Enantiomer

The aim of this article is to describe the results of our investigation on the chiral crystallization of anti-tricyclic ladder polysilane, anti-dodecaisopropyltricyclo [4.2.0.0(2,5)] octasilane (1). In solid state, the silicon framework of 1 adopts twisted conformation in which the Si4 rings are puckered in the same direction. Crystallization of 1 afforded a large conglomerate crystal in high yield. X-ray crystallography revealed that the crystal consists of either right-handed (P) or left-handed (M) molecules. The ultraviolet and circular dichroism spectra of the chiral crystals of 1 reveal two absorption bands in the 400-250 nm region.

Circular dichroism spectra of a series of 4-(tetra-hydroxy-tetryl-1-yl)2-phenyl-2H-1,2,3-triazoles were measured and the sign of their Cotton effects was correlated to the absolute configuration of the hydroxyl group alpha to the triazole base moiety. Those having the D-glycero-configuration in their Fischer projection formula show positive Cotton effect and those with the L-glycero-configuration show negative Cotton effect. This correlation was extended for the assignment of the anomeric configuration of the corresponding glycofuranosyl-C-nucleosides.

The absolute configuration of rubroflavin has been determined indirectly by comparison of the measured and the calculated CD spectrum of its thermal decomposition product 3-methanesulfinyl-5-methylmercaptophenol. Performing geometry optimizations at the HF/6-31+G* level we found fifteen local minima for the (R)-isomer of 3-methanesulfinyl-5-methylmercaptophenol. The CD spectrum of the compound was then obtained as a superposition of the Boltzmann-weighted spectra for each structure calculated with the non-empirical CIS method. The corresponding Boltzmann factors have been calculated employing the relative energies of these minima determined at the ZPE + MP2/6-31+G*//HF/6-31+G* level of ab initio theory. Comparing the signs of the observed and calculated longest wave length Cotton effect we assign an absolute configuration to the thermolysis product. Since additional calculations revealed that the tricoordinate sulfur atom in rubroflavin and in its decomposition product is configurationally stable under the conditions of thermolysis we conclude that the absolute configuation at the corresponding sulfur atom of rubroflavin is the same.

X-ray crystallographic and semiempirical PM3 and AM1 studies of 1,1'-dichlorobi-9H-fluoren-9-ylidene (5) are reported. The X-ray molecular structure of (Z)-5 indicated an approximately C2 symmetric conformation with pure twist around C9 = C9' of 40.4 degrees. The fjord regions are somewhat overcrowded: r(C8...C8') = 315.3 pm, r(Cl(1)...Cl(1') = 341.7 pm, r(C(8)...H(8')) = 259.0 pm. The four chlorine atoms of two neighboring molecules of (Z)-5 form a chain. The PM3 calculations showed that the global minimum of 5 is the C2 symmetric twisted conformation t(E)-5, which is 2.4 kJ/mol more stable than its diastereomer C2-t(Z)-5. The corresponding AM1 relative stability is reversed: C2-t(Z)-5 is 1.1 kJ/mol more stable than C2-t(E)-5. The pure twists of t(Z)-5 and t(E)-5 are 37.0 degrees and 37.2 degrees (PM3) and 40.5 degrees and 39.1 degrees (AM1). The corresponding (E) --> (Z) (PM3) and (Z) --> (E) (AM1) energy barriers of diastereomerization are 80.6 kJ/mol (PM3) and 75.8 kJ/mol (AM1). Two anti-folded local minima conformations C2-a(Z)-5 and C(i)-a(E)-5 were found to be 21.2 and 29.5 kJ/mol (PM3) and 25.8 and 35.2 kJ/mol (AM1) less stable than t(E)-5. The syn-folded conformations C(S)-s(Z)-5 and C2-s(E)-5 are transition states for the enantiomerization processes of C2-tz-5 and C2-tE-5, respectively, and lay 79.8 and 94.1 kJ/mol (PM3) and 108.3 and 107.4 kJ/mol (AM1) higher in energy than their corresponding twisted conformations. An alternative pathway for enantiomerization of C2-t(E)-5 via the anti-folded achiral intermediate C(i)-a(E) has a barrier of 56.0 kJ/mol (PM3) and 68.5 (AM1). An alternative pathway for enantiomerization of C2-t(Z)-5 via C2-t(E) and C(i)-a(E) has a barrier of 80.6 (PM3) and 75.8 (AM1) kJ/mol.

