Spontaneous head-to-tail cyclization of unprotected linear peptides with the KAHA ligation

The α-ketoacid–hydroxylamine (KAHA) ligation with 5-oxaproline enables the direct cyclization of peptides upon cleavage from a solid support, without coupling reagents, protecting groups, or purification of the linear precursors. This Fmoc SPPS-based method was applied to the synthesis of a library of 24 homoserine-containing cyclic peptides and was compared side-by-side with the synthesis of the same products using a standard method for cyclizing side-chain protected substrates. A detailed mechanistic study including ²H and ¹⁸O labeling experiments and the characterization of reaction intermediates by NMR and mass spectrometry is reported.     

Native peptide folding dominates over stereoelectronic effects of prolyl hydroxylation in loop 5 of the macrocyclic peptide kalata B1

Kalata B1 (4) is a prototypical, 29-residue, Möbius cyclotide with a cis prolyl peptide bond in loop 5. Two analogs were synthesized in which Pro²⁴ was substituted by trans-4-hydroxy-l-proline (peptide 5) and cis-4-hydroxy-l-proline (peptide 6). Linear peptides were assembled by solid phase peptide synthesis using Fmoc/tBu chemistry. Head-to-tail cyclization was performed using HATU, side-chain protecting groups removed and the cyclic peptides 2 and 3 isolated by RP-HPLC. Oxidation led to the formation of peptides 5 and 6, each incorporating three disulfide bonds. Analysis of TOCSY and NOESY spectra of the purified peptides enabled assignment of the backbone amide and Hα resonances. These showed a striking correlation with those of native kalata B1, indicating that folding had produced the same disulfide bridge topology. While somewhat surprising that stereoelectronic effects introduced by the hydroxyl substituents in this key region of the peptide had little impact, this reflects the strong thermodynamic driving force toward formation of the cyclic cystine knot scaffold.     

Synthesis of Cyclohexapeptides Containing Pro and Aib Residues

Cyclization reactions on hexapeptides containing several α‐aminoisobutyric acid (=2‐amino‐2‐methylpropanoic acid; Aib) residues and the turn‐promoting glycine (Gly) and proline (Pro) residues were investigated. Eight linear hexapeptides were synthesized, and their cyclization was attempted with various coupling reagents. The macrolactamization step proved to be difficult since only three hexapeptides could be cyclized. Two of these latter peptides were the linear precursors of the same cyclic hexapeptide, cyclo(Aib‐Aib‐Phe‐Pro‐Aib‐Gly) ( 1 ). Surprisingly, they gave 1 in almost the same yield. Thus, 1 was obtained in 35% yield upon ring closure at the Phe/Pro site by using DEPBT as the coupling reagent, whereas the cyclization at the Aib/Phe site led to 1 in 28 and 34% yield by using PyAOP and DEPC, respectively (DEPBT=3‐[(diethoxyphosphoryl)oxy]‐1,2,3‐benzotriazin‐4(3H)‐one, PyAOP=(1H‐7‐azabenzotriazol‐1‐yloxy)tripyrrolidin‐1‐ylphosphonium hexafluorophosphate, DEPC=diethyl phosphorocyanidate). Another cyclic hexapeptide, cyclo(Aib‐Aib‐Gly‐Aib‐Pro‐Gly) ( 2 ) was prepared in 34% yield when DEPC was used in the cyclization step. The solid‐state conformation of 1 was established by X‐ray crystallography.     

Synthesis of cyclo-PLAI using a combination of solid- and solution-phase methods

Cyclo-PLAI was successfully synthesized using a combination of solid- and solution-phase methods. This current synthesis was found to be faster than the previously reported synthesis for the cyclic peptide. The linear precursor was synthesized on 2-chlorotrityl resin with Fmoc/t-Bu strategy. HATU/HOAt was employed as the coupling reagent in the amide bond formation on the resin. Cyclization of the linear precursor was experimented with HATU/HOAt reagents with different conditions. However, the linear precursor was best cyclized using HATU reagent in DIPEA by stirring the reaction mixture at 0?°C for 1?h and followed by stirring the reaction mixture at room temperature for 30?min, giving the cyclic product in 70% yield (calculated from the linear peptide). Both linear and cyclic products were characterized using HR-TOF-ESMS, ¹H-NMR, ¹³C-NMR, and compared with previously reported spectral data for the cyclic product.     

