Showing papers on "Intramolecular force published in 1978"

Cryptates: inclusion complexes of macropolycyclic receptor molecules

Abstract: Molecular receptors use intermolecular interactions for the selective binding of substrates. Macropolycyclic molecules containing appropriate binding sites and cavities of suitable size and shape, may be designed so as to display molecular recognition in the formation of selective inclusion complexes, cryptates, with metal cations, anions and molecules. Macrocyclic receptors which form stable and selective complexes with primary ammonium and guanidinium groups are discussed; they display central and lateral discrimination. Enhanced rates of intramolecular thiolysis and hydrogen transfer have been observed when suitable reactive groups are attached to the receptor. Macrobicyclic ligands form very stable and selective cryptates with alkali and alkaline-earth cations; they may be modified so as to selectively complex toxic heavy metal cations. Binuclear cryptates of two types have been synthesized: macrobicyclic complexes of an ellipsoidal Bis-Tren ligand and cylindrical macrotricyclic complexes. They display interesting properties (like cation-cation interactions, copper protein type spectral parameters etc.) and are suitable for formation of “cascade complexes” by interaction of substrates with the bound cations. Spherical macrotricyclic receptors form cryptates with cations, anions and small inorganic species. They display tetrahedral recognition and may be considered as topologically optimal receptors for the ammonium ion, the water molecule, the halide ions, with which they form cryptates where the substrate is held in the intramolecular cavity by a tetrahedral array of hydrogen bonds. Finally, the macrobicyclic Bis-Tren system in its protonated form, complexes triatomic species like the azide anion. It represents a further step in the design of abiotic molecular receptors for polyatomic molecules or ions. The main lines of further developments in the chemistry of macropolycycles comprise the design of receptors for other important groups (carboxylate, phosphate), of polynuclear complexes and cascade complexes of potential use in polynuclear catalysis, of molecular catalysts as enzyme models and new chemical reagents.

Intramolecular Ene Reactions in Organic Synthesis

Abstract: Thermal cyclizations of appropriate dienes, enynes and related unsaturated systems, some of them carried out on an industrial scale, demonstrate increasingly the preparative power of the intramolecular ene reaction. A variety of substituted, fused and bridged ring systems, including natural products, are thus easily accessible in a regio- and stereo-selective manner. Numerous examples are discussed systematically illustrating the possibilities, limitations, and common features of this cyclization reaction and its reverse ring-opening process.

Cyclodextrin chemistry. Selective modification of all primary hydroxyl groups of α- and β-cyclodextrins

Abstract: Two efficient methods are described for the selective modification of all six primary hydroxyl groups of α-cyclodextrin (α-CD, 11). One, using an indirect strategy, involves protection of all 18 hydroxyl functions as benzoate esters, followed by selective deprotection of the six primary alcohol groups. The other, using a direct strategy, involves selective activation of the primary hydroxyl groups via a bulky triphenylphosphonium salt, which is then substituted by azide anion as the reaction proceeds. A number of modified α-cyclodextrin derivatives have been prepared and fully characterized, among which are: the useful intermediate α-cyclodextrin-dodeca (2, 3) benzoate (3); hexakis (6-amino-6-deoxy)-α-cyclodextrin hexahydrochloride (7); hexakis (6-amino-6-deoxy)-dodeca (2, 3)-O-methyl-α-cyclodextrin hexahydrochloride (9), hexa (6)-O-methyl-α-cyclodextrin (13). The direct substitution is shown to be even more efficient for β-cyclodextrin (16), giving the heptakis (6-azido-6-deoxy)-β-CD-tetradeca (2, 3)acetate (17), while the indirect strategy fails. The compounds are characterized by extensive use of 13C- and 1H-NMR. spectroscopy. The steric and statistical problems of selective polysubstitution reactions for the cyclodextrins are discussed, and possible reasons for the observed differences in reactivity between α- and β-cyclodextrins are examined. The dodecabenzoate 3 presents a very marked solvent effect on physical properties (IR. and NMR. spectra, optical rotation); the effects observed may be ascribed to an unusually strong intramolecular network of hydrogen bonds which severely distorts the α-cyclodextrin ring and lowers the symmetry from six-fold to three-fold.