The benzamide chromophore is widely used as a Cottonogenic derivative of primary amines for stereochemical studies by circular dichroism. The assignments based on the exciton chirality method are reliable since the benzamide group has well-defined geometry and conformation. A recent report (J.D. Chisholm, J. Golik, B. Krishnan, J.A. Matson, D.L. Van Vranken, J. Am. Chem. Soc. 1999, 121: 3801-3802) claimed a caveat in the application of the exciton chirality method to benzamides derived from secondary amines. By the use of benzoyl derivatives of amino alcohols (1-4) and diamines (5, 6) of known absolute configuration we demonstrate that the 250-210 nm range exciton Cotton effects due to secondary and tertiary benzamides are generally of opposite sign. The origin of such disparity is traced to different conformational equilibria of the amide C-N bond in secondary and tertiary benzamides, as shown by semiempirical molecular modelling and NMR data. This feature can be useful in the determination of absolute configuration by analysis of the CD spectra due to exciton coupling of tertiary benzamides.

Ultraviolet absorption and Circular Dichroism spectra in the region 300-700 nm for highly regioregular samples of Poly[3[(S)-2-methylbutyl]-thiophene] are reported for solutions in good and poor solvents, at variable temperature, and for films in various conditions. In the latter case novel CD data up to the melting temperature provide some hints for the self-assembling process towards supramolecular organization. UV-visible and CD data are also reported on the related polymer Poly[3,4-di[(S)-2-methylbutyl]-thiophene].

The separation of [2R-[2alpha(R*),3alpha]]-5-[[2-[1-[3,5-bis-(trifluoromethyl)phenyl]ethoxy]-3(S)-4-fluorophenyl)4-morpholinyl]-methyl]-N,N-dimethyl-1H-1,2,3-triazole-4-methanamine hydrochloride from its enantiomer was achieved on an amylose tris-3,5-dimethylphenyl carbamate stationary phase. The retention of the enantiomers is dominated by weak hydrogen bonds while the enantioselectivity is governed by other kinds of interactions, e.g., inclusion in the amylose carbamate chains. Van't Hoffplots of 1nalpha vs. reciprocal temperature were non-linear and could be divided into two linear regions. One region at low temperature (5 degrees C- approximately 20 degrees C) and another one between 25 degrees C-70 degrees C with the change in slope occurring between 16 degrees C and 20 degrees C. DSC experiments suggested that the behavior can be attributed to breakage of H-bonds triggering a conformational change. Molecular simulation indicated a correlation between the interaction energies and the elution order obtained experimentally. The most retained enantiomer (R,R,S-enantiomer) interacts with the stationary phase through a hydrogen bond between the triazole proton and the C=O groups of the stationary phase, as well as through an inclusion in the cleft of the stationary phase. The other enantiomer exhibits a bifurcated H-bond between the triazolic proton and the C=O groups of the stationary phase leading to a less stable complex.

A high-performance photoelastic modulator (PEM) has been built. It consists of a home-made PEM head and a home-built PEM driver that is based on the principle of the phase-locked loop (PLL) (PLL PEM driver). It was installed in our new type circular dichroism (CD) spectrometer for visible light region and its performance was evaluated. With the phase angle being set exactly to zero, the PEM is found to be about eight times more efficient than commercially available one. The efficiency of PEM depends not only on the efficiency of PEM driver but also on the quality of PEM head. The latter can be estimated from admittance-parameter measurements without installing it into a spectrometer. With substituting the PLL PEM driver for a conventional one, the PEM drive voltage can be reduced from one-half to one-third and the baseline shift for air blank that is a measure of the total performance of polarization modulation spectrometers becomes quite small.

The aim of this article is to describe the results of our investigation on the chiral crystallization of anti-tricyclic ladder polysilane, anti-dodecaisopropyltricyclo [4.2.0.0(2,5)]octasilane (1). In solid state, the silicon framework of 1 adopts twisted conformation in which the Si-4 rings are puckered in the same direction. Crystallization of 1 afforded a large conglomerate crystal in high yield. X-ray crystallography revealed that the crystal consists of either right-handed (P) or left-handed (M) molecules. The ultraviolet and circular dichroism spectra of the chiral crystals of 1 reveal two absorption bands in the 400-250 nm region.

