A novel solid-phase synthesis of highly diverse guanidines: reactions of primary amines attached to the T2 linker

Development and applications of two colorimetric and fluorescent indicators for Hg2+ detection

Two rhodamine-active probes RBAI (Rhodamine B-di-Aminobenzene-phenyl Isothiocyanate) and RGAI (Rhodamine 6G-di-Aminobenzene-phenyl Isothiocyanate) were designed, synthesized and characterized. The probes were developed as fluorescent and colorimetric chemodosimeters in ethanol-water solution with a broad pH span (5-10) and high selectivity toward Hg²⁺ but no significant response toward other common competitive cations. The Hg²⁺-promoted ring opening of spirolactam of the rhodamine moiety induced cyclic guanylation of the thiourea moiety, which resulted in the dual chromo- and fluorogenic observation (off-on). Cytotoxicity and bioimaging studies by L929 living cells and living mice indicated that the probes were negligible cytotoxicity, cell permeable and suitable for detecting Hg²⁺ in biological environments. Moreover, the new probes not only displayed excellent abilities for the successful detection of Hg²⁺ in L929 living cells and living mice but also able to detect Hg²⁺ by adsorbing on solid surfaces and quantitative detection of Hg²⁺ in real water samples with good recovery (more than 90%), indicating that they have promising prospect for application for Hg²⁺ sensing in environmental and biological sciences.

Selective Protection of Secondary Amines as the N-Phenyltriazenes. Application to Aminoglycoside Antibiotics

Selective protection of secondary amines as triazenes in the presence of multiple primary amines is demonstrated, with subsequent protection of the primary amines as either azides or carbamates in the same pot. Aminoglycoside antibiotic examples reveal broad functional group compatibility. The triazene group is removed with trifluoroacetic acid and, because of the low barrier to rotation, affords sharp (1)H NMR spectra at room temperature.

Water-soluble triazabutadienes that release diazonium species upon protonation under physiologically relevant conditions

Triazabutadienes are an understudied structural motif that have remarkable reactivity once rendered water-soluble. It is shown that these molecules readily release diazonium species in a pH-dependent manner in a series of buffer solutions with pH ranges similar to those found in cells. Upon further development, we expect that this process will be well suited to cargo-release strategies and organelle-specific bioconjugation reactions. These compounds offer one of the mildest ways of generating diazonium species in aqueous solutions.

Solid-phase synthesis of peptide thioureas and thiazole-containing macrocycles through Ru-catalyzed ring-closing metathesis

N-Terminally modified α-thiourea peptides can selectively be synthesized on solid support under mild reaction conditions using N,N'-di-Boc-thiourea and Mukaiyama's reagent (2-chloro-1-methyl-pyridinium iodide). This N-terminal modification applies to the 20 proteinogenic amino acid residues on three commonly used resins for solid-phase synthesis. Complementary methods for the synthesis of α-guanidino peptides have also been developed. The thiourea products underwent quantitative reactions with α-halo ketones to form thiazoles in excellent purities and yields. When strategically installed between two alkene moieties, said thiazole core was conveniently embedded in peptide macrocycles via Ru-catalyzed ring-closing metathesis reactions. Various 15-17 membered macrocycles were easily accessible in all diastereomeric forms using this methodology. The developed "build/couple/pair" strategy is well suited for the generation of larger and stereochemically complete screening libraries of thiazole-containing peptide macrocycles.

Azides – Diazonium Ions – Triazenes: Versatile Nitrogen-rich Functional Groups

For more than 100 years, nitrogen-rich compounds such as azides, diazonium ions, and triazenes have proved to be extremely valuable. Because these functional groups can be easily introduced into various substrates, they are frequently used nowadays. More importantly, they can be converted into a great number of other functional groups. The scope of this article is thus to summarize possible synthetic routes for the formation of these functional groups as well as to highlight some of the most prominent applications of these exciting moieties in chemical biology and combinatorial chemistry. Many of the most famous name reactions such as the Staudinger reduction, Staudinger ligation, Sandmeyer reaction, Wallach reaction, Mitsunobu reaction, Huisgen reaction, Balz–Schiemann reaction, Meerwein arylation, Pschorr reaction or Gomberg–Bachmann reaction are covered.

Modern reaction-based indicator systems

Traditional analyte-specific synthetic receptors or sensors have been developed on the basis of supramolecular interactions (e.g., hydrogen bonding, electrostatics, weak coordinative bonds). Unfortunately, this approach is often subject to limitations. As a result, increasing attention within the chemical sensor community is turning to the use of analyte-specific molecular indicators, wherein substrate-triggered reactions are used to signal the presence of a given analyte. This tutorial review highlights recent reaction-based indicator systems that have been used to detect selected anions, cations, reactive oxygen species, and neutral substrates.

