Convenient asymmetric synthesis of beta-substituted alpha,alpha-difluoro-beta-amino acids via Reformatsky reaction between Davis' N-sulfinylimines and ethyl bromodifluoroacetate

Chromium Catalyzed Asymmetric Reformatsky Reaction

This study describes an unprecedented chromium-catalyzed asymmetric Reformatsky reaction, enabling the synthesis of chiral β-hydroxy carbonyl compounds from α-chlorinated or α-brominated esters and amides. By employing a chiral chromium/diarylamine bis(oxazoline) catalyst, we achieved relatively broad functional group tolerance. Distinct from known reports, the protocol operates under both classical and photoredox conditions, facilitated by the in situ formation of a nucleophilic chiral chromium intermediate through a radical-polar crossover mechanism. Preliminary mechanistic insights, supported by DFT calculations, identify the nucleophilic aldehyde addition as the key stereo-determining step. This approach not only overcomes the limitations of existing Reformatsky reactions but also provides a versatile strategy for accessing complex chiral molecules.

Synthesis of α,α-Difluoro-β-amino Ketones from N-Boc-α-Amidosulfones and Pentafluoro-gem-diols

To circumvent the synthesis and isolation of imines, a method was devised to construct α,α-difluoro-β-amino ketones from N-Boc-α-amidosulfones. The reactive nucleophiles, difluoroenolates, are generated in situ from the pentafluoro-gem-diols using cesium fluoride in pyridine. NMR studies confirm the role of the α-amidosulfones in this process. Incubation of the α,α-difluoro-β-amino ketones in rat serum demonstrates the relative stability of this structure as well as its value as a chemical probe or lead.

Difluoroalkylation of Tertiary Amides and Lactams by an Iridium-Catalyzed Reductive Reformatsky Reaction

An iridium-catalyzed, reductive alkylation of abundant tertiary lactams and amides using 1-2 mol % of Vaska's complex (IrCl(CO)(PPh₃)₂), tetramethyldisiloxane (TMDS), and difluoro-Reformatsky reagents (BrZnCF₂R) for the general synthesis of medicinally relevant α-difluoroalkylated tertiary amines is described. A broad scope (46 examples), including N-aryl- and N-heteroaryl-substituted lactams, demonstrated an excellent functional group tolerance. Furthermore, late-stage drug functionalizations, a gram-scale synthesis, and common downstream transformations proved the potential synthetic relevance of this new methodology.

Asymmetric Synthesis of Tailor-Made Amino Acids Using Chiral Ni(II) Complexes of Schiff Bases. An Update of the Recent Literature

Tailor-made amino acids are indispensable structural components of modern medicinal chemistry and drug design. Consequently, stereo-controlled preparation of amino acids is the area of high research activity. Over last decade, application of Ni(II) complexes of Schiff bases derived from glycine and chiral tridentate ligands has emerged as a leading methodology for the synthesis of various structural types of amino acids. This review article summarizes examples of asymmetric synthesis of tailor-made α-amino acids via the corresponding Ni(II) complexes, reported in the literature over the last four years. A general overview of this methodology is provided, with the emphasis given to practicality, scalability, cost-structure and recyclability of the chiral tridentate ligands.

Applications of fluorine-containing amino acids for drug design

Fluorine-containing amino acids are becoming increasingly prominent in new drugs due to two general trends in the modern pharmaceutical industry. Firstly, the growing acceptance of peptides and modified peptides as drugs; and secondly, fluorine editing has become a prevalent protocol in drug-candidate optimization. Accordingly, fluorine-containing amino acids represent one of the more promising and rapidly developing areas of research in organic, bio-organic and medicinal chemistry. The goal of this Review article is to highlight the current state-of-the-art in this area by profiling 42 selected compounds that combine fluorine and amino acid structural elements. The compounds under discussion represent pharmaceutical drugs currently on the market, or in clinical trials as well as examples of drug-candidates that although withdrawn from development had a significant impact on the progress of medicinal chemistry and/or provided a deeper understanding of the nature and mechanism of biological action. For each compound, we present features of biological activity, a brief history of the design principles and the development of the synthetic approach, focusing on the source of tailor-made amino acid structures and fluorination methods. General aspects of the medicinal chemistry of fluorine-containing amino acids and synthetic methodology are briefly discussed.

