Showing papers by "Vadim A. Soloshonok published in 2012"

Recent Advances in the Asymmetric Synthesis of α-(Trifluoromethyl)-Containing α-Amino Acids

Abstract: This review article provides a comprehensive update on the development of new methods for the asymmetric synthesis of α-(trifluoromethyl)-α-amino acids, covering the literature from 2006 to mid-February 2012. Most of the methods discussed are based on asymmetric additions across the carbon–nitrogen double bond of the imines derived from esters of 3,3,3-trifluoropyruvic acid. Medium to high levels of stereocontrol can be achieved using phenylglycinol-derived chiral auxiliaries attached to the nitrogen of the corresponding imines. Among the enantioselective approaches, impressive results were achieved through the application of the Strecker­ reaction using chiral thioureas, as well as chiral Bronsted acids, as organocatalysts. 1 Introduction 2 Stoichiometric Asymmetric Syntheses of α-(Trifluoromethyl)-α-amino Acids 3 Catalytic Asymmetric Syntheses of α-(Trifluoromethyl)-α-amino Acids 4 Miscellaneous 5 Conclusions

Self-disproportionation of enantiomers via achiral chromatography: a warning and an extra dimension in optical purifications

Abstract: This tutorial review describes the self-disproportionation of enantiomers (SDE) of chiral, non-racemic compounds, subjected to chromatography on an achiral stationary phase using an achiral eluent, which leads to the substantial enantiomeric enrichment and the corresponding depletion in different fractions, as compared to the enantiomeric composition of the starting material. The physicochemical background of SDE is a dynamic formation of homo- or heterochiral dimeric or oligomeric aggregates of different chromatographic behavior. This phenomenon is of a very general nature as the SDE has been reported for different classes of organic compounds bearing various functional groups and possessing diverse elements of chirality (central, axial and helical chirality). The literature data discussed in this review clearly suggest that SDE via achiral chromatography might be expected for any given chiral enantiomerically enriched compound. This presents two very important issues for organic chemists. First, chromatographic purification of reaction products can lead to erroneous determination of the stereochemical outcome of catalytic asymmetric reactions and second, achiral chromatography can be used as a new, nonconventional method for optical purifications. The latter has tremendous practical potential as the currently available techniques are limited to crystallization or chiral chromatography. However, a further systematic study of SDE is needed to develop understanding of this phenomenon and to design practical chromatographic separation techniques for optical purification of non-racemic mixtures by achiral-phase chromatography.

Self-Disproportionation of Enantiomers of Chiral, Non-Racemic Fluoroorganic Compounds: Role of Fluorine as Enabling Element

Abstract: This review discusses all available literature on the self-disproportionation of enantiomers (SDE) of fluorine-containing compounds, resulting in a separation of racemic form from the excess enantiomer. Included are examples of the SDE via distillation, sublimation and achiral chromatography. The role of fluorine on the magnitude and preparative efficiency of the SDE, as a new, nonconventional method for optical purifications, is emphasized and critically discussed. 1 Introduction 2 General Aspects of Fluorine Influence on the Self-Disproportionation of Enantiomers 3 Self-Disproportionation of Enantiomers via Distillation 4 Self-Disproportionation of Enantiomers via Sublimation 5 Self-Disproportionation of Enantiomers via Achiral Chromatography 6 Conclusions

Alkylations of Chiral Nickel(II) Complexes of Glycine under Phase‐Transfer Conditions

Abstract: Alkylation reactions of nickel(II) complex 6 derived from glycine and 2-[(1-benzyl-L-prolyl)amino]benzophenone (BPBP) were studied under phase-transfer-catalysis (PTC) conditions. The goal of this work was to find an alternative suitable solvent for these reactions to replace the commonly used CH2Cl2 which leads to the formation of several by-products, thus lowering the yield of target compounds. We demonstrate that 1,2-dichloroethane is a markedly better solvent providing higher yields (75–99%) of the desired products 10 with 36–88% diastereoisomer purity (Scheme 3 and Table). Furthermore, we show that the stereochemical outcome of these PTC reactions (kinetic control) can be easily improved to >95% de by treatment of the PTC products with MeONa/MeOH. The scope of these reactions includes alkylations with methyl iodide as well as activated halides such as benzyl, allyl or propargyl, bromides and most notably ethyl 2-bromoacetate (Table).

