Search

The preparation of four per-O-benzyl-d- or l-glycero-d-galacto and d- or l-glycero-d-gluco heptopyranosyl sulfoxides and the influence of their side-chain conformations on reactivity and stereoselectivity in glycosylation reactions are described. The side-chain conformation in these donors is determined by the relative configuration of its point of attachment to the pyranoside ring and the two flanking centers in agreement with a recent model. In the d- and l-glycero-d-galacto glycosyl donors, the d-glycero-d-galacto isomer with the more electron-withdrawing trans,gauche conformation of its side chain was the more equatorially selective isomer. In the d- and l-glycero-d-gluco glycosyl donors, the l-glycero-d-gluco isomer with the least disarming gauche,gauche side-chain conformation was the most equatorially selective donor. Variable temperature NMR studies, while supporting the formation of intermediate glycosyl triflates at -80 °C in all cases, were inconclusive owing to a change in the decomposition mechanism with the change in configuration. It is suggested that the equatorial selectivity of the l-glycero-d-gluco isomer arises from H-bonding between the glycosyl acceptor and O6 of the donor, which is poised to deliver the acceptor antiperiplanar to the glycosyl triflate, resulting in a high degree of SN2 character in the displacement reaction.

Glycosylation reactions were performed on a series of bicyclic C2-substituted pyranoside models to isolate and analyze factors that control the glycosylation stereoselectivities observed in carbohy...

International audience; Using a molecular jet Fourier transform spectrometer, the microwave spectrum of E-3-penten-2-one, also called methyl propenyl ketone, was recorded in a range from 2.0 to 26.5 GHz. Two conformers, antiperiplanar (ap) and synperiplanar (sp), could be identified. Complicated splitting patterns arising due to the internal rotation of the acetyl methyl and propenyl methyl groups could be resolved, and all measured rotational transitions were fitted to measurement accuracy. The barrier heights of the acetyl methyl group are 434.149(37) cm−1 and 358.076(26) cm−1 for the ap and sp conformers, respectively. For the propenyl methyl group, the barrier is 581.903(45) cm−1 for the ap conformer and 595.271(71) cm−1 for the sp conformer. By comparing these results to those of other ketones categorized in a class system connecting the internal rotation of the acetyl methyl group to the structures of the molecules, we extended the categorization with a new class for α,β-unsaturated ketones.

The glycosylation stereoselectivities for a series of bicyclic furanoside models have been carried out in the presence of weak nucleophiles. These results were analyzed through the bent bond/antiperiplanar hypothesis (BBAH) orbital model to test its validity. According to the BBAH, incoming nucleophiles displace one of the two bent bonds of bicyclic oxocarbenium ion intermediates in an antiperiplanar fashion. The glycosylation stereoselectivity is then governed by the displacement of the weaker bent bond as determined by the presence of electron-withdrawing or -donating substituents at C

To study the influence of a linker rigidity and changes in donor-acceptor properties, three series of nucleotide analogs containing a P-X-HN-C(O)- residue (X=CH(OH)CH2, CH(OH)CH2CH2, CH2CH(OH)CH2) as a replacement for the P-CH2-O-CHR- fragment in acyclic nucleoside phosphonates, e.g., adefovir, cidofovir, were synthesized. EDC proved to provide good yields of the analogs from the respective ω-amino-1- or -2-hydroxyalkylphosphonates and nucleobase-derived acetic acids. New phosphorus-nucleobase linkers are characterized by two fragments of the restricted rotation within amide bonds and in four-atom units (P-CH(OH)-CH2-N, P-CH(OH)-CH2-C and P-CH2-CH(OH)-C) in which antiperiplanar disposition of P and N/C atoms was deduced from 1H and 13C NMR spectral data. The synthesized analogs P-X-HNC(O)-CH2B [X=CH(OH)CH2, CH(OH)CH2CH2, CH2CH(OH)CH2] appeared inactive in antiviral assays on a wide variety of DNA and RNA viruses at concentrations up to 100 μM, while two phosphonates showed cytostatic activity towards myeloid leukemia (K-562) and multiple myeloma cells (MM.1S) with IC50 of 28.8 and 40.7 μM, respectively. ispartof: MONATSHEFTE FUR CHEMIE vol:150 issue:4 pages:733-745 ispartof: location:Austria status: published

Adenosine diphosphate (ADP) plays a crucial role in cell biochemistry, especially in metabolic pathways and energy storage. ADP itself, as well as many of its analogues, such as adenosine hypodiphosphate (AhDP), has been studied extensively, in particular in terms of enzymatic activity. However, structural studies in the solid state, especially for AhDP, are still missing. An analogue of ADP, i.e. adenosine hypodiphosphate ester, has been synthesized and characterized in the crystalline form as two hydrated sodium salts of 2′:3′-isopropylideneadenosine 5′-hypodiphosphate (H3AhDP, C13H19N5O9P2 for the neutral form), namely pentasodium tetrakis(2′:3′-isopropylideneadenosine 5′-hypodiphosphate) tetracosahydrate, 5Na+·3C13H18N5O9P2 −·C13H17N5O9P2 2−·24H2O or Na5(H2AhDP)3(HAhDP)·24H2O, (I), and sodium tetrakis(2′:3′-isopropylideneadenosine 5′-hypodiphosphate) pentadecahydrate, Na+·C13H20N5O9P2 +·2C13H18N5O9P2 −·C13H19N5O9P2·15H2O or Na(H4AhDP)(H3AhDP)(H2AhDP)2·15H2O, (II). Crystal structure analyses of (I) and (II) reveal two nucleoside hypodiphosphate ions in the asymmetric units with different ionization states of the hypodiphosphate unit and adenine base. For all AhDP nucleotides, the same anti conformation about the N-glycosidic bond and similar puckering of the ribose ring have been found. AhDP geometry and interactions have been compared to ADP nucleotides deposited in the Cambridge Structural Database. The adenine–hypodiphosphate interactions, identified as defining nucleotide self-assembly, have been analysed in model systems, i.e. the adenine (Ade) salts of hypodiphosphoric acid, namely bis(adeninium) hypodiphosphate dihydrate, 2C5H6N5 +·H2P2O6 2−·2H2O or (AdeH)2(H2P2O6)·2H2O, (III), and bis(adeninium) hypodiphosphate, 2C5H6N5 +·H2P2O6 2− or (AdeH)2(H2P2O6), (IV).

