N(epsilon) functionalization of metal and organic protected L-histidine for a highly efficient,direct labeling of biomolecules with [Tc(OH2)3(CO)3]+

Two different pathways for the introduction of an acetyl group at N(epsilon ) in a N(alpha), N(delta), and -COO protected histidine to afford N(epsilon)-(CH(2)COOH)-histidine derivative 7 b are presented. The purpose of this study is the coupling of 7 b to amino groups in bioactive molecules such as peptides. After full deprotection of such a bioconjugate, histidine provides three coordination sites which efficiently coordinate to [(99m)Tc(OH(2))(3)(CO)(3)](+) or [Re(OH(2))(3)(CO)(3)](+) in a facial geometry. This allows the development of novel radiopharmaceuticals. Selective derivatization at the N(epsilon) position has conveniently been achieved by concomitant protection of N(alpha) and N(delta) with a carbonyl group forming a six-membered urea. Cyclic urea ring opening with Fm-OH, coupling of phenylalanine as a model to 7 b through its primary amine and removing of all protecting groups in one step gave a histidine derivative of phenylalanine which could be labeled at 10(-5) M with (99m)Tc in very high yield and even in about 50 % yield at 10(-6) M. The Xray structure of a complex with [Re(CO)(3)](+) in which anilin is coupled to 7 b confirms the facial arrangement of histidine. A second pathway applies directly the [Re(CO)(3)](+) moiety as a protecting group. This is one of the rare examples in which a metal fragment is used as a protecting group for organic functionalities. The coordination to histidine protects the N(alpha), N(delta) and COO group in one single step, subsequent alkylation with BrCH(2)COOH(R) at N(epsilon), coupling to phenylalanine and oxidative deprotection of [Re(CO)(3)](+) to [ReO(4)](-) gave the corresponding bioconjugate in which histidine is coupled to phenylalanine through an acetylamide at N(epsilon). Both methods offer convenient pathways to introduce histidine in a biomolecule under retention of its three coordination sites. The procedures are adaptable to any biomolecule with pendant amines and allow the development of novel radiopharmaceuticals or inversed peptides.     

An Environmentally Benign System for Synthesis of β-Hydroxylketones： L-Histidine Asymmetrically Catalyzed Direct Aldol Reactions in Aqueous Micelle and Water-like Media

The first histidine catalyzed direct aldol reactions of ketones with nitrobenzaldehydes in water and in poly（ethylene glycol） （PEG） were reported. It reveals that histidine is a good aldol catalyst for synthesis of β-hydroxylketones in water and in PEG, giving good to excellent yields of the respective products. Better enantioand regioselectivity were achieved using low molecular weight PEG as the media. The results show that histidine and PEG-200 or -300 may constitute a promising environmentally benign system for asymmetric synthesis of β-hydroxylketones.     

Cyclododecane and cyclotridecane complexes with cobalt as potent histidine chelators: A theoretical study

Density functional theory calculations have been employed to study the interaction between cyclododecane and cyclotridecane derivatives complexing cobalt and interacting with histidine. The results suggest that some of these derivatives have high molecular affinities toward histidine, higher than in the commonly used iminodiacetate–cobalt system. Those derivatives may become new, potent chelators for use in the immobilized-metal-ion-affinity chromatography to immobilize and/or purify peptides and proteins.     

Syntheses and relaxation properties of mixed gadolinium hydroxypyridinonate MRI contrast agents

