Oxazoline−Oxazinone Oxidative Rearrangement. Divergent Syntheses of (2S,3S)-4,4,4-Trifluorovaline and (2S,4S)-5,5,5-Trifluoroleucine

Tinker, Tailor, Soldier, Spy: The Diverse Roles That Fluorine Can Play within Amino Acid Side Chains

Side chain-fluorinated amino acids are useful tools in medicinal chemistry and protein science. In this review, we outline some general strategies for incorporating fluorine atom(s) into amino acid side chains and for elaborating such building blocks into more complex fluorinated peptides and proteins. We then describe the diverse benefits that fluorine can offer when located within amino acid side chains, including enabling 19F NMR and 18F PET imaging applications, enhancing pharmacokinetic properties, controlling molecular conformation, and optimizing target-binding.

Recent advances in the synthesis of fluorinated amino acids and peptides

The site-selective modification of amino acids, peptides, and proteins has always been an intensive topic in organic synthesis, medicinal chemistry, and chemical biology due to the vital role of amino acids in life. Among the developed methods, the site-selective introduction of fluorine functionalities into amino acids and peptides has emerged as a useful approach to change their physicochemical and biological properties. With the increasing demand for life science, the direct fluorination/fluoroalkylation of proteins has also received increasing attention because of the unique properties of fluorine atom(s) that can change the protein structure, increase their lipophilicity, and enable fluorine functionality as a biological tracer or probe for chemical biology studies. In this feature article, we summarized the recent advances in the synthesis of fluorinated amino acids and peptides, wherein two strategies have been discussed. One is based on the fluorinated building blocks to prepare fluorinated amino acids and peptides with diversified structures, including the transformations of fluorinated imines and nickel-catalyzed dicarbofunctionalization of alkenes with bromodifluoroacetate and its derivatives; the other is direct fluorination/fluoroakylation of amino acids, peptides, and proteins, in which the selective transformations of the functional groups on serine, threonine, tyrosine, tryptophan, and cysteine lead to a wide range of fluorinated α-amino acids, peptides, and proteins, featuring synthetic convenience and late-stage modification of biomacromolecules. These two strategies complement each other, wherein transition-metal catalysis and new fluoroalkylating reagents provide powerful tools to selectively access fluorinated amino acids, peptides, and proteins, showing the prospect of medicinal chemistry and chemical biology.

Design of ionic liquids containing glucose and choline as drug carriers, finding the link between QM and MD studies

Designing drug delivery systems for therapeutic compounds whose receptors are located in the cytosol of cells is challenging as a bilayer cell membrane is negatively charged. The newly designed drug delivery systems should assist the mentioned drugs in passing the membrane barriers and achieving their targets. This study concentrated on developing novel ionic liquids (ILs) that interact effectively with cell membranes. These ILs are based on glucose-containing choline and are expected to be non-toxic. The binding energies of the known pharmaceutically active ionic liquids were calculated at the B3LYP/6-311++G(d,p) level in the gas phase and compared with those of our newly designed carbohydrate-based ionic liquids. Subsequently, we employed MD simulations to obtain information about the interactions of these known and designed ILs with the cell membrane. In our approach, we adopted QM and MD studies and illustrated that there could be a link between the QM and MD results.

Gram-Scale Asymmetric Synthesis of Fluorinated Amino Acids Using a Chiral Nickel(II) Complex

Fluorinated amino acids play an important role in the field of peptide and protein engineering. Although numerous syntheses have been published in recent decades, strategies that allow routine access to fluorinated amino acids on a gram-scale have been poorly described. Furthermore, the described pathways that gain fluorinated amino acids are based on different synthetic strategies, making a uniform approach that uses similar starting materials highly beneficial. Chiral Ni(II) complexes were introduced as powerful tools in the synthesis of noncanonical amino acids. In this work, we present a strategy for the synthesis of a diverse range of fluorinated amino acids based on the corresponding Ni(II) complex from which the products can be obtained in enantiopure form (99% ee) on a gram-scale. In addition, we describe an optimized procedure for the synthesis of alkyl iodide building blocks that are required for the alkylation reactions with the corresponding Ni(II) complex. Finally, we characterized the synthesized fluorinated amino acids with regard to their hydrophobicity and α-helix propensity.