TCNQ is one of the most important organic acceptors and lots of its derivatives have been prepared. However the reports on their crystal polymorphs are limited to their complexes, and simple polymorphs of TCNQ derivatives are uncommon. We succeeded in preparation of a novel TCNQ derivative, 2,2'-(2-(prop-2-yn-1-yloxy)cyclohexa-2,5-diene-1,4-diylidene)dimalononitrile, having a propynyloxy group on a substituent. This compound was found to have two crystal polymorphs depending on a solvent for recrystallization. In polymorph I, dimeric hydrogen bonds are formed between acetylenic hydrogens and cyano nitrogens with the molecule in an inversion symmetry. While, in polymorph II, the molecules make intermolecular hydrogen bonds between acetylenic hydrogens and cyano nitrogens with the molecule in 21 symmetry, forming a hydrogen bonded molecular helix along the b axis. Besides patterns of the intermolecular hydrogen bonds, difference was recognized in conformation of propynyloxy group. The molecule has an anti conformation in polymorph I and a gauche conformation in polymorph II. DFT calculation indicates that the anti conformer is less stable than the gauche one. But a solvation model suggests the anti conformer is estimated to be more stable in a toluene solution.

Conformational effects, including some controversial examples, have been reported in this work for 2-substituted tetrahydropyrans and piperidines, and for the respective protonated compounds [substituent = F, OH, OCH3, NH2, NHCH3 and N(CH3)2]. Hyperconjugation plays a major role in most cases, either due to endo or exo-anomeric interactions, especially when nitrogen is the electron donor to an antiperiplanar σ* orbital. This interaction also seems to contribute for the Perlin and reverse fluorine Perlin-like effects, which are related to the relative magnitude of 1JC,H and 1JC,F coupling constants, respectively, in axial and equatorial conformers. However, the preference for an equatorial conformation appears when a hydrogen or methyl group of the substituent faces the ring, increasing the steric repulsion, even if concomitant with a hyperconjugative interaction in the axial isomer, such as for the well-known 2-aminotetrahydropyran. Unlike some reports in the literature, the gas phase 2-hydroxypiperidine experiences the herein called reverse anomeric effect, although the highly stabilizing nN → σ*C-O interaction in the axial isomer. Overall, steric effects should be taken into account when deciding for the normal or reverse anomeric effects as determinant factors of the stereochemical control of carbohydrate-like molecules.

The geometries of the two hydroxy groups in 1,2-ethanediol or 2,3-butanediols are more stable in a gauche orientation than those in an anti orientation. This has been generally explained in terms of the gauche effect, which is stabilization due to antiperiplanar electron delocalization between an antibonding orbital of the C–O bond (σCO*) and a bonding orbital of the C–H or C–C bond (σCH or σCC). However, a C–C single bond rotation simultaneously determines the geometries of the six vicinal bonds. Therefore, it is important to understand the effects on conformational stability of other interactions of the bond orbitals adjacent to the rotating C1–C2 bond. Bond model analysis revealed that antiperiplanar bond orbital interactions as a whole contribute to the higher stabilities of hydroxy/hydroxy gauche conformers, where the C–O/C–H or C–O/C–C combination including the σCO*/σCH or σCO*/σCC delocalization is not the dominant interaction stabilizing hydroxy/hydroxy gauche conformers. Rather, our results show ...

Protonation dependent base flipping in RNA has never been studied theoretically. In this work we studied protonation-dependent behavior of the base flipping in the catalytic triad of a single-stranded RNA which was previously characterized by NMR experiment. Molecular dynamics simulation reveals that the GA mismatch in this region accounts for this behavior. Free energy profiles show that the stable point for flipping dihedral shifts about 35° and the free energy barrier along the flipping pathway is elevated upon protonation. The orientation of Guanine from syn to anti conformation is coupled with protonation-dependent base flipping and G-HA + base pair is formed under acidic condition.

We describe the synthesis and coordination chemistry of the (1R,2S,5R)-menthyl-substituted N,N′-diaryl-α-diimine ligands N,N′-(2-Men-4-Me-Ph)2-BIAN (L1, Men = menthyl, BIAN = bis(imino)acenaphthene) and N,N′-(2-Men-4,6-Me2-Ph)2-BIAN (L2), the conformational properties of these ligands and their metal complexes, and the ethylene and 1-hexene polymerization behavior of the corresponding (α-diimine)Ni complexes. Free ligands L1 and L2 and square-planar (L1)PdCl2 and (L1,2)Ni(acac)+ complexes exhibit a preference for the syn conformation, in which the two menthyl units are located on the same side of the N═CC═N plane, while tetrahedral (L1,2)MX2 (MX2 = ZnCl2, NiBr2) complexes exhibit a preference for the anti conformation, in which the menthyl units are located on opposite sides of the N═CC═N plane. Both the anti and the syn conformers of [(L2)Ni(acac)][B(C6F5)4] can be activated by Et2AlCl to generate highly active ethylene polymerization catalysts (activity (2.5–6.6) × 106 g of PE/((mol of Ni) h) at 15 psi ...

Attempts to prepare [Se(CH2CH2COOH)3]+Cl− from Se(CH2CH2COOH)2 and H2C=CHCOOH in concentrated hydrochloricacid, for the corresponding sulfonium salt, led exclusively to the Se-betaine,Se(CH2CH2COOH)2(CH2CH2COO). The Se-betaine crystallises in the space group P2l/c with the cell dimensions at 223 K, a = 5.5717(1), b = 24.6358(4), c = 8.4361(1) A, β = 104.762(1)°, V = 1119.74(3) A3, Z = 4, Dcalc = 1.763 Mgm− 3,μ = 3.364 Mm−1. The structure refined to RI = 0.0223 for 2801 reflections with F0 > 4σ(F0). In the crystalline state the molecule is intermolecularly linked to neighbouring molecules by a number of hydrogen bonds; a very strong carboxylic-carboxylate bond with an O⋯O distance of 2.4435(16) A, a medium strong carboxylic-carboxylate bond with an O⋯O distance of 2.6431(16) A and several weak O⋯H(CH2) with O⋯C distances between 3.2 and 3.3 A. In the carboxylicgroup involved in the very strong hydrogen bond the O⋯H bond is antiperiplanar to the C O bond while the OH bond is periplanar to the C=O bond in the second carboxylic group. Based upon the C-O bond lengths and theelongation of the O-H bond involved in the strong hydrogen bond one may describe the compound as strongly linked units of Se(CH2CH2COOH)(CH2CH2COO)2 rather than Se(CH2CH2COOH)2(CH2CH2COO). The selenium atom forms two strong intramolecular 1,5-Se⋯O contacts, with a carboxylate oxygen atom, 2.9385(12) A, and with a carboxylicoxygen atom, 2.8979(11) A. To allow for these contacts the two organic fragments have been forced into the periplanar conformation. The molecule is only slightly asymmetric with regard to the C-Se-C bond angles but is very asymmetric with regard to the torsion angles.

