Design, synthesis, and evaluation of a new generation of modular nucleophilic glycine equivalents for the efficient synthesis of sterically constrained alpha-amino acids

Substituent-controlled preference of carbonyl group–metal coordination in d8 metal complexes with non-symmetric pentadentate ligands. Structural and stereochemical aspects

The sidearm substitution of chirally switchable Ni(ii) and Pd(ii) complexes permits stereochemical inclinations to be controlled in the solid state.

Design and synthesis of quasi-diastereomeric molecules with unchanging central, regenerating axial and switchable helical chirality via cleavage and formation of Ni(II)-O and Ni(II)-N coordination bonds

We describe herein the design and synthesis of asymmetric, pentadentate ligands, which are able to coordinate to Ni(II) cations leading to quasi-diastereomeric complexes displaying two new elements of chirality: stereogenic axis and helix along with configurational stabilization of the stereogenic center on the nitrogen. Due to the stereocongested structural characteristics of the corresponding Ni(II) complexes, the formation of quasi-diastereomeric products is highly stereoselective providing formation of only two, (R_a*,M_h*,R_c*) and (R_a*,P_h*,R_c*), out of the four possible stereochemical combinations. The reversible quasi-diastereomeric transformation between the products (R_a*,M_h*,R_c*) and (R_a*,P_h*,R_c*) occurs by intramolecular trans-coordination of Ni–NH and Ni–O bonds providing a basis for a chiral switch model.

Concise asymmetric synthesis of configurationally stable 4-trifluoromethyl thalidomide

BACKGROUND

Thalidomide is one of the promising multidimensional drugs that possess high activity against serious diseases such as rheumatoid arthritis, Crohn's disease, leprosy, AIDS and various cancers. However, its medicinal applications are plagued by its configurational instability, as it easily undergoes racemization under the physiological conditions (t(1/2) = 8 h at pH 7.1, 37°C in water). Consequently, the design and synthesis of configurationally stable analogs of thalidomide continue to be an important area of research in bioorganic and medicinal chemistry.

DISCUSSION

4-trifuoromethyl thalidomide-3c was identified as an important synthetic target, which was expected to be a configurationally stable analog of thalidomide. Synthetic challenges in preparation of compound 3c were truly multipronged, considering the unique steric, electronic as well as electrostatic characteristics of trifluoromethyl group, significantly affecting properties of parent amino acids. After numerous experiments and unsuccessful attempts, both (3S,4R) and (3R, 4S) enantiomers of 4-trifluoromethyl-substituted thalidomide were effectively synthesized in six steps starting from enantio- and diastereomerically pure 3-(trifluoromethyl)pyroglutamates, prepared by highly diastereoselective Michael addition reactions between achiral glycine equivalents and chiral 3-(trifluoromethyl)acrylate.

CONCLUSION

We have developed a reliable asymmetric approach for preparation of hitherto unknown 4-trifluoromethyl-substituted thalidomide in (3S,4R) and (3R,4S) enantiomerically pure forms. These thalidomide derivatives were shown to be configurationally stable and therefore may serve as useful lead compounds for the development of a new generation of thalidomide-based pharmaceuticals.

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.

Resolution/deracemization of chiral alpha-amino acids using resolving reagents with flexible stereogenic centers

This work has demonstrated that a previously unexplored approach to separation of enantiomers via formation of diastereomeric derivatives with three stereogenic centers has obvious practical potential and deserves further systematic study. The design reported here is based on the unusual application of a configurationally unstable stereogenic nitrogen, which plays a key role in setting up the stereochemical match between the three stereogenic centers in the corresponding products.

Novel Michael addition products of bis(amino acidato)metal(II) complexes: synthesis, characterization, dye degradation, and oxidation properties

Michael addition reactions of bis(amino acidato)metal(II) complexes (metal = copper, nickel, zinc; amino acid = glycine, dl-alanine, l-alanine) with acrylonitrile have been carried out under various experimental conditions in the absence of a base, resulting in mono- and disubstituted products in high yield, including partially hydrolyzed products. A reaction mechanism for the Michael addition on the nitrogen atom of the coordinated amino acid moiety, replacing the amino hydrogen atom(s), is proposed. All of the products have been characterized by Fourier transform infrared spectroscopy, electron paramagnetic resonance spectra, and elemental and electrochemical analyses. The single-crystal structures of bis( N-cyanoethylglycinato)copper(II) monohydrate ( 1a), diaquabis( N-cyanoethylglycinato)nickel(II), aquabis( N, N-dicyanoethylglycinato)copper(II) ( 2a), and bis[( N-propionamido- N-cyanoethyl)glycinato]copper(II) dihydrate ( 4a) have been confirmed by X-ray diffraction techniques. The products 1a, 2a, 4a, and bis( N-propionamidoglycinato)copper(II) monohydrate ( 3a) have been used as catalysts for the degradation of a phenol red dye and mild oxidation of various organic substrates in the presence of hydrogen peroxide. The monosubstituted complexes have been found to catalyze the reactions to a greater extent than the disubstituted complexes.

Efficient asymmetric synthesis of novel 4-substituted and configurationally stable analogues of thalidomide

The preparation of new thalidomide derivatives 4-methyl-(3S,4R)-3a and 4-phenyl-(3S,4S)-3b starting from pyroglutamic acids (2R,3R)-7a and (2R,3S)-7b, possessing an inappropriate stereochemistry, was successfully realized due to stereochemically complete epimerization at the alpha-stereogenic center upon formation of the corresponding N-phthaloyl anhydrides 9a,b. The demonstrated conformational stability of these new thalidomide derivatives provides solid experimental evidence for practical feasibility of the approach described here to overcome the inherent problem of configurational instability of thalidomide by introducing an alkyl or aryl group in the C4 position. [reaction: see text].

Synthesis, Configuration, and 15N NMR Spectra of Iminoaziridines. Synthons Equivalent to Three Components of the Ugi Reaction

Monocyclic iminoaziridines and exo-endo diastereomers of spirocyclic iminoaziridines that are derived from norbornane are prepared in batches of up to 10 g to foster applications as building blocks in syntheses. N,N'-Disubstituted alpha-haloamidines, which are readily available in two steps from N-substituted alpha-halocarboxamides, are 1,3-dehydrohalogenated by strong bases such as alkali-metal hydrides or tert-butoxides to afford distillable oils or low-melting solids, which consist of slowly interconverting E-Z diastereomers of the title compounds. The scope and limitations are outlined for this reaction. The configurations E and Z that were assigned on the basis of homoallylic 1H-1H coupling and asymmetric solvent-induced shifts required that, in 13C NMR spectra, the observed gamma-effects of substitutents at the imino nitrogens were deshielding, contrary to the well-known shielding gamma-effects in all other types of C=N compounds. However, an NOE NMR study demonstrated unequivocally that the previous assignments are correct and hence the observed gamma-effects actually deshielding. The ranges of 15N NMR chemical shifts span more than 60 ppm. Neither the beta- nor the gamma-effects of substituents on both types of nitrogen follow a uniform increment pattern.