The tripodal ligand tris[(3-hydroxy-1-methyl-2-oxo-1,2- didehydropyridine-4-carboxamido)ethyl]amine (TREN-Me-3,2-HOPO) forms a stable Gd3+ complex that is a promising candidate as a magnetic resonance imaging (MRI) contrast agent. However, its low water solubility prevents detailed magnetic characterization and practical applicability. Presented here are a series of novel mixed ligand systems that are based on the TREN-Me-3,2-HOPO platform. These new ligands possess two hydroxypyridinone (HOPO) chelators and one other chelator, the latter of which can be easily functionalized. The ligands described use salicylamide, 2-hydroxyisophthalamide, 2,3-dihydroxyterephthalamide, and bis(acetate) as the derivatizable chelators. The solution thermodynamics and relaxivity properties of these new systems are presented. Three of the four complexes (salicylamide-, 2-hydroxyisophthalamide-, and 2,3-dihydroxyterephthalamide-based ligands) possess sufficient thermodynamic stability for in vivo applications. The relaxivities of the three corresponding Gd3+ complexes range from 7.2 to 8.8 mM-1 s-1 at 20 MHz, 25 degrees C, and pH 8.5, significantly higher than the values for the clinically employed polyaminocarboxylate complexes (3.5-4.8 mM-1 s-1). The high relaxivities of these complexes are consistent with their faster rates of water exchange (< 100 ns), higher molecular weights (> 700), and greater numbers of inner-sphere coordinated water molecules (q = 2) relative to those of polyaminocarboxylate complexes. A mechanism for the rapid rates of water exchange is proposed involving a low energy barrier between the 8- and 9-coordinate geometries for lanthanide complexes of HOPO-based ligands. The pathway is supported by the crystal structure of La[TREN-Me-3,2-HOPO] (triclinic P1: Z = 4, a = 15.6963(2) A, b = 16.9978(1) A, c = 17.1578(2) A, alpha = 61.981(1) degrees, beta = 75.680(1) degrees, gamma = 71.600(1) degrees), which shows both 8- and 9-coordinate metal centers in the asymmetric unit, demonstrating that these structures are very close in energy. These properties make heteropodate Gd3+ complexes promising candidates for use in macromolecular contrast media, particularly at higher magnetic field strengths.     

A stereochemical surprise at the late stage of the synthesis of fully N-differentiated heparin oligosaccharides containing amino, acetamido, and N-sulfonate groups

The glucosamine residues in heparin-like glycosaminoglycans have been found to exist as amines, acetamides, and N-sulfonates. To develop a completely general, modular synthesis of heparin, three degrees of orthogonal nitrogen protection are required. Reported herein is a strategy for the synthesis of fully N-differentiated heparin oligosaccharides in the context of target octasaccharide 1, which contains an N-acetate, N-sulfonates, and a free amine. The protecting group scheme used in the synthesis blocked the N-acetate as a N-diacetate, the N-sulfonates as azido groups, and the amine as a N-CBz; free hydroxyls were masked as benzyl ethers and O-sulfonates as acetate esters. Disaccharide and tetrasaccharide modules were synthesized using this strategy; however, the union of tetrasaccharide trichloroacetimidate 4 with disaccharide acceptor 5 unexpectedly formed the undesired beta-linked glycoside in addition to the alpha-linkage anticipated for iduronic acid nucleophiles, resulting in an inseparable 6:1 alpha/beta mixture of products. Detailed studies into the basis for this unexpected result were conducted and are also reported.     

Synthesis of highly functionalized strained bicyclic dilactam derivatives

An efficient and modest protocol has been utilized for the synthesis of bicyclic dilactam derivatives through one-pot multicomponent domino reaction starting from various cyclic ketones under mild condition. The synthesized motif shows four stereogenic centers with two quaternary amine functionalities and such molecular arrangements are very fascinating and rare to obtain. We propose a mechanism for the formation of bicyclic dilactams through aldol condensation/condensation of cyanoacetamides/cyclization of condensed intermediate.     

Potential Building Blocks for 1,4-Dihydro-N-heteroacenes

Studies have been performed aimed at the synthesis of N-heteroacenes via substitution reactions of 4,5-difluoro-1,2-dinitrobenzene with a diamine. The fluorine atoms are displaced first, followed by an activated nitro group. Two intermediates have been characterised by X-ray single-crystal structure determinations. Their intermolecular interactions were examined by Hirshfeld surfaces to assess their suitability for organic molecular electronics. The high reactivity of the phenazine, which is prone to oxidise and rearrange, as are displacement products prepared from it, is explained by the formation of a cis-aci-nitro form from the secondary amine of the phenazine and a nitro group.     

