Cinchona alkaloids as privileged chiral solvating agents for the enantiodiscrimination of N-protected aminoalkanephosphonates—a comparative NMR study

Scalable Synthesis of All Stereoisomers of 2-Aminocyclopentanecarboxylic Acid─A Toolbox for Peptide Foldamer Chemistry

Although the construction of peptides with well-defined three-dimensional structures and predictable functions, including biological activity, using conformationally constrained β-amino acids has been shown to be a very successful strategy, their broad application is limited by access to the appropriate building blocks. In particular, trans- and cis-stereoisomers of 2-aminocyclopentanecarboxylic acid (ACPC) are of high interest. The scalable synthesis of all four stereoisomers of Fmoc derivatives of ACPC is presented with NMR-based analysis methods for their enantiomeric purity.

Quinine as a highly responsive chiral sensor for the 1H and 19F NMR enantiodiscrimination of N-trifluoroacetyl amino acids with free carboxyl functions

Hydrogen-bond accepting and enantiodiscriminating abilities of quinine (Qui) have been exploited in the enantiodiscrimination of N-trifluoroacetyl (TFA) derivatives of amino acids by nuclear magnetic resonance (NMR) spectroscopy. ¹H and ¹⁹F NMR resonances of derivatives of alanine, valine, leucine, norvaline, phenylalanine, phenylglycine, methionine, glutamic acid, proline, and tryptophan were well differentiated employing CDCl₃ and/or C₆D₆ as solvent, with Qui acting in some cases not only as enantiodiscriminating agent, but also as solubility promoter. For derivatives soluble in both solvents, the best results were obtained in benzene-d6, with very high nonequivalence values, which were detectable not only starting from very low equimolar concentrations of 0.1 mM, but also in the presence of sub-stoichiometric amounts of Qui. The quality of enantiodifferentiation has been also evaluated by means of the enantioresolution quotient E. The method has been applied to the detection and quantification of mixtures of amino acid derivatives by single point measurements.

Synthesis and Inhibitory Studies of Phosphonic Acid Analogues of Homophenylalanine and Phenylalanine towards Alanyl Aminopeptidases

A library of novel phosphonic acid analogues of homophenylalanine and phenylalanine, containing fluorine and bromine atoms in the phenyl ring, have been synthesized. Their inhibitory properties against two important alanine aminopeptidases, of human (hAPN, CD13) and porcine (pAPN) origin, were evaluated. Enzymatic studies and comparison with literature data indicated the higher inhibitory potential of the homophenylalanine over phenylalanine derivatives towards both enzymes. Their inhibition constants were in the submicromolar range for hAPN and the micromolar range for pAPN, with 1-amino-3-(3-fluorophenyl) propylphosphonic acid (compound 15c) being one of the best low-molecular inhibitors of both enzymes. To the best of our knowledge, P1 homophenylalanine analogues are the most active inhibitors of the APN among phosphonic and phosphinic derivatives described in the literature. Therefore, they constitute interesting building blocks for the further design of chemically more complex inhibitors. Based on molecular modeling simulations and SAR (structure-activity relationship) analysis, the optimal architecture of enzyme-inhibitor complexes for hAPN and pAPN were determined.

Alkaloids for cancer prevention and therapy: Current progress and future perspectives

Alkaloids are important chemical compounds that serve as a rich source for drug discovery. Numerous alkaloids screened from medicinal plants and herbs showed antiproliferative and anticancer effects on wide category of cancers both in vitro and in vivo. Vinblastine, vinorelbine, vincristine, and vindesine have already been successfully developed as anticancer drugs. The available and up-to-date information on the ethnopharmacological uses in traditional medicine, phytochemistry, pharmacology and clinical utility of alkaloids were collected using various resources (PubMed, ScienceDirect, Google Scholar and Springerlink). In this article, we provide a comprehensive and critical overview on naturally-occurring alkaloids with anticancer activities and highlight the molecular mechanisms of action of these secondary metabolites. Furthermore, this review also presents a summary of synthetic derivatives and pharmacological profiles useful to researchers for the therapeutic development of alkaloids. Based on the literature survey compiled in this review, alkaloids represent an important group of anticancer drugs of plant origin with enormous potential for future development of drugs for cancer therapy and management.

