Recent advances in the synthesis and application of fluorescent α-amino acids

Fluorescence spectroscopy has become a powerful technique for probing a range of complex biological processes including enzyme mechanisms and protein-protein interactions. While the application of this technique uses a number of strategies, many of these rely on the use of fluorescent α-amino acids. This review highlights the recent synthetic methods developed for the incorporation of highly conjugated chromophores into the side-chain of α-amino acids and the application of these compounds as probes for imaging in medicine and biology. In particular, the design and synthesis of α-amino acids bearing coumarin, flavone and polyaromatic derived chromophores is described

Access to 2,6-disubstituted 4-oxopiperidines using a 6-endo-trig cyclization: stereoselective synthesis of spruce alkaloid and (+)-241D

A synthetic route to cis-2-methyl-4-oxo-6-alkylpiperidines has been developed using a 6-endo-trig cyclization of E-enones. The base-mediated intramolecular cyclization was found to be general for both alkyl and aryl substituted enones, providing the corresponding 4-oxopiperidines in high yields (80–89%). Stereoselective reduction of the 2,6-cis-disubstituted 4-oxopiperidines then gave the 2,4,6-cis,cis-trisubstituted 4-hydroxypiperidines in high diastereoselectivity. The general nature of this approach was demonstrated with the synthesis of the natural products, spruce alkaloid and (+)-241D

Synthesis of novel fluorescent heterocyclic-derived α-amino acids and the total syntheses of piperidine natural products

During the course of this PhD, methodology for the synthesis of a series of novel, highly fluorescent pyridine-derived α-amino acids was developed. Enone-derived α-amino acids were subjected to an inverse electron demand hetero-Diels-Alder cycloaddition and aromatisation reaction, which led to a twelve-membered library of pyridine analogues. The optical properties of these compounds were analysed, with several exhibiting interesting fluorescent characteristics. One of the analogues was incorporated into a cell penetrating pentapeptide via solid phase peptide synthesis. The resulting hexapeptide was incubated with human fibroblast cells and fluorescence microscopy was used to show accumulation of the peptide in the cells. The total syntheses of piperidine containing natural products, spruce alkaloid and (+)-241D were also examined during this PhD. A short nine-step linear sequence was developed giving spruce alkaloid and (+)-241D in 21% and 19% overall yield, respectively. A base mediated 6-endo-trig cyclisation was employed as the key-step followed by stereoselective ketone reduction to complete the total syntheses. The scope of the cyclisation and reduction was examined with a range of enone side-chains resulting in a small library of novel 4-hydroxy-2,6-disubstituted piperidines

Stereoselective synthesis of 2,6-trans-4-oxopiperidines using an acid-mediated 6-endo-trig cyclisation

An acid-mediated 6-endo-trig cyclisation of amine-substituted enones has been developed for the stereoselective synthesis of trans-6-alkyl-2-methyl-4-oxopiperidines. Performed under conditions that prevent removal of the Boc-protecting group or acetal formation, the key cyclisation was found to generate cleanly the 4-oxopiperidine products in high overall yields from a wide range of alkyl substituted enones. The synthetic utility of the trans-6-alkyl-2-methyl-4-oxopiperidines formed from this process was demonstrated with the total synthesis of the quinolizidine alkaloid, (+)-myrtine and the piperidine alkaloid, (–)-solenopsin A

Synthesis and fluorescent properties of β-pyridyl α-amino acids

The preparation of a new class of β-pyridyl α-amino acid is described using a highly regioselective, ytterbium-catalyzed hetero Diels-Alder reaction of enones with vinyl ethers followed by a modified Knoevenagel-Stobbe reaction as the key heterocycle forming steps. Investigation of the properties and applications of these amino acids showed that they could be utilized in solid phase peptide synthesis for the preparation of a biologically relevant hexapeptide, while pyridines bearing electron-rich substituents exhibited strongly fluorescent properties with high quantum yields and MegaStokes shifts. A solvatochromic study with the most fluorogenic amino acid, a p-methoxyphenyl analogue revealed that this charge-transfer based chromophore is highly sensitive to solvent polarity with a bathochromic shift of 115 nm on changing from THF to phosphate-buffered saline

