A direct method for the synthesis of orthogonally protected furyl- and thienyl- amino acids

Nickel-Catalyzed Cross-Coupling of Amino-Acid-Derived Alkylzinc Reagents with Alkyl Bromides/Chlorides: Access to Diverse Unnatural Amino Acids

Unnatural α-amino acids are important synthetic targets in the field of peptide science. Herein we report an efficient, versatile, and straightforward strategy for the synthesis of homophenylalanine derivatives via the nickel-catalyzed Csp3-Csp3 cross-coupling of (fluoro)benzyl bromides/chlorides with natural α-amino-acid-derived alkylzinc reagents. The current protocol features the advantages of a low-cost nickel catalyst system, synthetic convenience, and the tolerance of rich functionality and stereochemistry.

Synthesis of Differentially Protected Azatryptophan Analogs via Pd2(dba)3/XPhos Catalyzed Negishi Coupling of N-Ts Azaindole Halides with Zinc Derivative from Fmoc-Protected tert-Butyl (R)-2-Amino-3-iodopropanoate

Unnatural amino acids play an important role in peptide based drug discovery. Herein, we report a class of differentially protected azatryptophan derivatives synthesized from N-tosyl-3-haloazaindoles 1 and Fmoc-protected tert-butyl iodoalanine 2 via a Negishi coupling. Through ligand screening, Pd₂(dba)₃/XPhos was found to be a superior catalyst for the coupling of 1 with the zinc derivative of 2 to give tert-butyl (S)-2-((((9H-fluoren-9-yl)methoxy)carbonyl)amino)-3-(1-tosyl-1H-pyrrolo[2,3-b]pyridin-3-yl)propanoate derivatives 3 in 69-91% isolated yields. In addition, we have demonstrated that the protecting groups, namely, Ts, Fmoc, and ^tBu, can be easily removed selectively.

Total Synthesis of Endolides A and B

The total synthesis of endolides A and B has been achieved in a concise, highly stereoselective fashion (12 steps, 16.2% and 16.0% overall yields, respectively). Key features of the route include a modified Negishi coupling between 3-bromofuran and an organozinc reagent derived from an iodoalanine derivative for the synthesis of 3-(3-furyl)-alanine derivative, and a judicious choice of reaction conditions to surmount the conformational constraints placed by converting a linear peptide into the corresponding macrocycle.

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.

Tetrafluoropyridyl (TFP): a general phenol protecting group readily cleaved under mild conditions

Herein we introduce tetrafluoropyridyl (TFP) as a new general protecting group for phenols. The TFP protecting group is readily cleaved under mild conditions.

Phenols are extremely valuable building blocks in the areas of pharmaceuticals, natural products, materials and catalysts. In order to carry out modifications on phenols, the phenolic oxygen is routinely protected to prevent unwanted side reactions. Presently many of the protecting groups available can require harsh conditions, specialist equipment, expensive or air/moisture-sensitive reagents to install and remove. Here we introduce the use of the tetrafluoropyridyl (TFP) group as a general protecting group for phenols. TFP can be installed in one step with no sensitivity to water or air, and it is stable under a range of commonly employed reaction conditions including acid and base. The TFP protecting group is readily cleaved under mild conditions with quantitative conversion to the parent phenol, observed in many cases in less than 1 hour.

Synthesis of pentafluorosulfanyl (SF5) containing aromatic amino acids

Herein, a series of aromatic pentafluorosulfanyl (SF5) containing amino acids are reported. A Negishi cross-coupling strategy utilising a catalyst system of Pd(dba)2 and SPhos afforded the aforementioned SF5 amino acids in yields between 32% and 42%. Two dipeptides utilising both the amine and carboxylic functionalities of the synthesised SF5 containing amino acids were prepared, demonstrating their compatibility with common amide/peptide coupling reagents and strategies.

Negishi cross-couplings in the synthesis of amino acids

The Negishi cross-coupling is a powerful C-C bond-forming reaction widely utilised in many areas of organic synthesis. This review details the use of Negishi cross-couplings in the synthesis of unnatural amino acids. The application of this reaction in the preparation of aromatic, heteroaromatic, and, complex amino acid derivatives are reviewed and presented herein.