Total synthesis of five proline-enriched cyclic heptapeptides from the marine sponge Stylissa carteri

The total synthesis of five naturally occurring cyclic proline-enriched heptapeptides from the marine sponge Stylissa carteri was reported. The five cyclic heptapeptides were synthesized by applying a two-step solid-phase/solution synthesis strategy. The linear heptapeptides were assembled by standard Fmoc chemistry on 2-chlorotrityl chloride resin, cleaved off-resin with acetic acid/trifluoroethanol/dichloromethane to keep side-chain protecting groups intact, and subsequently cyclization was achieved by a solution method. The final products were purified by a preparative RP-HPLC system, and their structures were characterized by HR-QTOF-MS NMR. The spectral data of synthetic peptides were found to be identical to that reported for the natural products.     

Catalytic Double Carbon–Boron Bond Formation for the Synthesis of Cyclic Diarylborinic Acids as Versatile Building Blocks for π‐Extended Heteroarenes

The first catalytic synthesis of cyclic diarylborinic acids is developed using a dihydroaminoborane reagent as the boron source. Unlike previously reported methods that use organolithium reagents, this method allows the easy synthesis of cyclic diarylborinic acids bearing a range of functionalities including CN, CO₂Et, CONEt₂ and NMeCO₂^t Bu. Furthermore, these cyclic diarylborinic acids provide rapid access to skeletal diversity, in particular they enable the synthesis of six‐ to nine‐membered π‐extended heteroarenes through simple cross‐coupling reactions, which are important synthetic targets in both advanced materials and pharmaceuticals.     

Versatile soluble oligomeric styrene supports for peptide synthesis

Soluble oligomeric styrene supports are reported here with high loading capacities of 1.5–1.6 mmol/g similar to resins used in solid phase peptide synthesis. Oligoether and alkyl chains are incorporated into the scaffold to improve the support solubility and act as spacers between the attachment sites. Amino acids have been attached to the support in 59–85% yields and 0.87–1.3 mmol/g loading. The supports have been used to synthesize tri‐ to hexapeptides in 38–64% yields using only 2 equivalents of coupling reagents, which is much lower than the amount of reagents typically used in solid phase synthesis. A modular synthetic approach is used to obtain the supports so that any efficient styrene‐based attachment site can be readily incorporated into our soluble support scaffold. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2501–2509     

Rubiyunnanins A and B, two novel cyclic hexapeptides from Rubia yunnanensis

Two novel cyclic hexapeptides, named rubiyunnanins A (1) and B (2), were isolated from the roots of Rubia yunnanensis (Franch.) Diels. Their structures were elucidated extensively by spectroscopic analysis and theoretical computation. Possible biosynthetic pathways for RAs, 1, and 2 were proposed. Compound 2 showed moderate cytotoxicities against 11 cancer cell lines and inhibited nitric oxide (NO) production in LPS and IFN-γ-induced RAW 264.7 murine macrophages.     

5'-O-(2-Isopropoxyprop-2-yl)-protected Phosphoramidite Building Blocks in the Liquid Phase Oligonucleotide Synthesis

5'-O-(2-isopropoxyprop-2-yl) (IIP)-protection was introduced to 5’-OH function of nucleosides in high yields by an acid-catalysed transacetalization with 2,2-diisopropoxypropane. The applicability of this temporal 5’-O-protecting group was demonstrated in the liquid phase oligonucleotide synthesis (LPOS) using the corresponding phosphoramidite building blocks (dA, dG, dC and dT) and a tetrapodal precipitative soluble support. Standard protecting groups were used on nucleobases. Tetrazole as an activator, followed by oxidation using m-chloroperbenzoic acid, was used for the coupling. The IIP was shown to be a capable choice to the 5’-O protection in solution phase synthesis. It could be readily removed with formic acid (t_1/2<10 s in 6 % HCOOH in dichloromethane/methanol (2/1) at RT), resulting in volatile byproducts (acetone and isopropanol).     