Acyclic αγα-tripeptides derived from fluorinated-furanoid sugar amino acid frameworks act as reverse-turn inducers with a U-shaped conformation, whereas the corresponding nonfluorinated αγα-tripeptides show random peptide conformations. The NMR studies showed the presence of bifurcated weak intramolecular hydrogen bonding (F···HN) and N+···Fδ- charge-dipole attraction compel the amide carbonyl groups to orient antiperiplanar to the C–F bond, thus, demonstrating the role of the fluorine substituent in stabilizing the U-shaped conformation. The NOESY data indicate that the U-shaped tripeptides self-assembly formation is stabilized by the intermolecular hydrogen bonding between C═O···HN with antiparallel orientation. This fact is supported by ESI-MS data, which showed mass peaks up to the pentameric self-assembly, even in the gas phase. The morphological analysis by FE-SEM, on solid samples, showed arrangement of fibers into nanorods. The antiparallel self-assembled pore of the fluorinated tripeptides illust...

1,1″-1,4-Phenylene-bis(methylene)bis-4,4′-bipyridinium cation [C28H24N4]2+ (c), an organic receptor that generally crystallizes in its anti conformation, has recently been shown to be isolated in its syn conformation in an ion-paired compound [C28H24N4][Zn(dmit)2]·2DMF (dmit2– = 1,3-dithiole-2-thione-4,5-dithiolate; DMF = dimethylformamide). In this article, we demonstrated that the same receptor [C28H24N4]2+ (c) can also be stabilized in an unusual intermediate conformation (neither syn nor anti) with PF6– anion in compound [C28H24N4](PF6)2·(1,4-dioxane) (1·(1,4-dioxane)). The energetically favored anti conformation has been described in its nitrate salt [C28H24N4](NO3)2·2H2O (2·2H2O). Compounds 1·(1,4-dioxane) and 2·2H2O, crystallizing in triclinic and monoclinic systems with space groups P1 and P21/n, respectively, were additionally characterized by Hirshfeld surface analysis. The density functional theory calculations are performed to understand the internal mechanism of the stability of various conf...

Polymorphism of four biologically active 3-(4,5-diaryl-4H-1,2,4-triazole-3-yl)propenoic acid derivatives has been investigated. Traditional solution-based solid-state forms screening revealed three anhydrous forms of 3-[4-phenyl-5-(2-pyridyl)-4H-1,2,4-triazole-3-yl]propenoic acid. Noteworthy, two pairs of concomitant polymorphs were detected for this system. Two other compounds were found to be dimorphic. The molecular and crystal structures of all obtained crystal forms were established by single-crystal X-ray diffraction. The resulting crystal structures were analyzed in terms of molecular conformation, intermolecular interaction patterns, and crystal packing motifs. The experimental studies were supported by extended lattice and interaction energy calculations. It was found that the carboxylic group adopts the anti conformation in all studied forms and is involved in the intramolecular O–H···Ntriazole hydrogen bonding. In consequence, the association modes are dominated by the weak C–H···O, C–H···N hyd...

The coordination chemistry of mercury(II) halides, HgX2, X = Cl, Br, I, with N-methyl-α-(2-pyridyl)nitrone, L1, and N-t-butyl-α-(2-pyridyl)nitrone, L2, is reported. The structures of 1 : 1 complexes [HgX2L], X = Cl, L = L1; X = Br, L = L2, 2 : 1 complexes [(HgX2)2L], X = Br or I, L = L1; X = Cl or I, L = L2, and a unique compound [(HgBr2)5(L2)3] have been determined. In the 1 : 1 and 1 : 2 complexes, the ligand L1 adopts the anti conformation, and is either monodentate or bridging, while the ligand L2 adopts the syn conformation and acts as a chelate ligand. In the compound [(HgBr2)5(L2)3] the ligand L2 is present in both syn-chelate and anti-bridging bonding modes. Secondary intermolecular bonding, involving O⋯Hg or X⋯Hg interactions, can lead to association of the molecular compounds to form polymers of several kinds. In solution, the complexes are labile and the crystalline products do not necessarily reflect the reaction stoichiometry.

A new convenient synthetic procedure for the preparation of tri- and tetrafluoropropanamides derived from sugars has been described. The conformations of fluorinated amides calculated for isolated molecules and those in the crystal state have been compared. In contrast to single α-fluorine substituted amides, which preferred the anti conformation around the F–C–CO bond, for tetrafluorinated amides the additional trifluoromethyl group forces the conformation of the F–C–CO bond to be nearly syn.

We report substituent effects on conformational preferences and hole mobilities of 2,5-bis-(thiophen-2-yl)thieno[3,2-b]thiophenes (BTTT) monomer and dimer, and hexyl derivatives. We employ single-crystal X-ray diffraction, quantum mechanical calculations, and thin-film transistors to explore the difference between monomer, dimer, and effect of hexyl substitution. The hexyl-substituted molecules show marked differences in solid-state packing compared to the unsubstituted analogs. Most notably, the alkylated monomer crystal structure exhibits terminal thiophenes in the syn conformation. In contrast, the unsubstituted monomer adopts the more common anti conformation. The hexyl-substituted dimer, however, features a mixture of syn and anti thiophenes. Gas phase conformations of oligomers rationalize the intrinsic conformational preferences. We use a multimode simulation to compute hole mobilities and find excellent agreement with experiment. Theoretical results support our hypothesis that alkyl side chains cause these small molecules to adopt orientations that enhance hole mobilities by an order of magnitude upon hexyl substitution of the monomer.

To address the electronic effect in nucleophilic addition, we often use the Felkin-Anh model, in which the most s-electron-withdrawing group (sEWG) at the a-position of the carbonyl should be located anti to the nucleophile approach. The diastereoselectivity is often explained by the antiperiplanar effect of the sEWG. Reetz and Frenking reported that the FMO, the LUMO in the a-substituted ketone, has greater expansion anti the sEWG. Thus, the FMO theory can also apply to this diastereoselectivity. However, there is still no explanation for why the LUMO has greater expansion anti to the sEWG and toward the nucleophile approach with this conformation, as is suggested by the FMO theory. We show here that the phase-continuous cyclic orbital interaction among nNu:--p*C=O-s*C-A/vic-s*CC/gem- controls the diastereoselectivity, which is confirmed with theoretical calculations and the bond model analysis using a conformationally-fixed bicyclic molecule, 2,3-bis(trifluoromethyl)bicyclo[2.2.1]heptan-7-one 1, as a model.