Using chiral HPLC and 13C NMR analyses, the optical purity of (+)-spiro[3.3]heptane-2,6-dicarboxylic acid (1) obtained by the known diastereomer method with brucine was first clarified to be 90% e.e., which was conventionally considered to be 100% e.e. Among the ester derivatives synthesized, dicinnamyl spiro[3.3]heptane-2,6-dicarboxylate (2) was found to show high optical separation ability on the chiral HPLC with cellulose phenyl carbamate stationary phase eluting with hexane/2-propanol (10/1, v/v) at a flow rate of 0.4 ml/min at 35 degrees C (separation factor, a, 1.14), and the isolated optically pure (+)- and (-)-2 show [alpha]D26 of + 1.84 degrees (c = 1.74, CHCl3) and -1.84 degrees (c = 1.74, CHCl3), respectively. Acidic hydrolysis of optically pure (+)-/(-)-2 without racemization yielded optically pure (+)-/(-)-1, exhibiting [phi]405(270 = + 21.1 degrees ([phi]D27 = +9.1 degrees) (c = 5.33, acetone) and [phi]405(27) = -21.1 degrees ([phi]D27 -9.1 degrees) (c = 5.32, acetone), respectively.

Rotamer population of S-tyrosinato and S-phenylalaninato ligands side groups in diastereomers of (1,2-diaminoethane)bis-(S-aminocarboxylato)cobalt(III) complexes is calculated by vicinal alpha and beta proton coupling constant analysis. The effect of noncovalent intra- and interligand interactions on the population of rotamers in D20 solution is discussed. It has been established that in all the complexes investigated the most abundant is rotamer t, in which aromatic voluminous moiety and carboxylic group are in an anti position. In almost all complexes the lowest content is of rotamer g, in which these two groups are in the nearest position. Relatively high population of rotamer h in complex 5 tyr, in spite of high steric hindrances, is due to intra- and interligand NH...pi interactions.

We review CD studies of a single-stranded DNA binding protein, gyp, of the Ff group of bacterial viruses. The CD spectrum of the gyp is dominated by a positive tyrosine L-a band at 229 rim, to which all five of the protein tyrosines contribute. The L-a band becomes much less positive upon binding of gyp to nucleic acids. CD spectra of mutant proteins identified a single tyrosine, Y34, that is largely responsible for this CD perturbation. At >250 nm, CD perturbations of nucleic acids can be monitored during gyp binding, and CD titrations have identified two distinct modes of binding of the gyp at physiological ionic strength (0.2 M NaCl). SELEX selection of sequences bound preferentially by gyp yielded a G-rich sequence that is closely related to telomere sequences and has CD properties of a G-tetraplex. CD spectroscopy showed that the presumed G-quadruplex form is maintained within saturated g5p. DNA complexes.

An improved methodology to prepare (S)2,3-O-cyclohexylideneglyceraldehyde is described. Starting from the commercially available (L)-cyclohexylidene protected ascorbic acid the enantiomerically pure aldehyde was synthesized in only two steps in 41% overall yield.

A novel device is described that permits fluorescence-detected circular dichroism (FDCD) detection in a conventional fluorescence spectrophotometer. FDCD is a phenomenon in which the emission intensity of a chiral analyte depends on the polarization state of the excitations. Although FDCD is a sensitive chiral detection technique, measurement of FDCD by using a special CD spectrometer has involved many difficulties. The proposed device consists of a polarizing prism and a retardation plate that fit on the excitation light side in the sample compartment of the fluorescence spectrophotometer. In this method, a FDCD wave superimposed on the fluorescence excitation spectrum of a chiral analyte was detected. We have also demonstrated the possibility of the enantiomeric purity determination of chiral analytes in real samples.

Carotenoids form structured self-assembly upon aqueous dilution of their organic solutions. In order to test the proposal predicting carotenoid aggregates to be organized in closely packed H-type (card-pack) manner by intermolecular hydrogen bonds, the trihydroxy derivative of capsanthin (1), (6'R)-capsanthol (2) ((all-E,3R,3'S,5'R,6'R)-beta,kappa-carotene-3,3',6'-triol) was acetylated to obtain all varieties of mono-, di- and triacetates and the corresponding supramolecules were studied by UV/Vis- and CD spectroscopy. It was verified that derivatives lacking hydroxyl functions at either of the end-groups form the loosely organized J-type (head-to-tail) aggregates. A model for the structure of the J-type self-assembly is proposed. Evidence was also obtained suggesting that close contacts of carotenoid molecules are not confined to hydrogen bonding.

