Synthesis and cation complexation properties of new macrolides

Inclusion complexes of the macrocycle nonactin with benchmark protonated amines: aniline and serine

The biological activity of the macrocycle nonactin is intimately related to its ionophore properties and ability to act as a selective cation carrier. While the focus of most investigations on nonactin has been on the binding of metal cations and small molecular ions, this study pursues the characterization of its inclusion complexes with primary amines with bulky structured side groups of different polarity. To this end, the complexes of nonactin with aniline and with the amino acid L-serine, both in protonated form, are considered as case studies and their relevant coordination arrangements are assessed by means of infrared action spectroscopy, quantum chemical density functional theory and Born-Oppenheimer molecular dynamics. The study suggests that the oxygen atoms from the oxolane (tetrahydrofuran) groups of nonactin constitute the preferential docking sites of the ammonium moiety of the guest cation, although conformational constraints promote interactions with the ester carbonyl backbone groups. In the aniline complex, the benzyl side ring is oriented outwards from the cavity, whereas in the case of L-serine, the side carboxylic acid and alcohol groups participate actively in the coordination process. Interestingly, the accommodation of L-serine is favoured when nonactin adopts an enantiomeric-selective folding, that promotes the tripodal coordination of the protonated amine group with oxolane rings from three nonactinic acid blocks with enantiomeric sequence (+)-(-)-(+), which allows for a facile coordination of the serine side groups. This is recognized as a general feature associated with the alternation of chiral domains in globally achiral natural nonactin, yielding mirror-symmetric complexes with the enantiomers of chiral amines.

Synthesis and Applications of Carbohydrate-Based Organocatalysts

Organocatalysis is a very useful tool for the asymmetric synthesis of biologically or pharmacologically active compounds because it avoids the use of noxious metals, which are difficult to eliminate from the target products. Moreover, in many cases, the organocatalysed reactions can be performed in benign solvents and do not require anhydrous conditions. It is well-known that most of the above-mentioned reactions are promoted by a simple aminoacid, l-proline, or, to a lesser extent, by the more complex cinchona alkaloids. However, during the past three decades, other enantiopure natural compounds, the carbohydrates, have been employed as organocatalysts. In the present exhaustive review, the detailed preparation of all the sugar-based organocatalysts as well as their catalytic properties are described.

Preferential host-guest coordination of nonactin with ammonium and hydroxylammonium

The biological activity of the macrocycle nonactin is intimately related to its ionophore properties and ability to act as a selective cation carrier. The competitive binding of small protonated amines constitutes a particularly key issue in the biochemistry of nonactin, which finds application in sensing and extraction technologies. In this study, isolated complexes of nonactin with ammonium and hydroxylammonium are investigated with infrared action spectroscopy and quantum chemical computations. The focus of the investigation is on the coordination achieved by the protonated guest with the oxygen atoms of either the oxolane groups or the carboxyl groups in the ester linkages of the macrocyle host and their relative contributions to the stability of the complexes. The experimental and computational data converge to a preferred coordination arrangement associated with a tight binding of the N-H ^δ+ bonds with the oxolane groups. In the N H 4 + complex, this results in a compact complex of S ₄ symmetry. In contrast, symmetry is disrupted in the NH₃OH⁺ complex, as it incorporates a bifurcated coordination of the -OH bond with a carbonyl group and an oxolane group of the host, involving also a more stretched arrangement of the nonactin backbone. These gas-phase conformations are in agreement with the structures postulated for these complexes in condensed phases, from previous Raman and crystallographic experiments.

Isolated alkali cation complexes of the antibiotic ionophore nonactin: correlation with crystalline structures

The stabilization of the nonactin–Na⁺ complex in a S₄ or C₂ conformation constitutes a challenging benchmark for experimental and modelling methods.

On the ionophoric selectivity of nonactin and related macrotetrolide derivatives

Microhydration determines the cation affinities of nactin macrocyles in aqueous–organic extraction processes.

Synthesis and conformational analysis of cyclic homooligomers from pyranoid ε-sugar amino acids

New pyranoid ε-sugar amino acids were designed as building blocks, in which the carboxylic acid and the amine groups were placed in positions C2 and C3 with respect to the tetrahydropyran oxygen atom. By using standard solution-phase coupling procedures, cyclic homooligomers containing pyranoid ε-sugar amino acids were synthesized. Conformation analysis was performed by using NMR spectroscopic experiments, FTIR spectroscopic studies, X-ray analysis, and a theoretical conformation search. These studies reveal that the presence of a methoxy group in the position C4 of the pyran ring produces an important structural change in the cyclodipeptides. When the methoxy groups are present, the structure collapses through interresidue hydrogen bonds between the oxygen atoms of the pyran ring and the amide protons. However, when the cyclodipeptide lacks the methoxy groups, a U-shape structure is adopted, in which there is a hydrophilic concave face with four oxygen atoms and two amide protons directed toward the center of the cavity. Additionally, we found important evidence of the key role played by weak electrostatic interactions, such as the five-membered hydrogen-bonded pseudocycles (C5) between the amide protons and the ether oxygen atoms, in the conformation equilibrium of the macrocycles and in the cyclization step of the cyclic tetrapeptides.

Synthesis and antiproliferative activity of (2R,3R)-disubstituted tetrahydropyrans. Part 2: Effect of side chain homologation

In this study, we synthesized a series of enantiomerically pure (2R,3R)- and (2R,3S)-disubstituted tetrahydropyrans bearing a CH2O spacer group on the side chain at position 2 of the heterocyclic ring. The in vitro antiproliferative activities of the compounds were examined in the human solid tumor cell lines A2780 (ovarian cancer), SW1573 (non-small cell lung cancer), and WiDr (colon cancer). Overall, the results point out the relevance for antiproliferative activity of the distance between the heterocycle and the unsaturated group.

Synthesis and antiproliferative activity of (2R,3R)-disubstituted tetrahydropyrans

In this study, we synthesized a series of enantiomerically pure (2R,3R)-disubstituted tetrahydropyrans with diverse functional groups. The in vitro antiproliferative activities were examined in the human solid tumor cell lines A2780 (ovarian cancer), SW1573 (non-small cell lung cancer), and WiDr (colon cancer). Overall, the results show the relevance for antiproliferative activity of the alpha,beta-unsaturated ester side chain at position 2 of the THP ring.

A Convenient and Chemoselective One-Pot Oxidation/Wittig Reaction for the C2-Homologation of Carbohydrate-Derived Glycols

[reaction: see text] A simple and convenient one-pot protocol for the chemoselective C2-homologation of carbohydrate-derived glycols is described. The method comprises the chemoselective oxidation of the glycol to the corresponding hydroxyaldehyde and the subsequent Wittig alkenylation. In addition, the method does not need selective protective group manipulation, and it is safe, economical, fast (5 to 6 h), and bench-friendly. Its general utility is discussed.