Concise enantioselective synthesis of 3,5-dialkyl-substituted indolizidine alkaloids via sequential cross-metathesis-double-reductive cyclization

An efficient stereoselective synthesis of two 3,5-dialkyl-substituted indolizidine alkaloids is reported. The convergent syntheses are based on a novel sequence of a cross-metathesis (CM) reaction of an alpha,beta-unsaturated ketone and a chiral homoallylic amine followed by a domino reaction involving hydrogenation, N-deprotection, and two diastereoselective reductive aminations. Our concept presents one of a few examples of a highly selective CM reaction in the synthesis of a natural product.     

Cascade intramolecular N-arylation/intermolecular carboamination reactions for the construction of tricyclic heterocycles

A new method for the stereoselective synthesis of tetrahydropyrroloindoles and hexahydropyrroloquinolines of general structure 8 is described. These products are formed through cascade Pd-catalyzed coupling reactions between aryl chlorides and unsaturated amine substrates 5. A single catalyst effects an intramolecular N-arylation reaction followed by an intermolecular alkene carboamination reaction to generate two rings, three bonds, and one stereocenter with good chemoselectivity, diastereoselectivity, and chemical yield.     

A convenient method for the preparation of fluorous tin derivatives for the fluorous labeling strategy

A convenient method for the preparation of fluorous aryl stannanes was developed as a means of expanding the general utility of the fluorous labeling strategy (FLS). Following the synthesis of a novel fluorous distannane, a palladium-catalyzed cross-coupling reaction was used to prepare the target compounds from aryl halides. The scope of the reaction was investigated by preparing a small library of model compounds where the reaction yields were similar to those reported for the analogous procedures employing hexamethyl- or hexabutyldistannanes. The utility of the reported methodology was demonstrated through the successful synthesis of fluorous precursors to two established molecular imaging and therapy agents (FIAU, IUdR). These were radiolabeled with iodine-125 and the desired products isolated in high yield and effective specific activity.     

Synthesis, characterisation, and biodistribution of radioiodinated C-hydroxy-carboranes

The synthesis, radiolabelling and biodistribution of iodinated C-hydroxy-nido-carborane ligands is described. Microwave heating by using NaF in aqueous ethanol was used to prepare {sodium [7-hydroxy-7,8-dicarba-nido-undecaborate], nido-carboranol} and {sodium [7-hydroxy-7,8-dicarba-nido-undecaborate-8-carboxylic acid], nido-salborin} in 97 and 90?% yield, respectively. Radioiodination of these nido-carboranes was completed by using both (125) I and (123) I, and the products were obtained in high radiochemical purity (>99?%) and yield (72 to 87?%). The structures of the radiolabelled products were validated through comparison to authentic standards. Biodistribution studies in BALB/c mice showed low accumulation of the labelled compounds in the liver and intestines, which are sites where labelled carboranes typically localise. The labelled cluster bearing hydroxy and carboxylic acid groups on the two carbon vertices demonstrated preferential clearance through the kidneys and low thyroid uptake. This compound had substantially reduced non-specific binding than the deshydroxy analogue making it an attractive bifunctional ligand for preparing targeted molecular imaging and therapy agents.     

Weinreb Amide,Ketone and Amine as Potential and Competitive Secondary Molecular Stations for Dibenzo-[24]Crown-8 in [2]Rotaxane Molecular Shuttles

This paper reports the synthesis and study of new pH-sensitive DB24C8-based [2]rotaxane molecular shuttles that contain within their axle four potential sites of interaction for the DB24C8: ammonium, amine, Weinreb amide, and ketone. In the protonated state, the DB24C8 lay around the best ammonium site. After either deprotonation or deprotonation-then-carbamoylation of the ammonium, different localizations of the DB24C8 were seen, depending on both the number and nature of the secondary stations and steric restriction. Unexpectedly, the results indicated that the Weinreb amide was not a proper secondary molecular station for the DB24C8. Nevertheless, through its methoxy side chain, it slowed down the shuttling of the macrocycle along the threaded axle, thereby partitioning the [2]rotaxane into two translational isomers on the NMR timescale. The ketone was successfully used as a secondary molecular station, and its weak affinity for the DB24C8 was similar to that of a secondary amine.     