The development of methodology appropriate for the rapid and precise assessment of enantiomeric purity is a critical need in the life sciences with impact in a number of areas including biomedical research, biotechnology, and pharmaceutical science. Real-time assessment of enantiomeric purity is critical to decisions related to possible product purity and/or the need for, and the type of additional processing. Recently, we have shown that laser-based polarimetric detection, in combination with ultraviolet detection, can be used to assess enantiomeric purity in real-time as an adjunct to the separation process. A mass-independent response function is obtained from the ratio of the normalized polarimetric signal relative to the normalized UV signal. This response ratio will be shown to be equivalent to the enantiomeric excess and independent of concentration and chromatographic resolution. The methodology will be evaluated as a function of injected mass, enantiomeric excess, chromatographic resolution, and peak asymmetry.

A new complex of diastereoisomeric pair, quinine and quinidine (QQd), was obtained from a mixture of saturated ethanol solutions of quinine and quinidine (0.5:1). The complex crystallises in the triclinic system, space group P1, and contains two molecules of quinine, two molecules of quinidine and four water molecules in the asymmetric unit. The X-ray structure analysis of a single crystal revealed that quinine and quinidine molecules occur in the so-called open conformation, characteristic for Cinchona alkaloids, whenever they are engaged in intermolecular hydrogen bonds. Quinine and quinidine molecules are organized in two very similar kinds of chains. In each chain the links that contain 14-membered rings can be distinguished. Within these rings quinine and quinidine molecules interact via intermolecular hydrogen bonds between the quinuclidine nitrogens and hydroxyl groups, mediated by water molecules. The links are connected with each other by hydrogen bonds between water molecules and nitrogens of the quinoline moieties, which interact via pi-pi stacking. The architecture of the hydrogen bond system in QQd, compared to those observed in the crystal structures of nonhydrated quinidine, cinchonine and cinchonidine, reveals the effect of the co-crystallizing water on the molecular packing. In nonhydrated alkaloid structures the hydrogen-bonded molecules form helical chains, different from those observed in the hydrated QQd complex and hydrated quinine toluene solvate (QTol). Comparison of QQd structure with that of QTol suggests that while the intermolecular hydrogen bonds in the system quinine-water-quinidine-water are very similar to those in quinine-water-quinine-water system, the mode of pi-pi interaction between their quinoline moieties depends on the absolute configuration of the interacting alkaloid molecules.

This work investigates the origins of enantioselectivity of polymers imprinted with the HIV protease inhibitor, Indinavir. For the preparation of imprints of the drug, the critical interactions between the functional monomer, methacrylic acid, and Indinavir were characterized by infrared (IR) spectroscopy to explore the optimum functional monomer concentration for the polymerization. It was shown that a polymer with high selectivity and minimum non-selective binding for Indinavir was obtained when prepared with enough functional monomer to hydrogen bond with all of the functional groups of the drug without using an excess of monomer. This observation is explained in terms of a balance that is achieved in the monomer-template equilibrium during the polymerization that yields a polymer with highly selective sites and minimal non-selective sites. This paper further demonstrates that IR spectroscopy can be a valuable tool in the design and syntheses of molecular imprinted polymers.

Racemic beta- and gamma-hydroxy sulfides were resolved by Humicola lanuginosa lipase catalyzed transesterification using vinyl acetate both as acyl donor and solvent. The effect of substituents and spacer length on rate of reaction and enantioselectivity is observed.

Two crystalline modifications of cinchonine cobalt complex, C19H23Cl3CoN2O, were obtained from mixture of saturated alcohol solutions of CoCl3 x 6H2O and cinchonine. The X-ray structure analysis revealed that the asymmetric unit of one modification, CoCn1, contains only zwitterionic molecules of the complex. In the asymmetric unit of the other, CoCn2, there are two molecules of the title compound and two molecules of ethanol. The influence of the absolute configuration, the CoCl3 coordination with quinoline, and the presence of alcohol molecules on the studied structures was established by comparison of the crystal and molecular structures of both cobalt complexes with the analogous quinine complex and zinc complex of cinchonine. The interactions that dominate in the packing of the molecules in both structures are intermolecular hydrogen bonds. They form characteristic ring systems, depending on the presence of the alcohol molecules. The ring features are also related to the absolute configuration of the alkaloid.