Pyran-2-ones 3 undergo a novel Pd0 -catalyzed 1,3-rearrangement to afford isomers 6. The reaction proceeds via an η2 -Pd complex, the pyramidalization of which (confirmed by quantum chemistry calculations) offers a favorable antiperiplanar alignment of the Pd-C and allylic C-O bonds (C), thus allowing the formation of an η3 -Pd intermediate. Subsequent rotation and rate-limiting recombination with the carboxylate arm then gives isomeric pyran-2-ones 6. The calculated free energies reproduce the observed kinetics semi-quantitatively. ispartof: CHEMISTRY-A EUROPEAN JOURNAL vol:25 issue:34 pages:8053-8060 ispartof: location:Germany status: published

The authors are grateful to the Tunisian Ministry of High Education and Scientific Research for financial support (LR99ES14) of this research. Two new bidentate phosphine dichalcogenide ligands of the type MeN(Pip2PE)2 [(Pip = piperidinyl; E = S ( 1 ) or Se ( 2 )] and their cadmium(II) and mercury(II) chloride complexes MCl2{MeN(Pip2PE)2} [M = Cd; E = S ( 3 ); Se ( 4 ) or M = Hg; L = S ( 5 ); Se ( 6 )] were prepared and characterized by elemental analysis, IR, multinuclear (31P, 77Se and 113Cd) NMR spectroscopy and single crystal X-ray analyses. Ligands 1 and 2 were found to adopt anti conformation structures in the solid state in which both P=E groups are trans to each other. Complexes 3 - 6 are comprised of one ligand coordinated in a bidentate fashion to the metal centre in a distorted tetrahedral arrangement [98.46(3)-121.24(4)°]. The P=S bond lengths of 1.999(15)-2.003(14) Å in the complex ( 3 ) are slightly elongated compared to those in free ligand [1.933(15)-1.950(14) Å]. The results are discussed and compared with closely related analogues. Postprint

NOE-based analysis of the disaccharide β-Gal-(1 → 3)-β-Glc-OMe ( 1 ), especially a diagnostic Gal1–Glc4 NOE detected in a HSQC-NOESY spectrum, reveals the existence of the anti-ψ conformer in aqueous solution in addition to the major syn conformer. This result provides experimental proof of conformational flexibility around the aglyconic bond of β-(1 → 3) disaccharides, in contrast to previous studies that suggested that the flexibility around this linkage was restricted to the syn conformational region.

The diiodobinorsnoutane, bi(5-iodopentacyclo[4.3.0.0(2,4).0(3,8).0(5,7)]non-4-yl) (5), exists in a sterically hindered gauche conformation rather than an anti or an averaged (freely rotating) C2v structure. Density functional theory (DFT) predictions place the gauche conformation 11 kcal/mol more stable than the anti conformation with a barrier of 17 kcal/mol connecting the minima. These are consistent with variable-temperature NMR (17.1 ± 0.8 kcal/mol) estimates and X-ray analysis. Predictions of the torsional profiles of the yet-unsynthesized bromo-, chloro-, and fluoro- analogues show a progressive lowering of the barriers.

Electrophilic attack to a double bond is often observed anti to the most electron-donating σ-bond at the α-position (hereafter, we refer to this as the extended anomeric effect). This preference is believed to result from the antiperiplanar effect between the bond that is formed between the double bond and the electrophilic reagent, and the donating vicinal σ-bond which is located on the substituent at the α-position. From an orbital viewpoint, however, it is still unclear why the approach of the electrophile anti to the substituent results in stabilization or why the frontier molecular orbital (FMO) deforms, expanding toward the reagent with this antiperiplanar interaction. We demonstrate here that cyclic orbital interaction including geminal bond participation plays an important role in the diastereoselectivity in electrophilic addition. We examined our idea using the electrophilic addition of chlorine to 3-substituted propenes as a model reaction. Our bond model approach should contribute to a better understanding of orbital mixing in FMO.

A novel silver(I) complex sustained by a flexible dicarboxylic acid ligand, [Ag2(L)]n (1, H2L = 2,2′-{[1,2-phenylenebis(methylene)]-bis(sulfanediyl)}dibenzoic acid), has been synthesized and structurally characterized. In 1, four anti conformation L2− ligands bind three silver(I) ions to form a trinuclear subunit through S···Ag and Ag···Ag interactions, and then those trinuclear subunits are further extended to an infinite 1D silver(I) chain through the anti L2− ligands. Thus, the thermal stability, luminescent behavior and antibacterial activity of 1 were also discussed.

The bent bond/antiperiplanar (BBA) hypothesis has been applied to the analysis of [1,3]-sigmatropic alkyl shifts. These thermal rearrangements, for which there is evidence that they proceed through diradical intermediates, can be interpreted by considering their transient allyl radical structures. For the thermolysis of cyclic molecules, the preferred generation of pyramidal allyl radicals in staggered conformations is postulated on the basis of the BBA hypothesis. This accounts for the preference of suprafacial rearrangement pathways as well as the extent of inversion or retention of configuration at the migrating carbons.

Herein two solid state phenomena have been observed in two chalcones. Firstly, polymorphism has been found in (E)-1-(2-aminophenyl)-3-(3,4,5-trimethoxyphenyl)prop-2-en-1-one (1). This compound crystallizes with only one conformer rather than three in the known reported structure. In the polymorphs, the conformation of the phenyl ring bonded to carbonyl differs slightly. The intermolecular hydrogen bonding from NH2 is the main interaction responsible for polymorphism. Our polymorph is assembled only with weak N–H⋯π interactions instead of strong N–H⋯O and N–H⋯N ones observed in the known structure. DFT calculations reveal that the three conformers of the known polymorph deviate from the minimum energy conformation which is adopted in our crystal form to compensate for the absence of strong intermolecular contacts. Second, conformerism is reported for (E)-1-(3-hydroxyphenyl)-3-(4-nitrophenyl)prop-2-en-1-one (2). Three crystallographically independent molecules are found in the structure of 2: two of them are similarly planar with an anti conformation around the single bond between the carbonyl and α carbons while the third one is twisted, with its phenyl ring bonded to carbonyl rotated by ca. 60° besides presenting a syn conformation around the corresponding rotatable bond. Both conformational features are related to the crystal packing, allowing accommodation of twisted molecules onto the layers made up of hydrogen bonded planar molecules. Furthermore, our potential energy surface scans indicate that the planar and twisted conformations of the phenyl ring bonded to carbonyl are not compatible with the syn and anti conformations of the chalcone skeleton.

