Non-canonical amino acids bearing thiophene and bithiophene: synthesis by an Ugi multicomponent reaction and studies on ion recognition ability

Electrochemical Synthesis of Unnatural Amino Acids Embedding 5- and 6-Membered Heteroaromatics

Using a commercially available potentiostat, the electrochemical synthesis of unnatural amino acids bearing heteroaromatics on the lateral chain has been accomplished. This strategy exploits the side-chain decarboxylative arylation of aspartic/glutamic acid, a reaction that becomes challenging with electron-rich coupling partners such as 5- and 6-membered heteroaromatics. These rings are underrepresented in unnatural amino acids, therefore allowing a wider exploration of the chemical space, given the abundance of the aryl bromides employable in this reaction.

New Amino Acid-Based Thiosemicarbazones and Hydrazones: Synthesis and Evaluation as Fluorimetric Chemosensors in Aqueous Mixtures

Bearing in mind the interest in the development and application of amino acids/peptides as bioinspired systems for sensing, a series of new phenylalanine derivatives bearing thiosemicarbazone and hydrazone units at the side chain were synthesised and evaluated as fluorimetric chemosensors for ions. Thiosemicarbazone and hydrazone moieties were chosen because they are considered both proton-donor and proton-acceptor, which is an interesting feature in the design of chemosensors. The obtained compounds were tested for the recognition of organic and inorganic anions (such as AcO-, F-, Cl-, Br-, I-, ClO4-, CN-, NO3-, BzO-, OH-, H2PO4- and HSO4-) and of alkaline, alkaline-earth, and transition metal cations, (such as Na+, K+, Cs+, Ag+, Cu+, Cu2+, Ca2+, Cd2+, Co2+, Pb2+, Pd2+, Ni2+, Hg2+, Zn2+, Fe2+, Fe3+ and Cr3+) in acetonitrile and its aqueous mixtures in varying ratios via spectrofluorimetric titrations. The results indicate that there is a strong interaction via the donor N, O and S atoms at the side chain of the various phenylalanines, with higher sensitivity for Cu2+, Fe3+ and F- in a 1:2 ligand-ion stoichiometry. The photophysical and metal ion-sensing properties of these phenylalanines suggest that they might be suitable for incorporation into peptide chemosensory frameworks.

Novel Crown Ether Amino Acids as Fluorescent Reporters for Metal Ions

Unnatural amino acids with enhanced properties, such as increased complexing ability and luminescence, are considered to be highly attractive building blocks for bioinspired frameworks, such as probes for biomolecule dynamics, sensitive fluorescent chemosensors, and peptides for molecular imaging, among others. Therefore, a novel series of highly emissive heterocyclic alanines bearing a benzo[d]oxazolyl unit functionalized with different heterocyclic π-spacers and (aza)crown ether moieties was synthesized. The new compounds were completely characterized using the usual spectroscopic techniques and evaluated as fluorimetric chemosensors in acetonitrile and aqueous mixtures in the presence of various alkaline, alkaline-earth, and transition metal ions. The different crown ether binding moieties as well as the electronic nature of the π-bridge allowed for fine tuning of the sensory properties of these unnatural amino acids towards Pd²⁺ and Fe³⁺, as seen by spectrofluorimetric titrations.

Review on the Ugi Multicomponent Reaction Mechanism and the Use of Fluorescent Derivatives as Functional Chromophores

In the present mini-review we discuss the findings, controversies, and gaps observed for the Ugi four-component reaction. The Ugi multicomponent reaction, performed by mixing an aldehyde, an amine, a carboxylic acid, and an isocyanide, is among the most important isocyanide-based multicomponent reactions (MCRs), allowing multiple bond formations (C-C and C-N) in a single synthetic step. The possibility of two reaction pathways and the little understood solvent effect over this transformation renders this reaction as one of the hardest challenges to overcome. The little knowledge of the mechanism of the Ugi MCR hinders the development of new and efficient chiral catalytic systems to further the application of the derivatives obtained by enantioselective versions. The asymmetric transformation is in this context a bigger challenge, and little is known about the mechanism of these few available versions. The new trend of functional chromophore synthesis by MCRs is also highlighted, and the few examples already disclosed in the literature exemplify the huge opportunity for investigation and creative ideas using the Ugi four-component reaction.

Raposo MM, Costa SPG. Heterocyclic amino acids as fluorescent reporters for transition metals: synthesis and evaluation of novel furyl-benzoxazol-5-yl-l-alanines

Novel benzoxazol-5-yl-l-alanines with heterocyclic pendants were synthesized and investigated as fluorescent reporters for metal cations through spectrofluorimetric titrations.