Achiral Isomer Research Articles

Rhodium-Catalyzed Asymmetric Addition to 4- or 5-Carbonyl-cycloenones through Dynamic Kinetic Resolution: Enantioselective Synthesis of (-)-Cannabidiol.

The reaction of 4/5-carbonyl-cycloalkenone 1 or its achiral isomer 1' with organoboronic acid 2 in the presence of a chiral diene (S,S)-Fc-tfb-rhodium catalyst gave disubstituted trans-cycloalkanone 3 with high diastereo- and enantioselectivity. This highly efficient dynamic kinetic resolution is achieved by fast racemization of 1 through the formation of a dienolate followed by kinetic resolution with the chiral catalyst. The utility is demonstrated by the synthesis of key intermediates en route to (-)-cannabidiol.

Synthesis and Properties of Achiral and Chiral Dipyrenoheteroles and Related Compounds.

Achiral and chiral isomers of dipyrenoheteroles were synthesized via alkyne benzannulation. The electronic properties of these compounds were examined using cyclic voltammetry and differential pulse voltammetry. The enantiomers of the chiral isomers were separated, and their optical properties were examined in circular dichroism and circularly polarized luminescence studies. The chiral isomers exhibited a large bathochromic shift, relative to the achiral isomer, in both absorbance and fluorescence, resulting from decreased symmetry, rather than a change in the size of the backbone.

Semirigid discotic dimers: flexible but not flexible enough?

Two diastereomeric dibenzo[a,c]phenazine 1,2-cyclohexyl bridged diesters were prepared and their phase properties examined. While the trans dimer exhibited a broad columnar liquid crystal phase, the cis dimer was amorphous at all temperatures studied. This difference was attributed to the conformational dynamics of the two systems. NMR and DFT studies indicate that both dimers adopt folded and unfolded conformers in solution. While their folded geometries were similar, the trans dimer adopts an extended, largely planar structure, whereas the cis dimer is limited to non-planar unfolded structures and likely disrupts columnar ordering. Geometric constraints imposed by the cylcic linker were also important for columnar stability, with the trans dimer clearing 40 °C lower than the corresponding acyclic 2R,3R-butyl linked dimer, likely because the cyclohexyl group hinders π-π stacking in the unfolded conformation of the former.

Algorithm for Combinatorial Bipartite Enumeration of Chiral and Achiral Stereoisomers of Homo- and Heteropolysubstituted Derivatives of the Cubane Molecule

This paper is devoted to the application of an algorithm for the enumeration of chiral and achiral stereoisomers of homo and heteropolysubstituted derivatives of the cubane molecule. The sequence of this algorithm is in three steps, the first of which is the determination of the permutations of 8 hydrogen atoms of the cuban molecule ( ) subjected to the action of the Oh group. The second step will be the composition of these permutations later into permutational representations and controlling respectively the chirality and achirality of this molecular system. The transformation of these 2 operators into a pair of generic formulae used for bipartite combinatorial enumeration giving chiral and achiral isomer numbers of the homo- and hetero-polysubstituted derivatives of the cubane molecule respectively symbolised by the empirical formulae and being the final step.

Ruthenium-Containing Linear Helicates and Mesocates with Tuneable p53-Selective Cytotoxicity in Colorectal Cancer Cells.

The ligands L1 and L2 both form separable dinuclear double-stranded helicate and mesocate complexes with RuII . In contrast to clinically approved platinates, the helicate isomer of [Ru2 (L1 )2 ]4+ was preferentially cytotoxic to isogenic cells (HCT116 p53-/- ), which lack the critical tumour suppressor gene. The mesocate isomer shows the reverse selectivity, with the achiral isomer being preferentially cytotoxic towards HCT116 p53+/+ . Other structurally similar RuII -containing dinuclear complexes showed very little cytotoxic activity. This study demonstrates that alterations in ligand or isomer can have profound effects on cytotoxicity towards cancer cells of different p53 status and suggests that selectivity can be "tuned" to either genotype. In the search for compounds that can target difficult-to-treat tumours that lack the p53 tumour suppressor gene, [Ru2 (L1 )2 ]4+ is a promising compound for further development.

Stereochemical Control of Nonamphiphilic Lyotropic Liquid Crystals: Chiral Nematic Phase of Assemblies Separated by Six Nanometers of Aqueous Solvents

Unlike conventional thermotropic and lyotropic liquid crystals, nonamphiphilic lyotropic liquid crystals consist of hydrated assemblies of nonamphiphilic molecules that are aligned with a separation of about 6 nm between assemblies in an aqueous environment. This separation raises the question of how chirality, either from chiral mesogens or chiral dopants, would impact the phase as the assemblies that need to interact with each other are about 6 nm apart. Here, we report the synthesis of three stereoisomers of disodium chromonyl carboxylate, 5'DSCG-diviol, and the correlation between the molecular structure, bulk assembly, and liquid crystal formation. We observed that the chiral isomers (enantiomers 5'DSCG-(R,R)-diviol and 5'DSCG-(S,S)-diviol) formed liquid crystals while the achiral isomer 5'DSCG-meso-diviol did not. Circular dichroism indicated a chiral conformation with bisignate cotton effect. The nuclear Overhauser effect in proton NMR spectroscopy revealed conformations that are responsible for liquid crystal formation. Cryogenic transmission electron microscopy showed that chiral 5'DSCG-diviols form assemblies with crossings. Interestingly, only planar alignment of the chiral nematic phase was observed in liquid crystal cells with thin spacers. The homeotropic alignment that permitted a fingerprint texture was obtained only when the thickness of the liquid crystal cell was increase to above ~500 μm. These studies suggest that hydrated assemblies of chiral 5'DSCG-diviol can interact with each other across a 6 nm separation in an aqueous environment by having a twist angle of about 0.22° throughout the sample between the neighboring assemblies.