A label-free amperometric immunosenor based on multi-layer assembly of polymerized o-phenylenediamine and gold nanoparticles for determination of Japanese B encephalitis vaccine

A label-free amperometric immunosensor for fast and sensitive assay of Japanese B encephalitis vaccine is presented. Antiserum of Japanese B encephalitis were immobilized on bilayer nano-Au/o-phenylenediamine polymer film with deposited Prussian blue as an electronic mediator on the Pt electrode. The electrochemical behavior of the biosensor was studied with Fe^2+/3+ as probe on the Pt surface using cyclic voltammetry technique. The variation of amperometric response to the concentration of Japanese B encephalitis vaccine, the target antigen, was evaluated by cyclic voltammetry in PBS. The immunosensor showed a specific response to Japanese B encephalitis vaccine in the range 1.1 × 10⁻⁸ to 1.9 × 10⁻⁶ lg pfu/ml (pfu/ml is plaque forming unit and lg is common logarithm) with a detection limit of 6 × 10⁻⁹ lg pfu/ml. The correlation coefficient is 0.9955. The incubation time, incubation temperature, pH, reproducibility and stability of the immunosensor were also studied. The present work supplied a promising test method for biological products.     

Copper‐Catalyzed Decarboxylative Radical Silylation of Redox‐Active Aliphatic Carboxylic Acid Derivatives

A decarboxylative silylation of aliphatic N ‐hydroxyphthalimide (NHPI) esters using Si−B reagents as silicon pronucleophiles is reported. This C(sp³)−Si cross‐coupling is catalyzed by copper(I) and follows a radical mechanism, even with exclusion of light. Both primary and secondary alkyl groups couple effectively, whereas tertiary alkyl groups are probably too sterically hindered. The functional‐group tolerance is generally excellent, and α‐heteroatom‐substituted substrates also participate well. This enables, for example, the synthesis of α‐silylated amines starting from NHPI esters derived from α‐amino acids. The new method extends the still limited number of C(sp³)−Si cross‐couplings of unactivated alkyl electrophiles.     

Directing/Protecting‐Group‐Free Synthesis of Tetraaryl‐Substituted Pyrazoles through Four Direct Arylations on an Unsubstituted Pyrazole Scaffold

A directing/protecting‐group‐free synthesis of 1,3,4,5‐tetraaryl‐substituted pyrazoles was achieved through four transition metal‐catalyzed direct arylations. Various pyrazoles with four different aryl rings were obtained using readily available reagents from an unsubstituted pyrazole. Two aryl‐substituted pyrazoles showed intense violet fluorescence, high quantum yields (Φ_f=0.68, 0.64), and large Stokes shifts (19000, 15200 cm^?1).     

A microwave-assisted stereoselective synthesis of polyandrocarpamines A and B

A stereoselective synthesis of the marine natural products, polyandrocarpamines A and B, has been achieved using a high-yielding one-step aldol condensation reaction under microwave conditions. The structures of both synthetic compounds were confirmed following 1D and 2D NMR, UV, IR and MS spectral analysis, and by comparison with literature data. Both synthetic natural products were assigned Z geometry about their exocyclic double bond on the basis of ¹³C/¹H long-range coupling constants, which were measured using a gHSQMBC experiment. Polyandrocarpamines A and B were evaluated for their cytotoxicity towards the tumour cell lines, MCF-7 (breast), H460 (lung) and SF268 (central nervous system). Polyandrocarpamine A displayed selective cytotoxicity towards the SF268 cell line with a GI₅₀ value of 65 μM.     

Highly diastereoselective preparation of anti-α,β-dialkyl β-amino acids containing natural α-amino acid side chains

An efficient synthesis of anti-2-alkyl β³-amino acids was developed starting from the fully protected β³-amino acids. The strategy allows the introduction of the side chain of natural α-amino acids such as Ala, Phe and Ser at the C-2 position, with high diastereoselectivity. The preparation of 2-methyliden-β³-amino acids is also reported. This methodology does not need the use of expensive chiral reagents and/or chiral auxiliaries, and leads to compounds with orthogonal protecting groups.     

