Comparison of various coupling reagents in solid-phase aza-peptide synthesis

Aza-peptides are promising drug leads, however extensive study of their properties is hampered by low yielding aza-peptide bond formation during conventional Fmoc SPPS. The kinetics of aza-peptide bond formation in the model peptide H-Ala-AzAla-Phe-NH₂ was compared with various conventional amino acid activators. The reaction rates and yields were dependent on the activator structure. The reaction time of aza-peptide formation using oxyma-based agents was approximately 30 times longer than in typical peptide synthesis. Therefore, new activators are required to increase the reactivity of the activated amino acid to achieve effective acylation of the semicarbazide moiety during aza-peptide bond formation.     

Preparation and properties of some α-aza-amino-acid derivatives,their possible use in peptide synthesis

Some derivatives of azaglycine and azaphenylalanine are described. Esters of acetyl- and benzoyl-aza-amino-acids rapidly cyclise to stable oxadiazolones and it is concluded that similar derivatives would be unsuitable for aza-peptide synthesis. t-Butyloxycarbonyl-azaglycine azide was too unreactive for use in peptide synthesis. Benzoyl-azaglycylphenylalanine ethyl ester and acetyl-azaphenylalanylphenylalanine ethyl ester were prepared by coupling benzoyl hydrazide and N-acetyl-N'-benzylhydrazide respectively with 2,4-dinitrophenyloxycarbonyl-phenylalanine ethyl ester.     

Synthesis of N′-substituted Ddz-protected hydrazines and their application in solid phase synthesis of aza-peptides

Hydrazine derivatives are of considerable scientific and industrial value. Substituted hydrazines are precursors for many compounds of great interest and importance, among them aza-peptides. (Aza-peptides are peptide analogues in which one or more of the α-carbons, bearing the side chain residues, has been replaced by a nitrogen atom.) Aza-amino acid residues conserve the pharmacophores necessary for biological activity while inducing conformational changes and increased resistance to proteolytic degradation. These properties make aza-peptides attractive tools for structure-activity relationship studies and drug design. We describe the synthesis of N′-substituted 2-(3,5-dimethoxyphenyl)propan-2-yloxycarbonyl (Ddz) protected hydrazines. A general approach for solid phase synthesis of aza-peptides has been developed based on the in-situ activation of the N-Ddz,N′-substituted hydrazines with phosgene, followed by introduction to the N-terminus of a resin-bound peptide. The Ddz-aza-amino building units include aliphatic, aromatic and functionalized side chains, protected for synthesis by the Fmoc strategy. Solid phase aza-peptide synthesis is demonstrated including selective mild deprotection of Ddz with Mg(ClO₄)₂ and coupling of the next amino acid with triphosgene. Ddz deprotection is orthogonal with the Fmoc and Boc protecting groups, making the solid phase Ddz-aza-peptide synthesis compatible with both the Fmoc and the Boc strategies. The Ddz-protected hydrazines have wide applications in the synthesis of substituted hydrazines and in the synthesis of aza containing peptidomimetics.     

Divergent Total Syntheses of C3 a−C7′ Linked Diketopiperazine Alkaloids (+)-Asperazine and (+)-Pestalazine A Enabled by a Ni-Catalyzed Reductive Coupling of Tertiary Alkyl Chloride

Short gram-scale asymmetric syntheses of asperazine, pestalazine A, and their unnatural congeners thereof, have been achieved in ≈10 steps by using readily accessible starting materials. The nickel-catalyzed reductive coupling protocol was utilized as a key step for the direct construction of C3a_sp3−C7′_sp2 bond furnishing the diaryl-substituted quaternary carbon centers with remarkable steric hindrance. The streamlined access to this core structure of heterodimeric tryptophans under the mild reaction conditions, makes this strategy hold a great promise in the concise synthesis of other relevant oligomeric pyrroloindoline alkaloids with unique C3a−C7′ linkages.     

炔酰胺类缩合剂研究进展

缩合剂是指用于促成羧酸与胺或者醇直接缩合构建酰胺键或酯键的一类试剂的总称.由于酰胺和酯的重要性,缩合剂的开发成为了学术界与工业界广泛关注的一个重要研究方向.多肽合成就是α-氨基酸在缩合剂的作用下反复形成酰胺键的过程,因此,缩合剂在多肽合成中发挥着至关重要的作用.当前多肽合成所使用的试剂和技术大多是20世纪50~80年代发展起来的,这些试剂和技术的天生弊端逐渐显现出来.比如传统多肽缩合剂过度活化α-氨基酸而诱发的外消旋化和其它副反应导致的副产物成为药物多肽生产过程中一个极为关切的问题.另外固相多肽合成的低原子经济性给可持续发展带来了极大的挑战.这些问题只能依靠原始创新的颠覆性技术和全新的缩合方法来解决.我们课题组致力于通过发展新试剂和新反应来解决多肽与蛋白质化学合成领域的难题.本文系统介绍了我们发展的一种结构全新的炔酰胺类缩合试剂及其在酰胺、酯、大环内酯、多肽、硫代多肽合成中的应用研究进展.     