Synthesis of an Achiral Isomer of Lipoic Acid As an Anchor Group for SAM Formation on Gold Surfaces

Isolipoic acid, a symmetrical and achiral isomer of the commonly used alpha-lipoic acid, has previously been overlooked as a tether group for the formation of self-assembled monolayers (SAMs). Here its ready synthesis through a new route and its functionalization with ferrocenyl groups for redox-active SAM formation on gold electrodes are described.

Chiral [Iminophosphoranyl]ferrocenes: Synthesis, Coordination Chemistry, and Catalytic Application

A series of new chiral [iminophosphoranyl]ferrocenes, {η 5 -C 5 H 4 -(PPh2=N-2,6-R 2 -C 6 H 3 )}Fe{η 5 -C 5 H 3 -1-PPh 2 -2-CH(Me)NMe 2 } (1: R = Me, i Pr), {η 5 -C 5 H 4 -(PPh 2 =N-2,6-R 2 1 -C 6 H 3 )}Fe{η 5 -C 5 H 3 -1-(PPh 2 =N-2,6-R2-C 6 H 3 )-2-CH(Me)R 2 } (2: R 1 = Me, i Pr; R 2 = NMe 2 , OMe), and (η 5 -C5H5)Fe{η 5 -C 5 H 4 -1-PR 2 -2-CH(Me)N=PPh 3 } (3: R = Ph, C 6 H 1 1 ) have been prepared from the reaction of [1,1'-diphenylphosphino-2-(N,N-dimethyl-amino)ethyl]ferrocene with arylazides (1 & 2) and the reaction of phosphine dichlorides (R 3 PCl 2 ) with [1,1'-diphenylphosphino-2-aminoethyl]ferrocene (3), respectively. They form palladium complexes of the type [Pd(C 3 H 5 )(L)]BF 4 (4-6: L = 1-3), where the ligand (L) adopts an η 2 -N,N (2) or η 2 -P,Ν (3) as expected. In the case of 1, a potential terdentate, an η 2 -P,N mode is realized with the exclusion of the -P=NAr group from the coordination sphere. Complexes 4-6 were employed as catalysts for allylic alkylation of 1,3-diphenylallyl acetate leading to an almost stoichiometric product yield with modest enantiomeric excess (up to 74% ee). Rh(I)-complexes incorporating 1-3 were also prepared in situ for allylic alkylation of cinnamyl acetate as a probe for both regio- and enantioselectivities of the reaction. The reaction exhibited high regiocontrol in favor of a linear achiral isomer regardless of the ligand employed.

Chiral tubule self-assembly from an achiral diynoic lipid.

Tubules possessing microm-scale chiral substructure self-assemble from an achiral isomer of the tubule-forming diynoic phosphatidylcholine, 1,2-bis(10,12-tricosadiynoyl)sn-glycero-3-phosphocholine [DC(8,9)PC], showing that molecular chirality is not essential for tubule formation. CD spectroscopy shows that these structures' helical sense of handedness instead originates in a spontaneous cooperative chiral symmetry-breaking process. We conclude that the chiral symmetry-breaking must originate in the unusual feature common to the chiral and achiral tubule-forming molecules, the diynes centered in their hydrocarbon tails.

Synthesis and X-ray structure determination of thionated biphthalimides

Thiation of N , N -biphthalimide in a search for new axially chiral ligands gave a mixture of oligothiobiphthalimides; X-ray crystallography shows that the achiral isomer of dithiodioxobiphthalimide, 1,3-dithioxo-1H-3H-[2,2 ' ]biisoindolyl-1 ' ,3 ' -dione is formed, which may imply a change in the mechanism of thiation between mono- and di-phthalimides.

Rotational isomerism in octahedral d6-iridium(III) complexes trans,mer-[IrCl 2(PMe 3) 3L] + where L = PMe 3, PMe 2Ph or PMePh 2; X-ray crystal structure of [IrCl 2(PMe 3) 3(PMe 2Ph)]ClO 4·CH 2Cl 2

The cationic d 6-complexes [IrCl 2(PMe 3) 3L] + where L = PMe 3, PMe 2Ph or PMePh 2 have been synthesized and the single-crystal X-ray structure of trans,mer-[IrCl 2(PMe 3) 3(PMe 2Ph)]ClO 4·CH 2Cl 2 determined; the cation is chiral because of the conformation adopted about the IrPMe 2Ph bond. The low-temperature 31P{ 1H} NMR spectrum (−75°C, CD 3COCD 3)of this salt shows a major ABCD component (95%), consistent with the chiral conformer found in the crystal, and a minor AB 2C component (5%) corresponding to a symmetrical achiral isomer. These isomers interconvert by rotation about the IrPMe 2Ph bond and at 45°C a sharp coalesced AB 2C spectrum is obtained. In the low-temperature 1H NMR spectrum, the major chiral isomer shows non-equivalent ortho-protons, whereas the minor achiral isomer gives a single resonance for these protons. Related rotamers are observed for the PMePh 2 complex but we could not obtain frozen-out spectra for [IrCl 2(PMe 3) 4] +. We conclude that, even for these small tertiary phosphines, it is possible to have rotamers that interconvert slowly enough for line-shape effects to be observed in NMR spectra at ambient temperatures provided that there are four phosphines in each complex. Chiral rotamers may be dominant in rotameric equilibria.