Synthesis and conformational analysis of 18-membered Aib-containing cyclohexapeptides

The synthesis and conformational analysis of two Aib-containing cyclic hexapeptides, cyclo(Gly-Aib-Leu-Aib-Phe-Aib) 1 and cyclo(Leu-Aib-Phe-Gly-Aib-Aib) 2, is described. The linear precursors of 1 and 2 were prepared using solution phase techniques, and the cyclization efficiency of three different coupling reagents (HATU, PyAOP, DEPC) was examined. The success of the cyclization was found to be reagent dependent. Solid-state conformational analysis of 1 and 2 was performed by X-ray crystallography and has revealed some unusual features as all three Aib residues of 1 assume nonhelical conformations. Furthermore, the residue Aib⁴ adopts an extended conformation (?=−175.9(3)°, ψ=+178.6(2)°), which is, to the best of our knowledge, the first observation of an Aib residue adopting an extended conformation in a cyclopeptide. The structure of 1 is also a rare example in which an Aib residue occupies the (i+1) position of a type II′ β-turn, stabilized by a bifurcated hydrogen bond. The cyclic peptide 2 adopts a more regular conformation in the solid state, consisting of two fused β-turns of type I/I′, stabilized by a pair of intramolecular hydrogen bonds. In addition, the conformational study of the cyclic peptide 1 in DMSO-d₆ by NMR spectroscopy and molecular dynamics simulations revealed a structure, which is very similar to its structure in the crystalline state.     

4‐(4,6‐Dimethoxy‐1,3,5‐triazin‐2‐yl)‐4‐methylmorpholinium Toluene‐4‐sulfonate (DMT/NMM/TsO−) Universal Coupling Reagent for Synthesis in Solution

4‐(4,6‐Dimethoxy‐1,3,5‐triazin‐2‐yl)‐4‐methylmorpholinium toluene‐4‐sulfonate (DMT/NMM/TsO⁻), a representative member of the inexpensive and environmentally‐friendly N‐triazinylammonium family of sulfonates, has been found to be a very effective coupling reagent for the synthesis of amides, esters and peptides in solution. This study confirms the usefulness of DMT/NMM/TsO⁻ for peptide synthesis in solution, starting from Z‐, Fmoc‐, and Boc‐protected substrates as well as unnatural building blocks. Peptide synthesis with DMT/NMM/TsO⁻ produced high yields, with high crude product purity and low risk of racemization. In all cases, stoichiometric amounts of reagents were used and the standard synthetic procedure, without the need for time‐consuming optimization stages or expensive chromatographic purification. DMT/NMM/TsO⁻ was also found to be very useful for the synthesis of oligopeptides using a fragment coupling strategy.     

Total Synthesis of Lajollamycin B

The first total synthesis of lajollamycin B, a structurally novel nitro-tetraene spiro-β-lactone/γ-lactone antibiotic, is described. The convergent synthesis involves the construction of the C8′–C11′ nitrodienylstannane and its coupling with the segment prepared from the C1′–C7′ ω-iodoheptadienoic acid and the right-hand heterocyclic fragment, which has been utilized for our previous syntheses of oxazolomycin A. The revision of the geometry of the terminal Δ^{10′, 11′}-double bond from E to Z is also described for the structure of natural lajollamycin B.     

Oxygen Activated,Palladium Nanoparticle Catalyzed,Ultrafast Cross‐Coupling of Organolithium Reagents

The discovery of an ultrafast cross‐coupling of alkyl‐ and aryllithium reagents with a range of aryl bromides is presented. The essential role of molecular oxygen to form the active palladium catalyst was established; palladium nanoparticles that are highly active in cross‐coupling reactions with reaction times ranging from 5 s to 5 min are thus generated in situ. High selectivities were observed for a range of heterocycles and functional groups as well as for an expanded scope of organolithium reagents. The applicability of this method was showcased by the synthesis of the [¹¹C]‐labeled PET tracer celecoxib.     

Oxygen Activated,Palladium Nanoparticle Catalyzed,Ultrafast Cross‐Coupling of Organolithium Reagents

The discovery of an ultrafast cross‐coupling of alkyl‐ and aryllithium reagents with a range of aryl bromides is presented. The essential role of molecular oxygen to form the active palladium catalyst was established; palladium nanoparticles that are highly active in cross‐coupling reactions with reaction times ranging from 5 s to 5 min are thus generated in situ. High selectivities were observed for a range of heterocycles and functional groups as well as for an expanded scope of organolithium reagents. The applicability of this method was showcased by the synthesis of the [¹¹C]‐labeled PET tracer celecoxib.     

Total synthesis and biological evaluation of dracaenins A and B

The first total synthesis of dracaenins A and B is achieved in four steps. The synthesis features the convergent coupling of three readily available fragments with minimized use of protecting groups. The chemical synthesis enables the discovery of their activity in stimulating platelet aggregation, and thus, sheds light on the possible origin of the hemostatic effect of dragon’s blood.