First Experimental Evidence of an Intramolecular H Bond between Aliphatic Cl and Aromatic C–H

Abstract: The first example of an intramolecular H bond between aliphatic Cl and aromatic C–H was observed in the crystal of C2-symmetric (4R,5R)-4,5-bis(diphenylchloromethyl)-1,3,2-dioxathiolane 2-oxide. In this case one Cl atom is engaged in H-bonding with two ortho-C–H groups in the axial and equatorial phenyl planes, with the recorded distances of 2.562 Ǻ and 2.804 Ǻ, respectively. The observed angle for Ca–H–Cl is 108.41° and that for Cq–H–Cl is 96.53°.

Recent Advances in the Asymmetric Synthesis of α‐(Trifluoromethyl)‐Containing α‐Amino Acids

Abstract: This review article provides a comprehensive update on the development of new methods for the asymmetric synthesis of α-(trifluoromethyl)-α-amino acids, covering the literature from 2006 to mid-February 2012. Most of the methods discussed are based on asymmetric additions across the carbon–nitrogen double bond of the imines derived from esters of 3,3,3-trifluoropyruvic acid. Medium to high levels of stereocontrol can be achieved using phenylglycinol-derived chiral auxiliaries attached to the nitrogen of the corresponding imines. Among the enantioselective approaches, impressive results were achieved through the application of the Strecker­ reaction using chiral thioureas, as well as chiral Bronsted acids, as organocatalysts. 1 Introduction 2 Stoichiometric Asymmetric Syntheses of α-(Trifluoromethyl)-α-amino Acids 3 Catalytic Asymmetric Syntheses of α-(Trifluoromethyl)-α-amino Acids 4 Miscellaneous 5 Conclusions

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.

Abstract: 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.

Organic Base‐Catalyzed Stereodivergent Synthesis of (R)‐ and (S)‐3‐Amino‐4,4,4‐trifluorobutanoic Acids.

Abstract: Both enantiomers of 3-amino-4,4,4-trifluorobutanoic acid (IV) can be prepared, simply by choosing an appropriate organic base catalyst for the Mannich reaction of chiral sulfinylimine (I) and alkyl malonates (II).

Self-Disproportionation of Enantiomers via Achiral Chromatography: A Warning and an Extra Dimension in Optical Purifications

Abstract: This tutorial review describes the self-disproportionation of enantiomers (SDE) of chiral, non-racemic compounds, subjected to chromatography on an achiral stationary phase using an achiral eluent, which leads to the substantial enantiomeric enrichment and the corresponding depletion in different fractions, as compared to the enantiomeric composition of the starting material. The physicochemical background of SDE is a dynamic formation of homo- or heterochiral dimeric or oligomeric aggregates of different chromatographic behavior. This phenomenon is of a very general nature as the SDE has been reported for different classes of organic compounds bearing various functional groups and possessing diverse elements of chirality (central, axial and helical chirality). The literature data discussed in this review clearly suggest that SDE via achiral chromatography might be expected for any given chiral enantiomerically enriched compound. This presents two very important issues for organic chemists. First, chromatographic purification of reaction products can lead to erroneous determination of the stereochemical outcome of catalytic asymmetric reactions and second, achiral chromatography can be used as a new, nonconventional method for optical purifications. The latter has tremendous practical potential as the currently available techniques are limited to crystallization or chiral chromatography. However, a further systematic study of SDE is needed to develop understanding of this phenomenon and to design practical chromatographic separation techniques for optical purification of non-racemic mixtures by achiral-phase chromatography.

Convenient Synthesis of Fluoroalkyl α‐ and β‐Aminophosphonates.

Abstract: The addition of alkyl phosphites (II) or the α-carbanion of phosphonate (V) to aldimines (I) provides access to fluoroalkyl aminophosphonates (IV) and (VII), respectively.