Recyclable and Stable α-Methylproline-Derived Chiral Ligands for the Chemical Dynamic Kinetic Resolution of free C,N-Unprotected α-Amino Acids

A novel special designed, stable, and recyclable chiral ligand bearing a quaternary carbon was developed for chemical dynamic kinetic resolution (DKR) of free C,N-unprotected racemic α-amino acids via Schiff base intermediates. This method furnishes high yields with excellent enantioselectivity, has a broad substrate scope, and uses operationally simple and convenient conditions. The present chemical DKR is a practical and useful method for the preparation of enantiopure α-amino acids.

Polyaniline Doping by α,α-Difluoro-β-amino Acids

Currently, polyaniline (PANI) is considered as a promising polymer that can be used in biosensors, drug delivery systems, bioelectronics, etc. Its biocompatibility can be strongly improved by using dopants with biofunctionality. This study reveals the protonation/doping of PANI by fluorinated analogs of natural amino acids, namely, α,α-difluoro-β-amino acids (DFAAs) with alkyl and aromatic tails in N-methylpyrrolidone solutions. We find that these acids can dope PANI due to both the weakened basicity of their amino groups because of two fluorine atoms in α,α-positions and specific intermolecular interactions (π-π stacking, alkyl-π, F-π) of their tails with units of PANI chains. These interactions did not give the doped PANI salts with high conductivity but led to formation of stable PANI-DFAA complexes, which were confirmed both by clear changes in the UV-Vis and Fourier transform infrared spectra of the protonated/doped PANI and by their conductivity of ∼10^-6 S/cm. Our results suggest an applicability of such PANI complexes as carriers of DFAA for their biomedical applications.

A Sc(OTf)3 catalyzed Mukaiyama–Mannich reaction of difluoroenoxysilanes with unactivated ketimines

The first example of a Mukaiyama–Mannich reaction of difluoroenoxysilanes with unactivated ketimines catalyzed by Sc(OTf)₃ is described.

Magnesium-Promoted Additions of Difluoroenolates to Unactivated Imines

Although there are many synthetic methods to produce fluorinated and trifluoromethylated organic structures, the construction of difluoromethylated compounds remains a synthetic challenge. We have discovered that unactivated imines will react with difluoroenolates under exceedingly mild conditions when using magnesium salts and organic bases. We have applied this approach to the iminoaldol reaction to produce difluoromethylene groups as α,α-difluoro-β-amino-carbonyl groups. This method provides synthetically useful quantities of difficult to access α,α-difluoro-β-aminoketones without the need of protecting groups or the use of activated imines. Moreover, we have applied this strategy to create analogues of the dual orexin receptor antagonist, almorexant, in only two synthetic steps.

Neighboring phenolic group-activated gem-difluoroallylboration of imines for the catalyst-free synthesis of gem-difluorohomoallylamines

An unprecedented imine allylation with pinacol gem-difluoroallylborates for the synthesis of a wide range of racemic gem-difluorohomoallylamine derivatives under catalyst-free conditions was enabled by a neighboring phenolic group in an N-protecting group.

Diastereoselective Addition of Metal α-Fluoroenolates of Carboxylate Esters to N-tert-Butylsulfinyl Imines: Synthesis of α-Fluoro-β-amino Acids

We report a diastereoselective addition reaction of fluoroacetate and α-alkylated fluoroacetate to N-tert-butylsulfinyl imines. This method provides a concise route to α-fluoro-β-amino acids containing fluorinated quaternary stereogenic carbon centers with very good yields and high diastereoselectivities. This protocol has the benefit of using abundant and readily accessible starting materials and is operationally simple. Additionally, the stereochemical outcome of the present reaction was different from that of the previously known addition of comparable nonfluorinated, brominated, and chlorinated enolates to N-sulfinyl imines, suggesting that an open transition state (rather than a closed one) is involved in the current fluoroalkylation reaction.

Scalable asymmetric synthesis of a key fragment of Bcl-2/Bcl-xL inhibitors

We describe a novel asymmetric synthesis, which is applicable on a large-scale, of a chiral diamine useful as a common fragment of numerous Bcl-2 and Bcl-_xL inhibitors.

Stabilization of β-peptide helices by direct attachment of trifluoromethyl groups to peptide backbones

Direct attachment of trifluoromethyl groups to peptide backbones was found to be a novel strategy for peptide folding stabilization, where the intramolecular hydrogen-bonding ability of backbone amides is enhanced.

β-Secretase (BACE1) inhibitors with high in vivo efficacy suitable for clinical evaluation in Alzheimer's disease

An extensive fluorine scan of 1,3-oxazines revealed the power of fluorine(s) to lower the pKa and thereby dramatically change the pharmacological profile of this class of BACE1 inhibitors. The CF3 substituted oxazine 89, a potent and highly brain penetrant BACE1 inhibitor, was able to reduce significantly CSF Aβ40 and 42 in rats at oral doses as low as 1 mg/kg. The effect was long lasting, showing a significant reduction of Aβ40 and 42 even after 24 h. In contrast to 89, compound 1b lacking the CF3 group was virtually inactive in vivo.