超支化聚(胺酯)的分子设计及其制备

以丙烯酸甲酯和二乙醇胺为原料由Michael加成反应制得N ,N 二羟乙基 3 胺基丙酸甲酯单体 ,再用“准一步法”和“发散法”使之与 1 ,1 ,1 三羟甲基丙烷 (核 )反应合成一种新型超支化聚 (胺 酯 ) .以核磁共振和元素分析方法对N ,N 二羟乙基 3 胺基丙酸甲酯单体的分子结构进行了表征 .GPC测定表明合成的超支化聚 (胺 酯 )分子量分布窄 ,具有单分散性 ;粘度小于同分子量的线形分子 ;耐热性能较好 ,失重温度高于2 0 0℃ .     

An improved approach for the synthesis of alpha,alpha-dialkyl glycine derivatives by the Ugi-Passerini reaction

A general and simple strategy for routine peptide synthesis with alpha,alpha-dialkyl glycines taking advantage of the four-component Ugi-Passerini reaction is presented. The isonitrile required for the reaction can be relatively simple and its selection based on cost, as the group it generates is easily removed under acidic conditions: in addition, this removal is not visibly affected by the bulkiness of the alpha-alkyl groups. Being a good leaving group from the N-terminal amino group of the amino acid, 4-methoxybenzyl was the choice for the amine component of the reaction. The method is illustrated with the synthesis of a series of acyl derivatives of several alpha,alpha-dialkyl glycines. The preparation of the latter compounds is also reported.     

Benzoylureas as removable cis amide inducers: synthesis of cyclic amides via ring closing metathesis (RCM)

Rapid and high yielding synthesis of medium ring lactams was made possible through the use of a benzoylurea auxiliary that serves to stabilize a cisoid amide conformation, facilitating cyclization. The auxiliary is released after activation under the mild conditions required to deprotect a primary amine, such as acidolysis of a Boc group in the examples given here. This methodology is a promising tool for the synthesis of medium ring lactams, macrocyclic natural products and peptides.     

Solid-phase synthesis of phenolic steroids: from optimization studies to a convenient procedure for combinatorial synthesis of biologically relevant estradiol derivatives

During the course of our studies on therapeutic agents for the treatment of breast cancer, we became interested in the solid-phase combinatorial synthesis of estradiol derivatives that contain a functionalized side chain at either position 16 beta or 7 alpha. Both types of compounds have already demonstrated inhibitory activity toward both biosynthesis and action of estradiol. As a first step, two versatile precursors bearing an azidoalkyl side chain at either position 16 beta or 7 alpha of estradiol were synthesized using standard solution-phase methods. Afterward, the effectiveness of five linkers to attach the phenolic function of these estradiol derivatives to a polystyrene resin was investigated; they were benzylic ether (Merrifield), 4-alkoxy-benzylic ethers (Wang, Sheppard), tetrahydropyranyl ether (Ellman), benzoic ester, and o-nitrobenzyl ether. To test the linker in a synthetic context, a short sequence of reactions, including reduction of the azide and acylation of the corresponding amine, was performed on the polymer-bound estradiol derivative. While all of the tested linkers proved effective in attaching the phenol functionality of the precursor, only the o-nitrobenzyl ether photolabile linker enabled the release of the final products in acceptable purities. Consequently, this linker was used to perform successfully the solid-phase synthesis of four different classes of estradiol derivatives in acceptable yields and excellent purities. This study was preliminary to the combinatorial synthesis of larger libraries of biologically relevant estradiol derivatives.     

Synthesis of Histidine‐Containing Oligopeptides via Histidine‐Promoted Peptide Ligation

Histidine‐containing peptides are valuable therapeutic agents for a treatment of neurodegenerative diseases. However, the synthesis of histidine‐containing peptides is not trivial due to the potential of imidazole sidechain of histidine to act as a nucleophile if unprotected. A peptide ligation method utilizing the imidazole sidechain of histidine has been developed. The key imidazolate intermediate that acts as an internal acyl transfer catalyst during ligation is generated by deprotonation. Transesterification with amino acids or peptides tethered with C‐terminal thioester followed by N→N acyl shifts led to the final ligated products. A range of histidine‐containing dipeptides could be synthesized in moderate to good yields via this method without protecting the imidazole sidechain. The protocol was further extended to tripeptide synthesis via a long‐range N→N acyl transfer, and tetrapeptide synthesis.     

