Enantioselective complexation of organic ammonium ions by simple tetracyclic podand ionophores

Predisposition in Dynamic Covalent Chemistry: The Role of Non-Covalent Interactions in the Assembly of Tetrahedral Boronate Cages

Directional bonding strategies guide the design of complex molecular architectures, yet challenges arise due to emergent behavior. Rigid structures face geometric constraints and sensitivity to mismatches, hindering the efficient assembly of molecular organic cages (MOCs). Harnessing intramolecular non-covalent interactions offers a promising solution, broadening geometrical possibilities and enhancing adaptability to boost assembly yields. However, identifying these interactions remains challenging, with their full potential sometimes latent until final assembly. This study explores these challenges by synthesizing boronic acid tripods with varied oxygen positions at the tripodal feet and investigating their role in assembling tetrahedral boronate MOCs. Our results reveal substantial differences in the assembly efficiency among tripods. While the building blocks with oxygen in the benzylic position relative to the central aromatic ring form the MOCs in high yields, those with the oxygen atom directly bound to the central aromatic ring, only yield traces. Through X-ray crystallography and DFT analyses, we elucidate how intramolecular interactions profoundly influence the geometry of the building blocks and cages in a relay-like fashion, highlighting the importance of considering intramolecular interactions in the rational design of (supra)molecular architectures.

Conformational control enables boroxine-to-boronate cage metamorphosis

The discovery of molecular organic cages (MOCs) is inhibited by the limited organic-chemical space of the building blocks designed to fulfill strict geometric requirements for efficient assembly. Using intramolecular attractive or repulsive non-covalent interactions to control the conformation of flexible systems can effectively augment the variety of building blocks, ultimately facilitating the exploration of new MOCs. In this study, we introduce a set of boronic acid tripods that were designed using rational design principles. Conformational control was induced by extending the tripod's arms by a 2,3-dimethylbenzene unit, leading to the efficient formation of a tetrapodal nanometer-sized boroxine cage. The new building block's versatility was demonstrated by performing cage metamorphosis upon adding an aromatic tetraol. This led to a quantitative boroxine-to-boronate transformation and a topological shift from tetrahedral to trigonal bipyramidal.

Conformationally Controlled sp3 -Hydrocarbon-Based α-Helix Mimetics

With over 60 % of protein-protein interfaces featuring an α-helix, the use of α-helix mimetics as inhibitors of these interactions is a prevalent therapeutic strategy. However, methods to control the conformation of mimetics, thus enabling maximum efficacy, can be restrictive. Alternatively, conformation can be controlled through the introduction of destabilizing syn-pentane interactions. This tactic, which is often adopted by Nature, is not a common feature of lead optimization owing to the significant synthetic effort required. Through assembly-line synthesis with NMR and computational analysis, we have shown that alternating syn-anti configured contiguously substituted hydrocarbons, by avoiding syn-pentane interactions, adopt well-defined conformations that present functional groups in an arrangement that mimics the α-helix. The design of a p53 mimetic that binds to Mdm2 with moderate to good affinity, demonstrates the therapeutic promise of these scaffolds.

Stereochemistry and biological activity of chlorinated lipids: a study of danicalipin A and selected diastereomers

The syntheses of (+)-16-epi- and (+)-11,15-di-epi-danicalipin A (2 and 3) are reported. The conformations of the parent diols 5 and 6 as well as the corresponding disulfates 2 and 3 were determined on the basis of J-based configuration analysis and supported by calculations. The impact of configuration on membrane permeability in Gram-negative bacteria and mammalian cell lines was assessed as well as cytotoxicity. Although diastereomer 2 showed similar behavior to natural (+)-danicalipin A (1), strikingly, the more flexible C11,C15-epimer 3 had no effect on permeability and proved equally or more toxic towards multiple cell lines.

Enantiomeric discrimination of chiral organic salts by chiral aza-15-crown-5 ether with C 1 symmetry: experimental and theoretical approaches

The work involves an experimental ((1)H NMR) and theoretical (MD, MM-PBSA and DFT) investigation of the molecular recognition and discrimination properties of a chiral aza-15-crown-5 against methyl esters of alanine, phenylalanine and valine hydrochloride salts. The results indicate that the receptor binds enantiomers with moderate binding constants (88-1,389 M(-1)), with phenylalanine being more discriminated. The difference in experimental binding free energies (ΔG(R) - ΔG(S)) for alanine, phenylalanine and valine enantiomers were calculated as -0.36, -1.58 and 0.80 kcal mol(-1), respectively. The differences in theoretical binding energies were calculated by MM-PBSA (ΔE(R)PB - ΔE(S)PB=) as -0.30, -1.45 and 0.88, by B3LYP/6-31+G(d) as -1.17, -0.84 and 0.74 and by M06-2X/6-31+G(d) as -1.40, -3.26 and 1.66 kcal mol(-1). The data obtained give valuable information regarding the molecular recognition mode of the organoammonium complexes of chiral aza-crown ether with C 1 symmetry, which may be relevant to biological systems.