Construction of C-N Atropisomers by Aminocatalytic Enantioselective Addition of Indole-2-carboxaldehydes to o-Quinone Derivatives

The first atroposelective aminocatalytic methodology for the construction of C−N atropisomers is presented. The synthesis of this class of axially chiral molecules typically encompasses substrates in which the C−N bond is pre-formed. In contrast, this work presents the direct coupling of indole-2-carboxaldehydes to ortho-quinones, to form the stereogenic C−N axis in an atroposelective way. Application of typical secondary amine catalysts furnished the desired product, however, in low yields and as a complex mixture arising from poor regiocontrol among the C₃- and N₁-sites of the indole core. A new aminocatalyst was designed and synthesized with increased outer-sphere steric bulk to address these challenges thereby providing good levels of regio- and enantioselectivity. A novel library of functionalized and enantioenriched C−N atropisomers was obtained and their synthetic utility was demonstrated by various transformations.     

Synthesis of Ψ[CH(RF)NH]Gly-peptides: The dramatic effect of a single fluorine atom on the diastereocontrol of the key aza-Michael reaction

We describe in full-detail the synthesis of new ψ[CH(R_F)NH]-peptidomimetics, having different fluoroalkyl groups R_F, as peptide bond surrogates. A key step in the synthesis is a stereoselective aza-Michael addition of chiral α-amino acid esters to β-fluoroalkyl-α-nitroethenes. The diastereoselection of the process was influenced by the electronegativity, rather than by the steric bulk, of the fluorinated residue R_F in the β-position of the nitroalkene acceptors. Replacement of a single F atom of R_F by a hydrogen or methyl group brings about a dramatic drop of stereocontrol, whereas Br, Cl and CF₃, albeit bulkier than F, provide inferior results in terms of stereocontrol. A mechanistic hypothesis is provided.     

A comprehensive survey of stille-type Csp-Csp single bond forming processes in the synthesis of retinoic acid and analogs

The synthesis of the retinoid skeleton has been exhaustively explored using the Stille coupling for the formation of the side-chain single bonds. On employing the experimental catalytic conditions developed by Farina [Pd₂(dba)₃, AsPh₃, NMP] we have modified the electronic and steric requirement of the coupling parters, alkenyl stannanes and electrophiles (alkenyl iodides and triflates). The comprehensive survey afforded appropriately matched components for every bond formation considered. Moreover, from the comparison of the reactivities of different coupling partners with different degrees of steric hindrance, the sensitivity of the Stille coupling to steric effects was confirmed. Besides providing a variety of building blocks for retinoid synthesis, the study highlights some trends that might be useful for the application of the Stille reaction to the synthesis of unsubstituted conjugated polyenes.     

Electrocatalytic Urea Synthesis with 63.5 % Faradaic Efficiency and 100 % N-Selectivity via One-step C−N coupling

Electrocatalytic urea synthesis via coupling N₂ and CO₂ provides an effective route to mitigate energy crisis and close carbon footprint. However, the difficulty on breaking N≡N is the main reason that caused low efficiencies for both electrocatalytic NH₃ and urea synthesis, which is the bottleneck restricting their industrial applications. Herein, a new mechanism to overcome the inert of the nitrogen molecule was proposed by elongating N≡N instead of breaking N≡N to realize one-step C−N coupling in the process for urea production. We constructed a Zn−Mn diatomic catalyst with axial chloride coordination, Zn−Mn sites display high tolerance to CO poisoning and the Faradaic efficiency can even be increased to 63.5 %, which is the highest value that has ever been reported. More importantly, negligible N≡N bond breakage effectively avoids the generation of ammonia as intermediates, therefore, the N-selectivity in the co-electrocatalytic system reaches100 % for urea synthesis. The previous cognition that electrocatalysts for urea synthesis must possess ammonia synthesis activity has been broken. Isotope-labelled measurements and Operando synchrotron-radiation Fourier transform infrared spectroscopy validate that activation of N−N triple bond and nitrogen fixation activity arise from the one-step C−N coupling process of CO species with adsorbed N₂ molecules.     