The synthesis of some novel non-symmetric liquid crystal dimers, {3-[4-(octyloxyphenyl)]-4,5-dihydroisoxazol-5-yl}alkyl 4-(decyloxy)benzoates (5a–d) and 4-{3-[4-(octyloxyphenyl)]-4,5-dihydroisoxazol-5-yl}alkyl 4-{[6-(octyloxy)naphthalen-2-yl]ethynyl}benzoate (9a–d), are reported. The liquid-crystalline properties, theoretical calculations based on the conformational aspects of the flexible alkyl spacer and X-ray experiments are discussed. The syntheses of the key intermediates, 2-{3-[4-(octyloxy)phenyl]-4,5-dihydroisoxazol-5-yl}alkanol (3a–d), presenting the flexible alkyl spacer were achieved through [3+2] cycloaddition reactions between nitrile oxides, which were generated in situ by oxidation of the respective aromatic oximes, and dipolarophile alkenols (CH2CH(CH2)nOH, n = 1, 2, 3, and 4). The benzoates 5a–d were synthesized through esterification of 3a–d and p-n-decyloxybenzoic acid (4). The esters 9a–d were synthesized through derivatization of isoxazolines 3a–d into 4-{3-[4-(octyloxyphenyl)]-4,5-dihydroisoxazol-5-yl}alkyl 4-bromobenzoate (7a–d) followed by a Sonogashira reaction with 2-ethynyl-6-octyloxynaphthalene (8). 5a and 5b showed a monotropic smectic C phase. 9a/c displayed a enantiotropic nematic (N) mesophase, whereas 9b/d showed a monotropic nematic mesophase. No mesophase was observed for 7a–d. An odd–even effect was observed for 5a–d and 9a–d associated with the crystal to isotropic phase transition and crystal to nematic phase, respectively, as the length of the spacer was increased from 1 to 4 carbon atoms. The transitional properties were higher for odd-numbered members (n = 1 and 3) for all of the series studied. The X-ray data of compounds 5a and 5b are in agreement with polarizing optical microscopy observations with the assignment of an SmC mesophase. Density functional theory calculations using the B3LYP hybrid functional with the level 6-311G(d,p) basis set were performed for molecules 5a–d to correlate the conformation of the flexible spacer and the transitional properties. The conformational analysis showed that the most stable conformation for 5a–d is one where all of the carbon atoms of the flexible spacer are orientated at 180° (antiperiplanar orientation) except for 5a because the spacer is too short. The odd-numbered members have a more bent shape and are less elongated molecules than the even-numbered members. Thus, mesomorphic behavior is dictated by the conformational constraint imposed by the flexible spacer on the mesogenic groups.

The stereoselectivity of nucleophilic addition on oxocarbenium ions derived from the bicyclic pyranoside model with or without a C2–OR group can be understood through the use of the bent-bond and the antiperiplanar hypothesis in conjunction with the concept of hyperconjugation as an alternative interpretive model of structure and reactivity.

3-Nitrobenzanthrone (3-NBA), an environmental mutagen found in diesel exhaust and a suspected carcinogen, undergoes metabolic reduction followed by reaction with DNA to form aminobenzanthrone (ABA) adducts, with the major alkylation product being N-(2'-deoxyguanosin-8-yl)-3-aminobenzanthrone (C8-dG-ABA). Site-specific synthesis of the C8-dG-ABA adduct in the oligodeoxynucleotide 5'-d(GTGCXTGTTTGT)-3':5'-d(ACAAACACGCAC)-3'; X = C8-dG-ABA adduct, including codons 272-275 of the p53 gene, has allowed for investigation into the structural and thermodynamic properties of this adduct. The conformation of the C8-dG-ABA adduct was determined using NMR spectroscopy and was refined using molecular dynamics (MD) calculations restrained by experimentally determined interproton distance restraints obtained from NOE experiments. The refined structure revealed that the C8-dG-ABA adduct formed a base-displaced intercalated conformation. The adducted guanine was shifted into the syn conformation about the glycosidic bond. The 5'- and 3'-neighboring base pairs remained intact. While this facilitated π-stacking interactions between the ABA moiety and neighboring bases, the thermal melting temperature (Tm) of the adduct-containing duplex showed a decrease of 11 °C as compared to the corresponding unmodified oligodeoxynucleotide duplex. Overall, in this sequence, the base-displaced intercalated conformation of the C8-dG-ABA lesion bears similarity to structures of other arylamine C8-dG adducts. However, in this sequence, the base-displaced intercalated conformation for the C8-dG-ABA adduct differs from the conformation of the N(2)-dG-ABA adduct reported by de los Santos and co-workers, in which it is oriented in the minor groove toward the 5' end of the duplex, with the modified guanine remaining in the anti conformation about the glyosidic torsion angle, and the complementary base remaining within the duplex. The results are discussed in relationship to differences between the C8-dG-ABA and N(2)-dG-ABA adducts with respect to susceptibility to nucleotide excision repair (NER).

In this manuscript the mol­ecular and supra­molecular structure of 2′,3,6′-tri­chloro­biphenyl-2-ylboronic acid tetra­hydro­furan monosolvate is presented, The title compound, C12H8BCl3O2·C4H8O, crystallizes as a tetra­hydro­furan monosolvate. The boronic acid group adopts a syn–anti conformation and is significantly twisted along the carbon–boron bond by 69.2 (1)°, due to considerable steric hindrance from the 2′,6′-di­chloro­phenyl group that is located ortho to the boronic acid substituent. The phenyl rings of the biphenyl are almost perpendicular to one another, with a dihedral angle of 87.9 (1)° between them. In the crystal, adjacent mol­ecules are linked via O—H⋯O inter­actions to form centrosymmetric dimers with R 2 2(8) motifs, which have recently been shown to be energetically very favourable. The hy­droxy groups are in an anti conformation and are also engaged in hydrogen-bonding inter­actions with the O atom of the tetra­hydro­furan solvent mol­ecule. Cl⋯Cl halogen-bonding inter­actions [Cl⋯Cl = 3.464 (1) Å] link neigbouring dimers into chains running along [010]. Further aggregation occurs due to an additional Cl⋯Cl halogen bond [Cl⋯Cl = 3.387 (1) Å].

Reactions of zinc(II) chloride with (S)-(−)-nicotine (Nic), cotinine, thionicotine and selenonicotine in 1:1 molar ratio yielded new metal complexes, which were characterized by elemental analysis, infrared, 13C and 1H NMR spectral methods. Single crystal X-ray diffraction studies of ZnCl2 complexes with thionicotine and selenonicotine revealed that the two complexes form isostructural crystals build of 1D coordination polymers (CP) and that coordination to metal centers alters the conformation of the parent organic ligand from syn to anti. Quantum chemical calculations indicate that in the isolated state the anti conformation of the uncoordinated chalcogen nicotine lactams is slightly more preferred energetically and it does not undergo substantial changes upon coordination to the metal center. The preference for the syn conformation in crystals of thio- and selenolactams of nicotine might be ascribed to their involvement in columnar stacking interactions, which do not operate in the isolated state and are also absent in crystals of their adducts with ZnCl2. This study has provided useful information regarding functionalization of (S)-(−)-nicotine molecule as a mixed-donor-atom ligand to enforce formation of 1D coordination polymers.