The journey of L-tartaric acid in the world of enantiomerically pure bis- and trisadducts of C60 with the inherently chiral trans-3 and all-trans-3 addition patterns

Inherently chiral multi-adducts of [60]fullerene represent unique chiral molecular tectons featuring fascinating optoelectronic properties. Herein we discuss the most recent progress in the synthesis of enantiomerically pure bis- and trisadducts of C60 with the inherently chiral trans-3 and all-trans-3 addition patterns utilizing cyclo-[n]-malonate tethers derived from (-)-dimethyl-2,3-O-isopropylidene-L-tartrate. Some future perspectives regarding the investigation of these chiral building blocks in modern areas of research are discussed.

Assembly-line synthesis of organic molecules with tailored shapes

Molecular assembly lines, where molecules undergo iterative processes involving chain elongation and functional group manipulation are hallmarks of many processes found in Nature. We have sought to emulate Nature in the development of our own molecular assembly line through iterative homologations of boronic esters. Here we report a reagent (α-lithioethyl triispopropylbenzoate) which inserts into carbon-boron bonds with exceptionally high fidelity and stereocontrol. Through repeated iteration we have converted a simple boronic ester into a complex molecule (a carbon chain with ten contiguous methyl groups) with remarkably high precision over its length, its stereochemistry and therefore its shape. Different stereoisomers were targeted and it was found that they adopted different shapes (helical/linear) according to their stereochemistry. This work should now enable scientists to rationally design and create molecules with predictable shape, which could have an impact in all areas of molecular sciences where bespoke molecules are required.

Total syntheses of parthenolide and its analogues with macrocyclic stereocontrol

The first total synthesis of parthenolide (1) is described. The key feature of this synthesis is the formation of a 10-membered carbocylic ring by a macrocyclic stereocontrolled Barbier reaction, followed by a photoinduced Z/E isomerization. The biological evaluation of a small library of parthenolide analogues (19, 33, and 34) disclosed a preliminary structure-activity relationship (SAR). The results revealed that the C1, C10 double bond configuration of parthenolide has little or no effect on the activity, and the C6 and C7 configurations of the lactone ring have a moderate impact on the activities against some cancer cell lines.

Molecular recognition of organic ammonium ions in solution using synthetic receptors

Ammonium ions are ubiquitous in chemistry and molecular biology. Considerable efforts have been undertaken to develop synthetic receptors for their selective molecular recognition. The type of host compounds for organic ammonium ion binding span a wide range from crown ethers to calixarenes to metal complexes. Typical intermolecular interactions are hydrogen bonds, electrostatic and cation-π interactions, hydrophobic interactions or reversible covalent bond formation. In this review we discuss the different classes of synthetic receptors for organic ammonium ion recognition and illustrate the scope and limitations of each class with selected examples from the recent literature. The molecular recognition of ammonium ions in amino acids is included and the enantioselective binding of chiral ammonium ions by synthetic receptors is also covered. In our conclusion we compare the strengths and weaknesses of the different types of ammonium ion receptors which may help to select the best approach for specific applications.

Olefin metathesis-iodoetherification-dehydroiodination strategy for spiroketal subunits of polyether antibiotics

The convergent synthesis of two pentacyclic analogues of the polyether monensin A is described. Although different with respect to the configuration of the alcohol at the 3 position of the six-membered ring of the spiroketal subunit, the configuration at the acetal center in both structures is unchanged and is consistent with the anomeric effect. The key synthetic steps are the coupling of two complex segments via an olefin metathesis, and the subsequent conversion of a dihydroxyalkene to the spiroketal through an iodoetherification-dehydroiodination sequence. The compatibility of these transformations with a variety of functional groups makes the overall strategy appropriate for highly substituted frameworks.