Synthesis and photophysical properties of benzotriazole-derived unnatural α -amino acids

The synthesis of a new class of benzotriazole-derived α-amino acid is described using a highly efficient nucleophilic aromatic substitution of ortho-fluoronitrobenzenes with l-3-aminoalanine and a polymer-supported nitrite reagent-mediated diazotization and cyclization of the subsequent 1,2-aryldiamines as the key steps. Further functionalization of the benzotriazole unit by preparation of halogenated analogues and Suzuki–Miyaura cross-coupling with aryl boronic acids allowed the synthesis of α-amino acids with conjugated side chains. Analysis of the photophysical properties of these α-amino acids revealed that incorporation of electron-rich substituents results in charge-transfer-based, fluorescent compounds with MegaStokes shifts

Conformationally rigid pyrazoloquinazoline α-amino acids: one- and two-photon induced fluorescence

The synthesis and photophysical properties of a new class of α-amino acid bearing a rigid pyrazoloquinazoline chromophore are described. Confromational constraint of the amino acid side-chains resulted in high emission quantum yields, while the demonstration of two-photon-induced fluorescence via near-IR excitation signifies their potential for sensitive bioimaging applications

Incorporating a polyethyleneglycol linker to enhance the hydrophilicity of mitochondria‐targeted triphenylphosphonium constructs

The targeting of bioactive molecules and probes to mitochondria can be achieved by coupling to the lipophilic triphenyl phosphonium (TPP) cation, which accumulates several hundred-fold within mitochondria in response to the mitochondrial membrane potential (Dym). Typically, a simple alkane links the TPP to its “cargo”, increasing overall hydrophobicity. As it would be beneficial to enhance the water solubility of mitochondria-targeted compounds we explored the effects of replacing the alkyl linker with a polyethylene glycol (PEG). We found that the use of PEG led to compounds that were readily taken up by isolated mitochondria and by mitochondria inside cells. Within mitochondria the PEG linker greatly decreased adsorption of the TPP constructs to the matrix-facing face of the mitochondrial inner membrane. These findings will allow the distribution of mitochondria-targeted TPP compounds within mitochondria to be fine-tuned

Synthesis of pyrazole containing α-amino acids via a highly regioselective condensation/aza-Michael reaction of β-aryl α,β-unsaturated ketones

A synthetic approach for the preparation of a new class of highly conjugated unnatural α-amino acids bearing a 5-arylpyrazole side-chain has been developed. Horner–Wadsworth–Emmons reaction of an aspartic acid derived β-keto phosphonate ester with a range of aromatic aldehydes gave β-aryl α,β-unsaturated ketones. Treatment of these with phenyl hydrazine followed by oxidation allowed the regioselective synthesis of pyrazole derived α-amino acids. As well as evaluating the fluorescent properties of the α-amino acids, their synthetic utility was also explored with the preparation of a sulfonyl fluoride derivative, a potential probe for serine proteases

Preparation of amino-substituted indenes and 1,4-dihydronaphthalenes using a one-pot multireaction approach: total synthesis of oxybenzo[c]phenanthridine alkaloids

Allylic trichloroacetimidates bearing a 2-vinyl or 2-allylaryl group have been designed as substrates for a one-pot, two-step multi-bond-forming process leading to the general preparation of aminoindenes and amino-substituted 1,4-dihydronaphthalenes. The synthetic utility of the privileged structures formed from this one-pot process was demonstrated with the total synthesis of four oxybenzo[c]phenanthridine alkaloids, oxychelerythrine, oxysanguinarine, oxynitidine, and oxyavicine. An intramolecular biaryl Heck coupling reaction, catalyzed using the Hermann–Beller palladacycle was used to effect the key step during the synthesis of the natural products