Rhodamine-based Hg2+-selective chemodosimeter in aqueous solution: fluorescent OFF-ON

[reaction: see text] N-(Rhodamine-6G)lactam-N'-phenylthiourea-ethylenediamine (1) was developed as a fluorescent and colorimetric chemodosimeter in aqueous solution with a broad pH span (5 approximately 10) and high selectivity toward Hg2+ but no significant response toward other competitive cations, such as Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Pb2+, Cd2+, Ca2+, Mg2+, K+, Na+, etc. The Hg2+-promoted ring opening of spirolactam of the rhodamine moiety induced cyclic guanylation of the thiourea moiety, which resulted in the dual chromo- and fluorogenic observation (OFF-ON).

Preparative Fluorous Mixture Synthesis of Diazonium-Functionalized Oligo(phenylene vinylene)s

[structure: see text] A series of building blocks for the synthesis of oligo(phenylene vinylene)s (OPVs) and hybrid oligomers were prepared, and alternating Heck coupling and Horner-Wadswoth-Emmons (HWE) reactions were used to couple the building blocks. Model studies were carried out to optimize the reaction strategies. The products were made to bear aryl diazonium functionalities that allow them to be used as surface grafting moieties in hybrid silicon/molecule assemblies. A library of OPV and hybrid oligomer tetramers was synthesized using fluorous mixture synthesis (FMS). The fluorous tags, which are secondary amines bearing different numbers of fluorine atoms, were synthesized and used as phase tags in mixture synthesis. The tags and substrates were anchored together by triazene linkages. The mixture synthesis was monitored by analytical HPLC on a fluorous column, and isolation of final OPV and hybrid oligomer tetramers was achieved by preparative HPLC. At the end of the FMS, after demixing, the tagged products were detagged by cleaving the triazene linkage and generating a series of aryl diazonium compounds. The fluorous tags could be recovered and reused. The NMR spectra of the 1-aryl-3,3-dialkyltriazenes are discussed.

A selective fluorescent ratiometric chemodosimeter for mercury ion

A selective fluorescent chemodosimeter for mercury ion based on the mercury-promoted intramolecular cyclic guanylation of thiourea connected on 1, 8-naphthalimide is described.

New Cellulose-Supported Reagent: A Sustainable Approach to Guanidines

[reaction: see text] A new cellulose-supported reagent for the synthesis of guanidine in aqueous medium is reported starting from commercially available functionalized cellulose beads. Primary and secondary amines, anilines, and amino acids were transformed to the corresponding guanidines in high yields and under very mild conditions.

The virtue of the multifunctional triazene linkers in the efficient solid-phase synthesis of heterocycle libraries

With the implementation of combinatorial chemistry into the modern drug discovery process, the approach to novel diverse heterocycle libraries is an indispensable requirement. Triazenes, which are concealed diazonium salts, can be used to link functionalized arenes and amines to generate various heterocyclic structures, namely, benzoannelated nitrogen heterocycles, upon cleavage from the resin. Since triazene anchors are stable toward various reagents and perform well under a range of reaction conditions, these multifunctional linkers are well suited for automated solid-phase syntheses and the syntheses of complex organic molecules, such as natural products, on solid supports.

Efficient synthesis of sulfonic, phosphoric, and phosphinic esters employing alkylating polymer-bound reagents

The efficient esterification of various sulfonic acids and sulfonates using polymer-bound triazenes based on the triazene T2 linker is described. Esterification of enantiopure alpha-substituted sodium sulfonates was performed in the presence of an alkylating resin without racemization. Racemization is a serious drawback in the esterification route via sulfonyl chlorides because of intermediate sulfene formation. To demonstrate the versatility of this protocol, phosphoric and phosphinic acids have been converted into the corresponding esters as well. All products were obtained in good yield and excellent purities without any further purification steps.

TFA-sensitive arylsulfonylthiourea-assisted synthesis of N,N'-substituted guanidines

An efficient synthesis of N,N'-substituted guanidine derivatives was developed via an aromatic sulfonyl-activated thiourea intermediate. The use of certain aromatic sulfonamides, such as PbfNH(2), as the key reagent to incorporate a TFA-labile guanidine protection group greatly facilitates solid-phase synthesis of N,N'-substituted guanidine compounds.

