Comparison of backbone modification in protein β-sheets by α→γ residue replacement and α-residue methylation

The mimicry of protein tertiary structure by oligomers with unnatural backbones is a significant contemporary research challenge. Among common elements of secondary structure found in natural proteins, sheets have proven the most difficult to address. Here, we report the systematic comparison of different strategies for peptide backbone modification in β-sheets with the goal of identifying the best method for replacing a multi-stranded sheet in a protein tertiary fold. The most effective sheet modifications examined led to native-like tertiary folding behavior with a thermodynamic folded stability comparable to the prototype protein on which the modified backbones are based.

Computing stoichiometric molecular composition from crystal structures

Crystallographic investigations deliver high-accuracy information about positions of atoms in crystal unit cells. For chemists, however, the structure of a molecule is most often of interest. The structure must thus be reconstructed from crystallographic files using symmetry information and chemical properties of atoms. Most existing algorithms faithfully reconstruct separate molecules but not the overall stoichiometry of the complex present in a crystal. Here, an algorithm that can reconstruct stoichiometrically correct multimolecular ensembles is described. This algorithm uses only the crystal symmetry information for determining molecule numbers and their stoichiometric ratios. The algorithm can be used by chemists and crystallographers as a standalone implementation for investigating above-molecular ensembles or as a function implemented in graphical crystal analysis software. The greatest envisaged benefit of the algorithm, however, is for the users of large crystallographic and chemical databases, since it will permit database maintainers to generate stoichiometrically correct chemical representations of crystal structures automatically and to match them against chemical databases, enabling multidisciplinary searches across multiple databases.

Synthesis of new simplified hemiasterlin derivatives with α,β-unsaturated carbonyl moiety

In this Letter, we report a convenient and efficient method for the synthesis of new simplified derivatives of hemiasterlin in which the α,α-dimethylbenzylic moiety A is replaced by α,β-unsaturated aryl groups as Michael acceptor. Most of these derivatives have a strong cytotoxic activity on three human tumor cell lines (KB, Hep-G2 and MCF7). Analogs 17b and 17f showed a high cytotoxicity against KB and Hep-G2 cancer cell lines comparable to paclitaxel and ellipticine.

Improved total synthesis and biological evaluation of potent apratoxin S4 based anticancer agents with differential stability and further enhanced activity

Apratoxins are cytotoxic natural products originally isolated from marine cyanobacteria that act by preventing cotranslational translocation early in the secretory pathway to downregulate receptor levels and inhibit growth factor secretion, leading to potent antiproliferative activity. Through rational design and total synthesis of an apratoxin A/E hybrid, apratoxin S4 (1a), we have previously improved the antitumor activity and tolerability in vivo. Compound 1a and newly designed analogues apratoxins S7-S9 (1b-d), with various degrees of methylation at C34 (1b,c) or epimeric configuration at C30 (1d), were efficiently synthesized utilizing improved procedures. Optimizations have been applied to the synthesis of key intermediate aldehyde 7 and further include the application of Leighton's silanes and modifications of Kelly's methods to induce thiazoline ring formation in other crucial steps of the apratoxin synthesis. Apratoxin S9 (1d) exhibited increased activity with subnanomolar potency. Apratoxin S8 (1c) lacks the propensity to be deactivated by dehydration and showed efficacy in a human HCT116 xenograft mouse model.

HBTU mediated 1-hydroxybenzotriazole (HOBt) conjugate addition: synthesis and stereochemical analysis of β-benzotriazole N-oxide substituted γ-amino acids and hybrid peptides

Synthesis and conformational analysis of β-benzotriazole N-oxide (β-BtO) substituted γ-amino acids by direct conjugate addition of HOBt to E-vinylogous γ-amino acids are reported.

Preparation of α-imino aldehydes by [1,3]-rearrangements of O-alkenyl oximes

The synthesis of α-imino aldehydes has been achieved through the thermal [1,3]-rearrangement of O-alkenyl benzophenone oximes. A copper-mediated C-O bond coupling between benzophenone oxime and alkenyl boronic acids provides facile access to the required O-alkenyl oximes and a Horner-Wadsworth-Emmons olefination can be applied to the α-imino aldehyde products to give γ-imino-α,β-unsaturated esters. The scope of the method is described and mechanistic experiments are discussed.