Diphenhydramine is an antihistamine used to treat some symptoms of allergies and the common cold. It is usually marketed as the hydrochloride salt, and both the neutral and cation forms have the O–C–C–N fragment. The gauche effect is well known in fluorine-containing chains, because its main origin is hyperconjugative and the σ ∗ C–F is a low-lying acceptor orbital, allowing electron delocalization in the conformation where F and an adjacent electronegative substituent in an ethane fragment are in the gauche orientation. Our experimental (NMR) and theoretical findings indicate that diphenhydramine exhibits the gauche effect, since the preferential conformations have the O–C–C–N moiety in this orientation due especially to antiperiplanar σ C–H → σ ∗ C–O and σ C–H → σ ∗ C–N interactions. This conformational preference is strengthened in the protonated form due to an incremental electrostatic gauche effect. Because the gauche conformation matches the bioactive structure of diphenhydramine complexed with histamine methyltransferase, it is suggested that intramolecular interactions, and not only induced fit, rule its bioactive form.

Hydrothermal reactions of 1,4-bis(imidazol-1-yl)benzene (bimb) and either 5-hydroxyisophthalic acid (HO-H2ip) or 5-methoxyisophthalic acid (MeO-H2ip), as well as transition metal salts, gave rise to six new coordination polymers: [Zn2(bimb)(H2O)(HO-ip)2]n (1), [Zn(bimb)(MeO-ip)]n·(H2O)n (2), [Mn2(bimb)2(HO-ip)2]n (3), [Mn2(bimb)2(H2O)2(MeO-ip)2]n (4), [Cu2(bimb)2(MeO-ip)2]n·(H2O)3n (5), and [Co(bimb)(H2O)(MeO-ip)]n·(DMF)0.5n·(H2O)n (6). Their structures have been defined by single-crystal X-ray diffraction analyses, IR spectra, elemental analyses and thermogravimetric analyses. Complex 1 is a 2D coordination network consisting of μ-oxygen-bridged dinuclear ZnII units, whereas 2 is an interdigitating 2D layer. ZnII ions in 1 and 2 have a distorted tetrahedral geometry, and two independent HO-ip and two bimb moieties in the dinuclear unit of 1 form π···π stacking interactions. Complexes 3 and 4 are 2D layers consisting of dinuclear MnII carboxylate units; bimb in an anti conformation serves as an exo-bidentate ligand in 3, whereas in 4 it acts as a terminal ligand, filling void space. Bis-monodentate MeO-ip in 5 connects CuII ions into a 3D coordination network, whereas MeO-ip in 6 bridges between two CoII ions to form a 2D layer, with the strong hydrogen bonds between water molecules of the adjacent 2D layers generating a 2D supramolecular bilayer. The dinuclear MnII units in 3 and 4 are well separated by organic ligands, and make up the basic magnetic units. Temperature-dependent magnetic susceptibility analyses show the existence of antiferromagnetic interaction in 3 and 4.

Bimolecular elimination reactions (E2) are fundamentally important processes in organic chemistry. Our current work focuses on a computational investigation of several interesting and unexpected experimental results previously obtained in our laboratory. In particular, we have examined the detailed mechanisms for generating CH2CHO− from the reaction of HO− + CH3CH2OCH2CH2OCH3, the unusually large isotope effect ( kH/ kD = 5.5) for the reaction of NH2− + CH3CH2OCH2CH3, and the possible kinetic barriers in the reaction of H− + CH3CH2OCH2CH3. Moreover, we have explored the high site selectivity in the reaction of NH2− + CH3CH2OC(CH3)3. In the HO− + CH3CH2OCH2CH2OCH3 reaction, three ion-neutral encounter complexes were located and fully optimized. The corresponding transition states were confirmed during the first E2 hydrogen-transfer process and they all possess E1cb-like antiperiplanar conformations. The formation of loosely bonded CH3O− and H2O moieties was found to be essential for the second E2-type hydrogen transfer, and an intriguing E1cb-like gauche transition state (CH3OH–C α–Cβ–OCHCH2 dihedral = 40.9°) was located, which results in the formation of ionic CH2CHO− and neutral CH3OH, H2O, and C2H4 products. The lowest kinetic barrier for the reaction of NH2− + CH3CH2OCH2CH3 is −5.3 kcal mol−1 (–22.2 kJ mol−1), which is 1.5 kcal mol−1 (6.3 kJ mol−1) higher in energy than the lowest barrier for the reaction HO− + CH3CH2OCH2CH3. The higher kinetic barrier of the NH2− + CH3CH2OCH2CH3 reaction is consistent with the observation of a larger isotope effect. The lowest kinetic barrier for the reaction of H− + CH3CH2OCH2CH3 is +5.4 kcal mol−1 (22.6 kJ mol−1), indicating that, although H− is a strong base, this reaction cannot occur at room temperature, which agrees well with the experimental results. The high selectivity in the formation of CH3CH2O− from the reaction of NH2− + CH3CH2OC(CH3)3 is explained by an electrostatic potential analysis of the ether molecule. Thus, this computational study provides important insight into the detailed mechanisms of elimination reactions.

N-(Substituted phen­yl)-4-oxo-4H-chromene-3-carboxamides have very similar conformations but show different inhibition activities against h-MAO-B so it may be assumed that the electronic environment provided by the substituents on the phenyl ring is the primary condition for the pharmacological activities displayed by these mol­ecules., Six N-substituted-phenyl 4-oxo-4H-chromene-3-carboxamides, namely N-(2-nitro­phen­yl)-4-oxo-4H-chromene-3-carboxamide, C16H10N2O5 (2b), N-(3-meth­oxy­phen­yl)-4-oxo-4H-chromene-3-carboxamide, C17H13NO4, (3a), N-(3-bromo­phen­yl)-4-oxo-4H-chromene-3-carboxamide, C16H10BrNO3, (3b), N-(4-methoxy­phen­yl)-4-oxo-4H-chromene-3-carboxamide, C17H13NO4, (4a), N-(4-methyl­phen­yl)-4-oxo-4H-chromene-3-carboxamide, C17H13NO3, (4d), and N-(4-hy­droxy­phen­yl)-4-oxo-4H-chromene-3-carboxamide, C16H11NO4, (4e), have been structurally characterized. All compounds exhibit an anti conformation with respect to the C—N rotamer of the amide and a trans-related conformation with the carbonyl groups of the chromone ring of the amide. These structures present an intra­molecular hydrogen-bonded network comprising an N—H⋯O hydrogen bond between the amide N atom and the O atom of the carbonyl group of the pyrone ring, forming an S(6) ring, and a weak Car—H⋯O hydrogen bond in which the carbonyl group of the amide acts as acceptor for the H atom of an ortho-C atom of the exocyclic phenyl ring, which results in another S(6) ring. The N—H⋯O intra­molecular hydrogen bond constrains the carboxamide moiety such that it is virtually coplanar with the chromone ring.