Stereochemistry of organic sulphur compounds. part 14 . synthesis and conformation analysis of 1-thioderivatives of 3,3-dimethyl-2-butanol and its acetates

The synthesis and conformational analysis of some thioderivatives But-CHX-CH₂Y (X = OH and OAc: Y = SH, SMe, SO₂Me and SMe₂) are reported. The conformational equilibra have been established from IH-NMR data and high dilution ir studies. All the compounds exhibited almost monoconformational behaviour around the C(1)-C(2) bond, due to strong steric factors. The observed vicinal coupling constants were used to check the different parameterizations of Altona's equation for our compounds with sulphur in distinct oxidation states. The possible dihedral angle deformations were also evaluated.     

Novel route in the synthesis of ψ[CH2NH] amide bond surrogate

An alternative method for the synthesis of pseudopeptides containing a ψ[CH₂NH] amide bond surrogate is reported. The synthetic approach is based on a nucleophilic displacement of the chiral N-protected β-iodoamines with conveniently protected amino acid esters. The compatibility of this method with both conventional and microwave-assisted peptide synthesis should increase the potentiality of the ψ[CH₂NH] peptide bond isostere in peptide chemistry.     

Efficient Synthesis of Diaryl Ketones by Nickel‐Catalyzed Negishi Cross‐Coupling of Amides by Carbon–Nitrogen Bond Cleavage at Room Temperature Accelerated by a Solvent Effect

The first Negishi cross‐coupling of amides for the synthesis of versatile diaryl ketones by selective C?N bond activation under exceedingly mild conditions is reported. The cross‐coupling was accomplished with bench‐stable, inexpensive precatalyst [Ni(PPh₃)₂Cl₂] that shows high functional‐group tolerance and enables the synthesis of highly functionalized diaryl ketone motifs. The coupling occurred with excellent chemoselectivity favoring the ketone (cf. biaryl) products. Notably, this process represents the mildest conditions for amide N?C bond activation accomplished to date (room temperature, <10 min). Considering the versatile role of polyfunctional biaryl ketone linchpins in modern organic synthesis, availability, and excellent functional‐group tolerance of organozinc reagents, this strategy provides a new platform for amide N?C bond/organozinc cross‐coupling under mild conditions.     

Ynamide‐Mediated Thiopeptide Synthesis

Exploration of the full potential of thioamide substitution as a tool in the chemical biology of peptides and proteins has been hampered by insufficient synthetic strategies for the site‐specific introduction of a thioamide bond into a peptide backbone. A novel ynamide‐mediated two‐step strategy for thiopeptide bond formation with readily available monothiocarboxylic acids as thioacyl donors is described. The α‐thioacyloxyenamide intermediates formed from the ynamides and monothiocarboxylic acids can be purified, characterized, and stored. The balance between their activity and stability enables them to act as effective thioacylating reagents to afford thiopeptide bonds under mild reaction conditions. Amino acid functional groups such as OH, CONH₂, and indole NH groups need not be protected during thiopeptide synthesis. The modular nature of this strategy enables the site‐specific incorporation of a thioamide bond into peptide backbones in both solution and the solid phase.     

Structures of Highly Twisted Amides Relevant to Amide N−C Cross‐Coupling: Evidence for Ground‐State Amide Destabilization

Herein, we show that acyclic amides that have recently enabled a series of elusive transition‐metal‐catalyzed N?C activation/cross‐coupling reactions are highly twisted around the N?C(O) axis by a new destabilization mechanism of the amide bond. A unique effect of the N‐glutarimide substituent, leading to uniformly high twist (ca. 90°) irrespective of the steric effect at the carbon side of the amide bond has been found. This represents the first example of a twisted amide that does not bear significant steric hindrance at the α‐carbon atom. The ¹⁵N NMR data show linear correlations between electron density at nitrogen and amide bond twist. This study strongly supports the concept of amide bond ground‐state twist as a blueprint for activation of amides toward N?C bond cleavage. The new mechanism offers considerable opportunities for organic synthesis and biological processes involving non‐planar amide bonds.     

Serine-Protease-Assisted Synthesis of Peptide Substrates for α-Chymotrypsin

δ-Chymotraypsin catalyzes peptide-bond formation between acylated amino-acid and peptide esters as the carboxyl component and amino-acid and peptide amides as the amino component. The conditions under which enzyme-catalyzed coupling can be used for fragment condensation in peptide synthesis is investigated. To illustrate the method the synthesis of tetra-, penta- and hexapeptides of the structure Ac-L_xn? …? ? L_xl? L_yl? …? ? L_ym? NH₂ with L_xl ? Tyr, designed as substrates for α-chymotrypsin, is described.     