Synthesis of α-halo-α,α-difluoromethyl ketones by a trifluoroacetate release/halogenation protocol

Three series of α-halo-α,α-difluoromethyl ketones are prepared from highly α-fluorinated gem-diols by exploiting the facile release of trifluoroacetate, followed by immediate trapping of the liberated α,α-difluoroenolate with an electrophilic chlorine, bromine, or iodine source. The products are typically isolated in good yields, even in the case of sensitive, α-iodo-α,α-difluoromethyl ketones. Also, we demonstrate that an α-iodo-α,α-difluoromethyl ketone will participate in a copper-promoted reaction to forge a new carbon-carbon bond.

Practical synthesis of fluorine-containing α- and β-amino acids: recipes from Kiev, Ukraine

Naturally occurring compounds containing a C-F bond are extremely rare; only a handful of fluorine-containing carboxylic acids have been described so far. By contrast, man-made fluorine-containing derivatives of all major classes of biologically important compounds are extremely promising medicinal targets used in the elucidation of biochemical, metabolic transformations and the development of new pharmaceuticals. Among the fluorine-containing derivatives of natural products, fluorinated analogs of amino acids are of particular interest and medicinal potential. This article presents a concise review of various synthetic methods, developed by the Kiev's school of bioorganic chemistry, for the preparation of fluorine-containing analogs of α- and β-amino acids, α-hydroxy acids, amines, as well as their phosphorus and sulfur-derived compounds, in enantiomerically pure form. One of the major methodological goals of the study was practicality, which is understood by us as stereochemical generality, operational convenience and synthetic affordance for each reaction step and isolation of the target products. The synthetic methods developed by our group can be roughly divided in two general categories: fluorine-adaptation of known synthetic approaches and discovery of new reactions. The former approach is most prominently represented by asymmetric homologation of nucleophilic glycine equivalents using fluorinated substrates via alkyl halide alkylations, aldol and Michael addition reactions. A plethora of discovered unexpected reaction outcomes, in particular stereochemical, are emphasized in this review and the particular role of fluorine, in altering the 'normal' reaction result, is explained. The latter direction is notably represented by the novel 1,3-proton shift reaction, a biomimetic reductive amination of fluorinated carbonyl compounds to the corresponding amines and amino acids, as well as the development of α-fluoroalkyl epoxides as true fluorinated synthons for generalized asymmetric synthesis of various biologically relevant compounds. Despite the highly anticipated potential of fluorine-containing amino compounds, their medicinal chemistry still remains underexplored. The major obstacle, in our opinion, is that these selectively fluorinated compounds are generally unavailable to the medicinal chemists for comprehensive, systematic study. We hope this review of synthetic methods will highlight and bring attention to particular types of fluorinated amino acids and related compounds readily available on a laboratory scale using methods developed by our group.

Enantioselective synthesis of α-fluoro-β(3)-amino esters: synthesis of enantiopure, orthogonally protected α-fluoro-β(3)-lysine

The scope of a tandem conjugate addition-fluorination sequence performed on α,β-unsaturated esters using the enantiopure lithium amide derived from (S)-N-benzyl-N-(α-methylbenzyl)amine, and the electrophilic fluorinating agent N-fluorobenzenesulfonimide has been investigated. Using this method, α-fluoro-β(3)-amino esters can be obtained in up to quantitative yield and 80:20 to >99:1 dr. This simple methodology does not rely on the use of α-amino acids from the chiral pool and thus provides the potential for the preparation of enantiopure α-fluoro-β(3)-amino acids with a wide variety of side chains. Its utility was demonstrated through the synthesis of orthogonally protected (2S,3S)-α-fluoro-β(3)-lysine.

Asymmetric synthesis of sterically and electronically demanding linear ω-trifluoromethyl containing amino acids via alkylation of chiral equivalents of nucleophilic glycine and alanine

An operationally convenient, scalable asymmetric synthesis of linear, ω-trifluoromethyl-containing amino acids, which were not previously produced in their enantiomerically pure form, has been developed via alkylation of chiral equivalents of nucleophilic glycine and alanine. The simplicity of the experimental procedures and high stereochemical outcome (yields up to 90% and diastereoselectivity up to 99%) of the presented method render these fluorinated amino acids readily available for systematic medicinal chemistry studies and de novo peptide design.