Synthesis and evaluation of new chiral nonracemic C2-symmetric and unsymmetric 2,2′-bipyridyl ligands

The synthesis of a series of chiral nonracemic and C2-symmetric 2,2'-bipyridyl ligands (R = Me, i-Pr and Ph) as well as the syntheses of the corresponding unsymmetric 2,2'-bipyridyl ligands (R = Me and Ph) is described. These bipyridyl ligands were prepared, in a notably direct and modular fashion, from the readily available and corresponding 2-chloropyridine acetals (R = Me, i-Pr and Ph). The bipyridyl ligands were evaluated in copper(I)-catalyzed cyclopropanation reactions of styrene with the ethyl and t-butyl esters of diazoacetic acid. The stereoselectivities, as well as the yields of the cyclopropanation reactions, were dependant on the ratio of the bipyridyl ligands and copper triflate that was employed. The best result was obtained in the asymmetric cyclopropanation reaction of styrene and tert-butyl diazoacetate with the C2-symmetric bipyridyl ligand (R = i-Pr). This afforded the corresponding trans-cyclopropane in good diastereoselectivity (4 : 1) and in moderate enantioselectivity (44% ee). The X-ray structure determination of a complex formed between the C2-symmetric 2,2'-bipyridyl ligand (R = Ph) and copper(I) chloride showed that two bipyridyl ligands had coordinated to the copper(I) ion. This information, along with the results of a series of cyclopropanation reactions and NMR data, led to the conclusion that the 2,2'-bipyridyl ligands had the propensity to form catalytically inactive bis-ligated copper(I) species in solution that were in equilibrium with catalytically active copper(i) triflate and the desired mono-ligated copper(I) species. Moreover, it was observed that the complex of the bipyridyl ligand (R = Ph) and copper(I) chloride had a particularly large optical rotation (sodium D-line). The maximum positive optical rotation was subsequently found to be +1.1 x 10(4) at 304 nm and the maximum negative optical rotation was -1.3 x 10(4) at 329 nm.

Synthesis and evaluation of 7-hydroxyindan-1-one-derived chiral auxiliaries

A series of novel chiral acetals were prepared from 7-hydroxyindan-1-one and a variety of substituted chiral nonracemic C2-symmetric 1,2-ethanediols (R = Me, Ph, CH2OMe, CH2OBn, CH2O(1-Np), and i-Pr). These acetals were evaluated as chiral auxiliaries for use in asymmetric synthesis. A high degree of stereochemical induction was observed in the diethylaluminum chloride-promoted Diels–Alder reaction of an acrylate derivative (R = i-Pr) with cyclopentadiene (91:9 dr). This demonstrated that these acetals could serve as effective chiral directors in asymmetric substrate-directed reactions.Key words: 7-hydroxyindan-1-one, chiral nonracemic C2-symmetric 1,2-diols, acetals, chiral auxiliaries, Diels–Alder reaction.

A New Class of Chiral P,N-Ligands and Their Application in Palladium-Catalyzed Asymmetric Allylic Substitution Reactions

An efficient and modular synthesis of a series of chiral nonracemic P,N-ligands is reported. The P,N-ligands were prepared from 2-chloro-4-methyl-6,7-dihydro-5H-[1]pyrindine-7-one and a series of substituted chiral C(2)-symmetric 1,2-ethanediols (R = Me, i-Pr, and Ph). The ligands were evaluated for use in catalytic asymmetric synthesis in the palladium-catalyzed allylic substitution reactions of a racemic allylic acetate and dimethyl malonate. In the case of the P,N-ligand (R = Ph), the reaction was found to be highly stereoselective (90% ee).

Preorganized ligand arrays based on spirotetrahydrofuranyl motifs. Synthesis of the stereoisomeric 1,8,14-trioxatrispiro[4.1.4.1.4. 1]octadecanes and the contrasting conformational features and ionic binding capacities of these belted ionophores

The cis,trans trispiro ether 4 is accessible from several synthetic directions as a consequence of a crossover in reaction selectivity when proceeding from nucleophilic attack on the cis dispiro ketone to oxygenation of the alpha,beta-unsaturated ester 17. Its cis,cis isomer 3 was obtained in 17 steps and 14.6% overall yield from 3, 5-dimethoxybenzoic acid by making use of the alicyclic side chain in N as a "conformational lock". Although 4 shows no measurable tendency to complex with alkali metal ions, 3 binds strongly to Li(+) and Na(+) ions, as well as to CH(3)NH(3)(+). Whereas the 3eq conformation is populated in the solid state and in solution, complex formation occurs readily. (13)C NMR studies have defined slow exchange limits as the 2:1 sandwich complex with lithium ion is initially formed and transformed progressively into a 1:1 species upon the addition of more LiClO(4). Only the 2:1 complex with sodium ion is formed during comparable titration with NaClO(4). Association constants, molecular mechanics calculations, and X-ray crystallographic studies provide insight into the binding capacity of this belted tridentate ionophore.