Deciphering the mechanistic dichotomy in the cyclization of 1-(2-ethynylphenyl)-3,3-dialkyltriazenes: competition between pericyclic and pseudocoarctate pathways

The mechanistic aspects of the cyclization of (2-ethynylphenyl)triazenes under both thermal and copper-mediated conditions are reported. For cyclization to an isoindazole, a carbene mechanistic pathway is proposed. The carbene intermediate can react with oxygen, dimerize to give an alkene, or be trapped either intermolecularly (using 2,3-dimethyl-2-butene to generate a cyclopropane) or intramolecularly (using a biphenyl moiety at the terminus of the acetylene to form a fluorene). Density-functional theory (DFT) calculations support a pseudocoarctate pathway for this type of cyclization. Thermal cyclization to give a cinnoline from (2-ethynylphenyl)triazenes is proposed to occur through a pericyclic pathway. DFT calculations predict a zwitterionic dehydrocinnolinium intermediate that is supported by deuterium trapping studies as well as cyclizations performed using a 2,2,6,6-tetramethylpiperidine moiety at the 3-position of the triazene.

Solid-phase synthesis of trisubsituted guanidines

The solid-phase library synthesis of trisubstituted guanidines was accomplished. Amines were loaded onto the 4-formyl-3,5-dimethoxyphenoxymethyl linker via reductive amination. Subsequent acylation with Fmoc-4-aminomethylbenzoic acid followed by Fmoc deprotection gave solid-supported primary amines. Alternatively, sulfonylation of resin-bound secondary amines with 4-cyanobenzenesulfonyl chloride followed by borane reduction also gave solid-supported primary amines. Both resins were acylated with isocyanates to furnish solid-supported ureas. Dehydration of ureas with p-toluenesulfonyl chloride in pyridine gave solid-supported carbodiimides. Nucleophilic addition of amines to the carbodiimide bond followed by cleavage off the solid support gave trisubstituted guanidines.

A novel approach for the solid-phase synthesis of substituted cyclic guanidines, their respective bis analogues, and N-acylated guanidines from N-acylated amino acid amides

An efficient method for the solid-phase synthesis of cyclic guanidines from N-acylated amino acid amides, bis cyclic guanidines from N-acylated dipeptides derived from orthogonally protected diamino acids, and N-acylated guanidines from disubstituted cyclic guanidines is described. The exhaustive reduction of N-acylated amino acid amides yields diamines that on treatment with cyanogen bromide lead to the formation of cyclic guanidines. Resin-bound orthogonally protected diamino acids (i.e., N(alpha)-Fmoc-N(x)-(Boc)-diamino acid, x = beta, gamma, delta, epsilon) were N-acylated following removal of the Fmoc group. Removal of the Boc functionality from the side chain then generated a primary amine. Subsequent coupling of Boc amino acids, followed by removal of the Boc group, generated dipeptides that were N-acylated. Exhaustive reduction of amide bonds of the N-acylated dipeptides generated tetraamines having four secondary amines, which upon cyclization with cyanogen bromide afforded the resin-bound trisubstituted bis cyclic guanidines. Treatment of the resin-bound disubstituted cyclic guanidines with carboxylic acids gave N-acylated guanidines. On the basis of their high yield and purity, bis cyclic guanidines derived from N(alpha)-Fmoc-N(epsilon)-Boc-lysine and N-acylated guanidines were chosen for preparation of mixture-based combinatorial libraries. Details of the preparation of these positional scanning libraries using the "libraries from libraries" concept are presented.

Efficient introduction of protected guanidines in boc solid phase peptide synthesis

[reaction: see text] Reaction of primary amines with pyrazole 1 results in rapid and efficient guanidinylation, either in solution or on solid phase. The reaction affords sulfonamide-protected products required for BOC solid phase peptide synthesis (SPPS) in a single step under mild conditions. Incorporation of orthogonally protected side chain amines permits the synthesis of peptides containing arginine analogues, one of which could not be prepared by coupling of preformed amino acids.

A Novel Traceless Resin-Bound Guanidinylating Reagent for Secondary Amines To Prepare N,N-Disubstituted Guanidines

[reaction: see text]. We report the development of a solid support-linked guanidinylating reagent. This reagent consists of a urethane-protected triflyl guanidine attached to the resin via a carbamate linker. It allows for rapid synthesis of guanidines from a variety of amines. It provides access to N-alkyl/aryl- or N,N-dialkylguanidines under mild conditions. Cleavage with 50% TFA produces target molecules in high yields and purity. The ability to guanidinylate secondary amines is a significant feature of this guanidinylating reagent.