Electrophilicity Scale of Activated Amides: 17O NMR and 15N NMR Chemical Shifts of Acyclic Twisted Amides in N−C(O) Cross-Coupling

The structure and properties of amides are of tremendous interest in organic synthesis and biochemistry. Traditional amides are planar and the carbonyl group non-electrophilic due to n_N→π*_C=O conjugation. In this study, we report electrophilicity scale by exploiting ¹⁷O NMR and ¹⁵N NMR chemical shifts of acyclic twisted and destabilized acyclic amides that have recently received major attention as precursors in N-C(O) cross-coupling by selective oxidative addition as well as precursors in electrophilic activation of N-C(O) bonds. Most crucially, we demonstrate that acyclic twisted amides feature electrophilicity of the carbonyl group that ranges between that of acid anhydrides and acid chlorides. Furthermore, a wide range of electrophilic amides is possible with gradually varying carbonyl electrophilicity by steric and electronic tuning of amide bond properties. Overall, the study quantifies for the first time that steric and electronic destabilization of the amide bond in common acyclic amides renders the amide bond as electrophilic as acid anhydrides and chlorides. These findings should have major implications on the fundamental properties of amide bonds.     

An economical approach for peptide synthesis via regioselective C–N bond cleavage of lactams

An economical, solvent-free, and metal-free method for peptide synthesis via C–N bond cleavage using lactams has been developed. The method not only eliminates the need for condensation agents and their auxiliaries, which are essential for conventional peptide synthesis, but also exhibits high atom economy. The reaction is versatile because it can tolerate side chains bearing a range of functional groups, affording up to >99% yields of the corresponding peptides without racemisation or polymerisation. Moreover, the developed strategy enables peptide segment coupling, providing access to a hexapeptide that occurs as a repeat sequence in spider silk proteins.

An economical, solvent-free, and metal-free method for peptide synthesis via C–N bond cleavage using lactams has been developed.     

Microwave-assisted solid-phase aza-peptide synthesis: aza scan of a PKB/Akt inhibitor using aza-arginine and aza-proline precursors

Aza-peptides are peptidomimetics in which one or more of the α-carbons, bearing the side-chain residues, has been replaced by a nitrogen. These peptidomimetics have been shown to be promising for the generation of drug leads and for structure-activity relationship studies. Aza-scan is the systematic replacement of amino acid residues in a given peptide with their aza counterparts. We report here an aza-scan of a potent, peptide-based PKB/Akt inhibitor, PTR6154. Procedures for microwave-assisted, Fmoc/t-Bu chemistry, solid-phase aza-peptide synthesis were developed which significantly reduce standard reaction time and are suitable for automation. Novel substituted hydrazines have been prepared for the straightforward incorporation of aza-arginine and aza-proline residues. This work will enable aza-scan to become a more common and standard method for structure-activity relationship studies of peptides.     

Recent development in peptide coupling reagents

Two decades of domination of benzotriazole-based chemistry stimulated the progress in peptide synthesis to a high level of effectiveness. However, the growing need for new and more complex peptide structures, particularly for biomedical studies and, very recently, for the large-scale production of peptides as drugs, required manufacturing peptide products by efficient synthetic strategies, at reasonably low prices. Therefore, the search for new, more versatile and low-cost reagents becomes a great challenge. Several comprehensive review articles summarized the great effort undertaken, but up to now, no versatile coupling reagent useful for both amide and ester bond formation, as well as for solution and solid-phase peptide synthesis has been yet developed. The most-widely used coupling reagents are carbodiimides on one hand and phosphonium and aminium salts on the other. Herein in this review article, we summarized the recent development in peptide coupling reagents during the last two decades.     

Effect of Alanine and Glycine on the Assembly of Surfactant‐like Peptides with Tyrosine as Hydrophilic Head in Basic Solution

Surfactant‐like peptides (SLPs ) can self‐assemble into various nanostructures, which have shown great potential for a variety of biomedical and biotechnological applications. In this work, two SLPs , V₄Y , and V₄AGY , were designed and synthesized, both of which had hydrophobic head valines (V) with large side‐chain steric hindrance effect and the hydrophilic head tyrosine (Y) with a rigid ring and two negative charges in the basic solution. Fourier transform infrared and circular dichroism studies confirmed their different secondary structures, whereas atomic force microscopy and dynamic light scattering characterized the difference in their morphologies. In solution, they formed different secondary structures. Correspondingly, V₄Y and V₄AGY formed noncompact spherical aggregates and a spiral clubbed structure, respectively. In V₄AGY , the introduction of alanine (A) and glycine (G) increased the molecule's flexibility and increased the distance between the tyrosine and four continuous valines, so as to weaken the synergistic action of electronic repulsion and steric hindrance and strengthen the intermolecular hydrogen bond beneficial to β‐sheet formation and the axial growth of the self‐assembly. Therefore, the flexibility of the molecule and the side‐chain steric effect of the two heads of SLPs are non‐negligible in the tuning process of peptide self‐assembly, in addition to hydrogen‐bonding, hydrophobic, and electrostatic interactions.