Role of proximal His93 in nitric oxide binding to metmyoglobin. Application of continuum solvation in Monte Carlo protein simulations

Monte Carlo protein simulations with continuum solvation were used to explore the conformational mobility of NO within the active site of metmyoglobin. To the best of our knowledge this is the first application of a continuum solvation model for exploring protein binding sites. The usefulness of the Monte Carlo conformational analysis was demonstrated in comparative molecular dynamics simulations. Analysis of conformer populations revealed that Monte Carlo conformational analysis is more effective in mapping the relevant region of the potential surface. Distribution of low-energy conformers obtained for the metmyoglobin-NO complex was found to depend on the orientation of proximal His93. Different charge distributions corresponding to the two experimentally verified possible torsions of this proximal residue result in strong binding of NO or its release to a nearby hydrophobic trap. Conformer populations obtained by Monte Carlo conformational analysis were grouped into three main families: one, with the NO directly bound to the iron, appears when the CA-CB-CG-CD2 torsion of His93 is at its ligand binding value (-113 degrees); and two conformers exist where NO is trapped in a nearby hydrophobic pocket, the same cavity that was determined to be the geminate trap of CO in ferrous Mb as a result of the torsional flip of His93 to its ligand releasing state (-125 degrees). Based on this analysis, we suggest that the electrostatic rearrangement coupled to the conformational fluctuation of the proximal His leads to the geminate trapping of the ligand. Conformational rearrangement of the proximal side would provide the possibility of rebinding of the ligand to Fe.

Definition of Several Control Elements Relevant to the Stereodefined Serial Elaboration of Belted Poly(spirotetrahydrofurans) Fitted with a Cyclohexane Core

The stereochemistry of the condensations of 2-cyclohexenones, alpha-arylidenecyclohexanones, and alpha-(tert-butyldimethylsiloxy)cyclohexanones carrying one or two (both syn and anti) spirotetrahydrofuran units adjacent to the carbonyl with allyl organometallics (especially indium) and with the Normant reagent (ClMgO(CH(2))(3)MgCl) is described. Good levels of anti stereoselection are observed in the alpha-arylidene series. Subsequent cyclization generates a second (or third) tetrahydrofuran ring possessing trans vicinal oxygens. Useful levels of matched and mismatched diastereoselection are also attainable by prior alpha-oxygenation. The intrinsic differences in diastereomer production between indium and magnesium organometallics are highlighted. A clear distinction regarding the anticipated direction of stereoselectivity is made on the grounds of chelation capabilities and the intra- or intermolecularity of carbon-carbon bond formation. Finally, the two protocols that are described in detail are shown to be iterative, a feature that augurs well for ultimately accessing the eight possible hexaspirocyclohexanes in an efficient and stereocontrolled manner.

Antenna-Initiated Photochemistry in Polyfunctional Steroids. Intramolecular Singlet and Triplet Energy Transfer between Aryl, Ketone, and Alkene Groups in 6beta-(Dimethylphenylsiloxy)-5beta-androstanes(1)

Steroids have been prepared that bear a dimethylphenylsiloxy (DPSO) group and additional C3 and/or C17 ketone functionalities. The DPSO group has been used to harvest 266 nm photons and then activate the ketone functionalities through intramolecular singlet-singlet energy transfer (intra-SSET). Thus, the monoketones 3,3-(ethylenedioxy)-6beta-(DPSO)-5beta-androstan-17-one (6) and 6beta-(DPSO)-5beta-androstan-3-one (8) both exhibit DPSO-initiated photochemistry at the carbonyl groups. Irradiation of the diketone, 6beta-(DPSO)-5beta-androstane-3,17-dione (5), gives two ring D-derived photoproducts, an epimer (19) and an enal (18), both coming from the C17 ketone excited singlet state. Here Phi(intra-SSET) from the aryl antenna to the carbonyl groups is ca. 88% efficients and occurs with a rate of ca. 6.5 x 10(9) s(-)(1), with the chemistry indicative of facile intra-SSET between the C3 and C17 ketones. The alkylidene group at C3 (i.e., as in 6beta-(DPSO)-3(E)-ethylidene-5beta-androstan-17-one (33) and its Z isomer (34)) has no effect on the rate or efficiency of aryl activation of the C17 ketone.