Cinchona Alkaloids-Derivatives and Applications

Major Cinchona alkaloids quinine, quinidine, cinchonine, and cinchonidine are available chiral natural compounds (chiral pool). Unlike many other natural products, these alkaloids are available in multiple diastereomeric forms which are separated on an industrial scale. The introduction discusses in short conformational equilibria, traditional separation scheme, biosynthesis, and de novo chemical syntheses. The second section concerns useful chemical applications of the alkaloids as chiral recognition agents and effective chiral catalysts. Besides the Sharpless ethers and quaternary ammonium salts (chiral PTC), the most successful bifunctional organocatalysts are based on 9-amino derivatives: thioureas and squaramides. The third section reports the main transformations of Cinchona alkaloids. This covers reactions of the 9-hydroxyl group with the retention or inversion of configuration. Specific Cinchona rearrangements enlarging [2.2.2]bicycle of quinuclidine to [3.2.2] products are connected to the 9-OH substitution. The syntheses of numerous esterification and etherification products are described, including many examples of bi-Cinchona alkaloid ethers. Further derivatives comprise 9-N-substituted compounds. The amino group is introduced via an azido function with the inversion of configuration at the stereogenic center C9. The 9-epi-amino-alkaloids provide imines, amides, imides, thioureas, and squaramides. The syntheses of 9-carbon-, 9-sulfur-, and 9-selenium-substituted derivatives are discussed. Oxidation of the hydroxyl group of any alkaloid gives ketones, which can be selectively reduced, reacted with Grignard reagents, or subjected to the Corey-Chaykovsky reaction. The alkaloids were also partially degraded by splitting C4'-C9 or N1-C8 bonds. In order to immobilize Cinchona alkaloids the transformations of the 3-vinyl group were often exploited. Finally, miscellaneous functionalizations of quinuclidine, quinoline, and examples of various metal complexes of the alkaloids are considered.

Applications of nuclear magnetic resonance spectroscopy for the understanding of enantiomer separation mechanisms in capillary electrophoresis

This review deals with the applications of nuclear magnetic resonance (NMR) spectroscopy to understand the mechanisms of chiral separation in capillary electrophoresis (CE). It is accepted that changes observed in the separation process, including the reversal of enantiomer migration order (EMO), can be caused by subtle modifications in the molecular recognition mechanisms between enantiomer and chiral selector. These modifications may imply minor structural differences in those selector-selectand complexes that arise from the above mentioned interactions. Therefore, it is mandatory to understand the fine intermolecular interactions between analytes and chiral selectors. In other words, it is necessary to know in detail the structures of the complexes formed by the enantiomer (selectand) and the selector. Any differences in the structures of these complexes arising from either enantiomer should be detected, so that enantiomeric bias in the separation process could be explained. As to the nature of these interactions, those have been extensively reviewed, and it is not intended to be discussed here. These interactions contemplate ionic, ion-dipole and dipole-dipole interactions, hydrogen bonding, van der Waals forces, π-π stacking, steric and hydrophobic interactions. The main subject of this review is to describe how NMR spectroscopy helps to gain insight into the non-covalent intermolecular interactions between selector and selectand that lead to enantiomer separation by CE. Examples in which diastereomeric species are created by covalent (irreversible) derivatization will not be considered here. This review is structured upon the different structural classes of chiral selectors employed in CE, in which NMR spectroscopy has made substantial contributions to rationalize the observed enantioseparations. Cases in which other techniques complement NMR spectroscopic data are also mentioned.

Synthesis of fluorescent (benzyloxycarbonylamino)(aryl)methylphosphonates

The synthesis of a library of structurally variable aromatic esters of (benzyloxycarbonylamino)(aryl)methylphosphonic acids is described by means of the Oleksyszyn reaction. The library was enlarged by the application of a Suzuki–Miayra approach and by preparation of mixed esters.

Supramolecular complexations of natural products

Complexations of natural products with synthetic receptors as well as the use of natural products as host compounds are reviewed, with an emphasis on possible practical uses or on biomedical significance. Applications such as separation, sensing, enzyme monitoring, and protection of natural drugs are first outlined. We then discuss examples of complexes with all important classes of natural compounds, such as amino acids, peptides, nucleosides/nucleotides, carbohydrates, catecholamines, flavonoids, terpenoids/steroids, alkaloids, antibiotics and toxins.

Novel chiral selector based on mefloquine--a comparative NMR study to elucidate intermolecular interactions with acidic chiral selectands

The synthesis, ab initio calculations, and a comparative nuclear magnetic resonance study of a novel chiral mefloquine-based selector (SO) are presented. On a series of variously N-acyl protected leucine selectands (SAs), a feasibility study of mefloquine carbamate as a basic chiral solvating agent, and potential fluorophilic high-performance liquid chromatography selector has been undertaken and evaluated. An analogy is drawn between the new SO and tert-butylcarbamoyl quinidine as a reference.

Host-Guest Chemistry of Alkaloids

Binding of alkaloids by different hosts (native and modified cyclodextrins, cucurbiturils, calixarenes, and metal complexes of porphyrin and Salphen-type ligands), as well as receptor properties of alkaloid based hosts are reviewed. With alkaloids as guests, the largest binding constants and most significant spectral changes, in particular strong fluorescence enhancements induced by complexation with isoquinoline alkaloids, are observed with cucurbituril hosts. Cyclodextrins are successfully employed for improvement of solubility and for chiral separation of alkaloids of different types. Receptor properties of native and modified cinchona and bisbenzylisoquinoline alkaloids have attracted considerable attention for development of chiral selectors for analysis and separation.