Stereodivergent entry to β-branched β-trifluoromethyl α-amino acid derivatives by sequential catalytic asymmetric reactions

Currently, conventional reductive catalytic methodologies do not guarantee general access to enantioenriched β-branched β-trifluoromethyl α-amino acid derivatives. Herein, a one-pot approach to these important α-amino acids, grounded on the reduction - ring opening of Erlenmeyer-Plöchl azlactones, is presented. The configurations of the two chirality centers of the products are established during each of the two catalytic steps, enabling a stereodivergent process.

Design of carboxylate-based ionic liquids (ILs) containing OH and CF3 groups; influence of intramolecular hydrogen bonds and inductive effect on the binding energy between the cation and anion of ILs, a DFT study

Ionic liquids, which are widely known as room temperature molten salts, have been the subject of much scientific debate for more than a decade.

Approaches to Obtaining Fluorinated α-Amino Acids

Fluorine does not belong to the pool of chemical elements that nature uses to build organic matter. However, chemists have exploited the unique properties of fluorine and produced countless fluoro-organic compounds without which our everyday lives would be unimaginable. The incorporation of fluorine into amino acids established a completely new class of amino acids and their properties, and those of the biopolymers constructed from them are extremely interesting. Increasing interest in this class of amino acids caused the demand for robust and stereoselective synthetic protocols that enable straightforward access to these building blocks. Herein, we present a comprehensive account of the literature in this field going back to 1995. We place special emphasis on a particular fluorination strategy. The four main sections describe fluorinated versions of alkyl, cyclic, aromatic amino acids, and also nickel-complexes to access them. We progress by one carbon unit increments. Special cases of amino acids for which there is no natural counterpart are described at the end of each section. Synthetic access to each of the amino acids is summarized in form of a table at the end of this article with the aim to make the information easily accessible to the reader.

Palladium-catalyzed cross-coupling of unactivated alkylzinc reagents with 2-bromo-3,3,3-trifluoropropene and its application in the synthesis of fluorinated amino acids

A palladium-catalyzed cross-coupling of unactivated alkylzinc reagents with 2-bromo-3,3,3-trifluoropropene (BTP) has been developed, which was used as a key step to prepare a series of trifluoromethylated and difluoromethylated amino acids.

Synthesis of enantiomerically pure (2S,3S)-5,5,5-trifluoroisoleucine and (2R,3S)-5,5,5-trifluoro-allo-isoleucine

A practical route for the stereoselective synthesis of (2S,3S)-5,5,5-trifluoroisoleucine (L-5-F₃Ile) and (2R,3S)-5,5,5-trifluoro-allo-isoleucine (D-5-F₃-allo-Ile) was developed. The hydrophobicity of L-5-F₃Ile was examined and it was incorporated into a model peptide via solid phase peptide synthesis to determine its α-helix propensity. The α-helix propensity of 5-F₃Ile is significantly lower than Ile, but surprisingly high when compared with 4’-F₃Ile.

Synthesis of optically pure 4-fluoro-glutamines as potential metabolic imaging agents for tumors

A versatile synthetic route to prepare all four stereoisomeric 4-fluoro-glutamines was developed by exploiting a Passerini three-component reaction. The skeleton of 4-substituted glutamine derivatives was efficiently constructed. Subsequent four-step reactions, highlighted by a "neutralized" TASF fluorination, provided the desired products with high yields and excellent optical purity. The optically pure fluorine-18 labeled 4-fluoroglutamines were also successfully prepared using either a 18-crown-6/KHCO(3) or K[222]/K(2)CO(3) catalysis system. Preliminary cell uptake and inhibition studies using the 9L tumor cells and SF188(Bcl-xL) tumor cells (a glutamine addicted tumor derived from glioblastoma) provided strong evidence for their potential application in conjunction with positron emission tomography (PET) for in vivo imaging of tumors, which use glutamine as an alternative energy source.

Diastereoselective allylstannane additions to (S)-5,6-dihydro-2H-5-phenyloxazin-2-one. A concise synthesis of (S)-beta-methylisoleucine

The addition of allyl stannanes to (S)-4,5-dihydro-5-phenyl-2H-oxazinone (3) was achieved under Brønsted acid catalysis to give 2-allylmorpholinones with high diastereoselectivity (up to dr 14.2:1). The product of dimethylallyltributylstannane addition to 3 was converted to l-beta-methylisoleucine, an alpha-amino acid residue found in the complex, biologically active marine-derived peptides polytheonamides A and B, and polydiscamides A-C.