Influence of amino acid relative position on the oxidative modification of histidine and glycine peptides

The radical oxidation of isomeric peptides containing one reactive amino acid [histidine (H)] and another less reactive amino acid [glycine (G)] in the form of dipeptides (HG and GH) and tripeptides (HGG, GHG, and GGH) was studied by mass spectrometry coupled to liquid chromatography (LC-MS) for detection and LC-MSⁿ for structural characterization. The oxidation products identified were keto, hydroxy, keto-hydroxy, and hydroperoxide derivatives for both di- and tripeptides. Among these, it was found that insertion of oxygen atoms occurred at histidine for HG and HGG, and both histidine and glycine for GH, GHG, and GGH. In addition, oxidation products formed by alkoxyl rearrangement reactions with cleavage of the peptide chain were also identified for GH, GHG, and GGH, corroborating hydrogen abstraction step in G residues. These findings were supported through the identification of radical intermediate species formed and trapped with 5,5-dimethyl-1-pyrrolidine-N-oxide (DMPO) spin trap. The observation of DMPO adducts bearing two spin trap molecules reinforced the abstraction of two hydrogen atoms from the same molecule.     

Molecular modeling and mutagenesis reveals a tetradentate binding site for Zn2+ in GABA(A) alphabeta receptors and provides a structural basis for the modulating effect of the gamma subunit

Gamma-aminobutyric acid type A receptors (GABA(A)-R) containing alpha1beta2gamma2 subunits are weakly inhibited by Zn2+, whereas receptors containing only the alpha1beta2 subunits are strongly inhibited. We built homology models of the ion pores of alpha1beta2 and alpha1beta2gamma2 GABA(A)-R using coordinates of the nicotinic acetylcholine receptor as a template. Threading the GABA(A)-R beta2 sequence onto this template placed the 17' histidine and the 20' glutamate residues at adjacent locations in the mouth of the pore, such that a nearly ideal tetradentate site for Zn2+ was formed from two histidine and two glutamate residues between adjacent beta subunits in the alpha1beta2 GABA(A)-R. Following optimization with CHARMM, the distance between the alpha-carbons of the adjacent histidine residues was approximately 9.2 A, close to the ideal distance for a Zn2+ binding site. Loss of inhibition by Zn2+ in alpha1beta2gamma2 GABA(A)-R can be explained by the geometry of these residues in the arrangement alpha1beta2gamma2alpha1beta2, in which the nearest C-alpha-C-alpha distance between the histidine residues is 15.5 A, too far apart for an energetically optimal Zn2+ binding site. We then mutated the gamma subunit at the 17' and/or 20' positions. Zn2+ inhibition was not restored in alpha1beta2gamma2 (I282H) receptors. A novel finding is that the modeling shows the native 20' lysine in gamma2 can compete with Zn2+ for binding to the inserted 17' histidine. Sensitivity to Zn2+ was restored in the double mutant receptor, alpha1beta2gamma2 (I282H; K285E), in which the competition with lysine was removed and a more favorable Zn2+ binding site was formed.     

Synthesis and metal binding properties of salicylate-, catecholate-, and hydroxypyridinonate-functionalized dendrimers

The synthesis, characterization, and metal-binding studies of chelate-functionalized dendrimers is reported. Salicylate, catecholate, and hydroxypyridinonate bidentate chelators have been coupled to the surface of both poly(propyleneimine) (Astramol) and poly(amidoamine) (Starburst, PAMAM) dendrimers up to the fifth generation (64 endgroups). A general method has been developed for the facile and high quality chromatographic purification of poly(propyleneimine) and poly(amidoamine) dendrimer derivatives. One- and two-dimensional (TOCSY) 1H NMR experiments and electrospray ionization mass spectrometry (ESI-MS) have confirmed the exhaustive coupling of these chelators to the primary amine functionalities of the dendrimers. Spectrophotometric titrations were used to investigate the metal binding ability of these macrochelates. Spectral analysis shows that ferric iron binding to these ligands is localized to the chelating endgroups. The ability of these dendritic polymers to bind large numbers of metal ions may lead to applications as metal sequestering agents for waste remediation technologies.