The title compound, C21H26FN3O7, is assembled by N—H...O and O—H...O hydrogen bonds into well-separated two-dimensional layers of about 15 Å thickness. The crescent conformation of the molecules is stabilized by weak intramolecular C—H...O and C—H...F hydrogen bonds. The uridine moiety adopts ananticonformation. The ribofuranose ring exists in an envelope conformation. All the endocyclic uracil bonds are shorter than normal single C—N and C—C bonds, and five of them have comparable lengths, which implies a considerable degree of delocalization of the electron density within this ring.

In the titled tricyclic orthocarbonate derivative, the three endocyclic C-O bonds are longer than the exo-cyclic C-O bond (similar to 4.40 angstrom vs. similar to 1.37 angstrom). This indicates an anomeric-type interaction between the two electron lone pairs on the exocyclic oxygen atom and the antibonding orbitals of the two antiperiplanar endocyclic C-O bonds. The remaining endocyclic C-O bond - marginally shorter than the other two apparently adds to this effect. Intriguingly, the antibonding orbital of the exocyclic C-O bond extends into the interior of the adamantyl cage, and is stereoelectronically prevented from overlapping with any of the six adjacent lone pairs. The results also seem to indicate a preference for interaction between a single donor oxygen atom and multiple acceptor antibonding orbitals rather than vice versa. The results add insightfully to the substantial body of evidence favouring the antiperiplanar lone pair hypothesis (ALPH). (C) 2014 Elsevier B.V. All rights reserved.

Two isomeric pyridine-substituted norbornenedicarboximide derivatives, namelyN-(pyridin-2-yl)-exo-norbornene-5,6-dicarboximide, (I), andN-(pyridin-3-yl)-exo-norbornene-5,6-dicarboximide, (II), both C14H12N2O4, have been crystallized and their structures unequivocally determined by single-crystal X-ray diffraction. The molecules consist of norbornene moieties fused to a dicarboximide ring substituted at the N atom by either pyridin-2-yl or pyridin-3-yl in ananticonfiguration with respect to the double bond, thus affordingexoisomers. In both compounds, the asymmetric unit consists of two independent molecules (Z′ = 2). In compound (I), the pyridine rings of the two independent molecules adopt different conformations,i.e. synandanti, with respect to the methylene bridge. The intermolecular contacts of (I) are dominated by C—H...O interactions. In contrast, in compound (II), the pyridine rings of both molecules have ananticonformation and the two independent molecules are linked by carbonyl–carbonyl interactions, as well as by C—H...O and C—H...N contacts.

The mechanism by which nucleophilic hydroxyls are attracted to activated glycopyranosyl donors is not known. Besides the intrinsic attraction of oxygen centred negative dipoles towards the developing electron deficiency at the anomeric carbon only a few suggestions have been given in the literature. By studying the effect on Density Functional Theory (DFT) modelled glycosylation reactions on the presence of polar additives as tested with acetonitrile two possible effects have been identified. One was noted in a previous publication (Carbohydr. Res. 2012, 356, 180–190) and two further examples discovered here that suggest that a lone pair of a nucleophile approaching a donor with a β-leaving group from the α-face can act as the antiperiplanar lone pair that assists leaving group departure. This interaction starts at just under a nucleophile C-1 separation of 3 A and has an incipient bond angle of O-5-C-1-Nuc(O or N) of very close to 90° which can be at C-1 with the p-type orbital at C-1–O-5 of the incipient oxacarbenium ion, that is, the LUMO of the activated donor. The 2nd interaction is less well studied and is suggested to be a similar bonding interaction which moves β-face nucleophiles to O-Nuc-C-1-leaving groups angles close to 180°.

The hexamethylenetetramine (HMT) framework displays interesting stereoelectronic interactions of the anomeric type. In the highly symmetrical parent system, the nitrogen centres act as both donors and acceptors. Protonation lowers symmetry and also leads to an enhancement of the anomeric interaction around the protonated centre. X-ray diffraction crystal structures of four derivatives of HMT - with succinic, (DL)-malic, phthalic and 4-hydroxybenzoic acids - reveal significant trends. (The first three form well-defined salts, 4-hydroxybenzoic acid forming a co-crystalline compound.) Each molecular structure is essentially characterised by a major anomeric interaction involving the protonated centre as acceptor. In two cases (succinic and 4-hydroxybenzoic), secondary protonation leads to a weaker anomeric interaction site that apparently competes with the dominant one. Bond length changes indicate that the anomeric interaction decreases as malic > phthalic > succinic > 4-hydroxybenzoic, which correlates with the degree of proton transfer to the nitrogen centre. Along with other bond length and angle changes, the results offer insight into the applicability of the antiperiplanar lone pair hypothesis (ALPH) in a rigid system. (C) 2014 Elsevier B.V. All rights reserved.

New tetramethoxy-(tetra-hydrazinecarboxamide) cyclophanes with unexpected anti /anti conformation were prepared from the addition of chiral di oxolane-based dicarbohydrazides to 4,4 ′-diisocyanato-3,3 ′-dimethoxy-1,1 ′-biphenyl in anhydrous THF. The molecular recognition of the macrocycles to a selection of chiral carboxylic gue sts, i.e. , L-(+)-tartaric, D-(-)-tartaric, D-(-)quinic, L-(-)-malic, D-(+)-galacturonic and D-glucuronic acids, was investigated by using direct injection electrospray ionization time-of-flight (E SI-TOF) and tandem mass spectrometry. A series of 1,3-dioxolane-4,5-dicarbonyl- bis -(N-substituted)-hydrazinecarboxamides, which mimic the backbone structure of the new cyclophanes, were also synthesized by addition of dioxolane dicarbohydrazides to aromatic isocyanates and their complexation with the aforementioned carboxylic guests was investigated by using direct inje ction ESI-TOF MS and MS/MS.

The Li, Rb and Cs complexes with the herbicide (2,4-dichlorophenoxy)acetic acid (2,4-D), namely poly[[aqua[μ3-(2,4-dichlorophenoxy)acetato-κ3O1:O1:O1′]lithium(I)] dihydrate], {[Li(C8H5Cl2O3)(H2O)]·2H2O}n, (I), poly[μ-aqua-bis[μ3-(2,4-dichlorophenoxy)acetato-κ4O1:O1′:O1′,Cl2]dirubidium(I)], [Rb2(C8H5Cl2O3)2(H2O)]n, (II), and poly[μ-aqua-bis[μ3-(2,4-dichlorophenoxy)acetato-κ5O1:O1′:O1′,O2,Cl2]dicaesium(I)], [Cs2(C8H5Cl2O3)2(H2O)]n, (III), respectively, have been determined and their two-dimensional polymeric structures are described. In (I), the slightly distorted tetrahedral LiO4coordination involves three carboxylate O-atom donors, of which two are bridging, and a monodentate aqua ligand, together with two water molecules of solvation. Conjoined six-membered ring systems generate a one-dimensional coordination polymeric chain which extends alongband interspecies water O—H...O hydrogen-bonding interactions give the overall two-dimensional layers which lie parallel to (001). In hemihydrate complex (II), the irregular octahedral RbO5Cl coordination about Rb+comprises a single bridging water molecule which lies on a twofold rotation axis, a bidentateOcarboxy,Cl-chelate interaction and three bridging carboxylate O-atom bonding interactions from the 2,4-D ligand. A two-dimensional coordination polymeric layer structure lying parallel to (100) is formed through a number of conjoined cyclic bridges, including a centrosymmetric four-membered Rb2O2ring system with an Rb...Rb separation of 4.3312 (5) Å. The coordinated water molecule forms intralayer aqua–carboxylate O—H...O hydrogen bonds. Complex (III) comprises two crystallographically independent (Z′ = 2) irregular CsO6Cl coordination centres, each comprising two O-atom donors (carboxylate and phenoxy) and a ring-substituted Cl-atom donor from the 2,4-D ligand species in a tridentate chelate mode, two O-atom donors from bridging carboxylate groups and one from a bridging water molecule. However, the two 2,4-D ligands are conformationally very dissimilar, with one phenoxyacetate side chain being synclinal and the other being antiperiplanar. The minimum Cs...Cs separation is 4.4463 (5) Å. Structure extension gives coordination polymeric layers which lie parallel to (001) and are stabilized by intralayer water–carboxylate O—H...O hydrogen bonds.

In four N-(4-halophen­yl)-4-oxo-4H-chromene-3-carboxamides, halo = -F, -Cl, -Br and -I, the mol­ecules are essentially planar and exhibit anti conformations with respect to the C—N rotamer of the amide and with cis geometries with respect to the relative positions of the C3arom—C2arom bond of the chromone ring and the carbonyl group of the amide., Four N-(4-halophen­yl)-4-oxo-4H-chromene-3-carboxamides (halo = F, Cl, Br and I), N-(4-fluoro­phen­yl)-4-oxo-4H-chromene-3-carboxamide, C16H10FNO3, N-(4-chloro­phen­yl)-4-oxo-4H-chromene-3-carboxamide, C16H10ClNO3, N-(4-bromo­phen­yl)-4-oxo-4H-chromene-3-carboxamide, C16H10BrNO3, N-(4-iodo­phen­yl)-4-oxo-4H-chromene-3-carboxamide, C16H10INO3, have been structurally characterized. The mol­ecules are essentially planar and each exhibits an anti conformation with respect to the C—N rotamer of the amide and a cis geometry with respect to the relative positions of the Carom—Carom bond of the chromone ring and the carbonyl group of the amide. The structures each exhibit an intra­molecular hydrogen-bonding network comprising an N—H⋯O hydrogen bond between the amide N atom and the O atom of the carbonyl group of the pyrone ring, forming an S(6) ring, and a weak Carom—H⋯O inter­action with the O atom of the carbonyl group of the amide as acceptor, which forms another S(6) ring. All four compounds have the same supra­molecular structure, consisting of R 2 2(13) rings that are propagated along the a-axis direction by unit translation. There is π–π stacking involving inversion-related mol­ecules in each structure.

Azine ligands show preference for the antiperiplanar conformation, but their 2:2 ligand-metal complexes can exhibit a central N4M2 core in the most stable chair arrangement, as in the case of the model lithium complex, with the ligand displaying a synclinal conformation that is not stable in the free ligand. According to DFT calculations, complexation of Cu2+ with bis(1-pyrenyl)azine (1) affords a C2-symmetric [1·Cu2+]2 species with a planar central N4Cu2 core exhibiting a weak cuprophilic interaction. The pendant pyrenyl substituents are brought close and parallel to each other, therefore accounting for the π-stacking that is responsible for excimer fluorescence. In case of Hg2+, complexation by the same ligand affords a C2-symmetric complex with an essentially linear N-Hg-C coordination environment at every metal center and without intermetallic interaction. T-Stacking interactions between pyrenyl groups explain the overall rigidity required for enhancement of fluorescence. Graphical abstract The structures of 2:2 complexes of Hg(II) and Cu(II) with bispyrenyl azine are reported.

The Friedel–Crafts-type reaction between properly functionalized inden-1-ones and 2-naphthols generates a hindered single bond which displays a unique preference for an antiperiplanar conformational diastereoisomer. The steric hindrance and the presence of an enantioenriched stereogenic center control the distribution of the two diastereomeric conformers at equilibrium and increase the energy for the rotation of the C(sp3)–C(sp2) single bond.

tn chelate rings in thecomplex adopted a stable anti conformation on the coor-dination plane not including syn conformer. However, thedetailed IR and electronic absorption spectral propertieshave not been published yet. In this study, the four elec-tronic bands due to spin-allowed transitions were assigned.Using the observed electronic transitions, a ligand fieldanalysis was performed to determine the metal-ligand bond-ing properties for the coordinated atoms toward chro-mium (III).

We performed X-ray structural analyses of the five- and six-layered [3.3]metacyclophanes (MCPs) 1 and 2 and the six-layered [3.3]MCP tetraone 3 . In the solid state, the MCP moieties of 1 , 2 , and 3 adopt different conformations from those of the free MCPs in solution. In the five-layered [3.3]MCP 1 , all the [3.3]MCP moieties adopt anti (chair/boat) conformations. In the six-layered [3.3]MCP 2 , two three-layered [3.3]MCPs are connected by a [3.3]MCP in the anti conformation with completely parallel benzene rings. In the six-layered [3.3]MCP tetraone 3 , the outer [3.3]MCP moieties and diones adopt general syn and anti geometries, respectively. However, the inner [3.3]MCP moiety adopts an anti geometry. Based on density functional theory (DFT) calculations, the most stable conformers of 1 , 2 , and 3 are syn (chair/chair) in the [3.3]MCP moieties and anti (twist boat/twist boat) in the dione moieties.

Stability, dynamics and function of nucleic acids are affected by nature of cations that are involved in interaction with specific functionalities. Introduction of inversion of polarity sites represents a very useful and chemically accessible backbone modification, which can alter the binding affinity of cations. NMR study on cation binding between G-quartets in tetramolecular G-quadruplex adopted by d(5′TGG3′–3′GGT5′) with 3′–3′ inversion of polarity sites in the middle of G-tract showed existence of two different G-quadruplex forms with all strands in parallel orientation, where all guanine residues adopt anti conformation around glycosidic bonds in the presence of 15 NH 4 + ions. In one of the forms all three binding sites are equally populated, while in the second form the binding site next to the inversion of polarity site is not fully populated by 15 NH 4 + ions.