The structure of bilirubin

Abstract: Crystals of bilirubin are triclinic, space group P1 with cell dimensions a = 19439, b = 11707, c = 15500 A, †, α = 9719, β =10022, γ = 11820°, Z = 4 The structure was solved by direct methods incorporating information derived from reflexion quartets and by using a set of diffractometer data containing 1408 reflexions with F obs ≥ 3σ( F obs ) The structure was refined with bond-length constraints to a final R of 0125 by using a set of microdensitometer-measured photographic data containing 1323 unique observed reflexions Both data sets were collected from a matured crystal whose diffraction pattern had improved considerably during a period of several weeks Bilirubin is shown to have the Z configuration at both the C5 and C15 bridges The two independent molecules of bilirubin in the crystal structure have very similar ‘ridge tile’ conformations: rings (A + B) and rings (C + D) form two good planes with an interplanar angle of approximately 97° and the molecule is stabilized by six intramolecular hydrogen bonds There is no evidence for any intermolecular hydrogen bonding Observed bond lengths suggest that bilirubin can be regarded as a 2,29 - dipyrrolylmethane with conjugating substituents at the α positions When freshly prepared, most crystals of bilirubin have reflexions hkl ( h odd) diffuse: this observation is discussed in terms of the molecular conformation of bilirubin and the particular packing arrangement which these molecules adopt in the crystal structure

Statistical mechanics of small chain molecules in liquids. I. Effects of liquid packing on conformational structures

Abstract: When a chain molecule can be viewed as a collection of overlapping spherical groups, the effect of a solvent on the conformational structure of the chain molecule is described by the distribution function for the cavity particles associated with the spherical groups. This article discusses the calculation of the cavitydistribution function for n‐butane disolved in various apolar solvents: the liquidscarbon tetrachloride, n‐butane, and n‐hexane. We consider the common picture where the CH3 and CH2 groups in n‐butane are simple spheres. For that model, the cavitydistribution function is a four‐point correlation function. We find that the superposition approximation for the four‐point function, while qualitatively correct, overestimates the effects of the solvent. An alternative scheme, which is called the two cavitymodel, yields results that agree quantitatively with a computer simulation study of liquidn‐butane. We find that a solvent medium produces significant shifts in the conformational equilibrium of n‐butane from that found in the gas phase. This phenomenon is the result of the nature of the local packing of solvent molecules neighboring the solute species under investigation. The conformational equilibrium is sensitive to this packing. The bulk packing fractions (molecular density times the volume of the molecule) of the liquids CCl4 and C4H10 are nearly identical. Even so there are noticeable differences between the intramolecular structure of n‐butane in liquidcarbon tetrachloride and in the neat liquid. Previous ideas on conformational equilibria have ignored the importance of steric (i.e., liquid packing) effects, and have assumed that solvent shifts in conformational structures can be attributed entirely to dielectric effects. Our calculations show that this assumption is wrong. The n‐butane molecule contains no significantly polar groups. yet solvent media produce substantial shifts. For example, the t r a n s–g a u c h e equilibrium constant, x g /x t , for n‐butane in the gas phase at room temperature is roughly 0.5, while we find it is about 1.0 when n‐butane is dissolved in liquid CCl4 at the same temperature and 1 atm pressure. We discuss why the phenomenon has been overlooked, and suggest experiments to document its existence.

Molecular pair-correlation function of liquid acetonitrile from X-ray and neutron-diffraction studies

Abstract: X-ray-diffraction studies on liquid acetonitrile at 20°C were carried out at a wavelength of 0·7 A. The data were corrected for polarization, absorption, Compton scattering and multiple scattering. The coherent distinct differential cross section was separated into intramolecular and intermolecular contributions. Whereas from the intramolecular coherent cross section the structure of the molecule in the liquid was determined, the intermolecular coherent cross section, together with intermolecular contributions from neutron-scattering data on CD3C14N and CD3C15N, was employed to determine three expansion coefficients of the molecular pair-correlation function.

An Enantioselective Total Synthesis of Natural (+)‐Luciduline

Abstract: The naturally occurring Lycopodium alkaloid (+)-luciduline (1) has been synthesized from (R)-5-methyl-cyclohex-2-en-1-one (12) by a sequence of seven steps (s. Scheme 6) in 33% overall yield. The key step 46 presumably involves a transient nitrone 5 which undergoes a highly regioselective intramolecular addition to a non-polarized olefinic bond.

A theoretical study of the OH stretching region of the vibrational spectrum of ice Ih

Abstract: We describe a theoretical interpretation of the OH stretching region of the vibrational spectrum of ice Ih derived from a study of extended models. The principle conclusions are the following: (i) The breadth and distribution of vibrational modes, and of intensity in the Raman and infrared spectra, are dominated by the influence of strong intermolecular coupling. The intermolecular coupling is comparable in strength to the intramolecular coupling. Long range couplings between molecules not directly hydrogen bonded mainly serve to make the features of the spectrum more diffuse than they would otherwise be, but have no great influence on the spectral distribution. (ii) The intramolecular OH stretch‐OH stretch coupling in ice Ih is of opposite sign to that in the gas phase. (iii) The vibrational modes of ice Ih are found to be complex mixtures of molecular motions, so identification of regions of the Raman or infrared spectra with particular isolated molecule modes is not useful. Rather good agreement is obtained between the Raman and infrared spectra predicted and those observed. Some deficiencies of the model, associated with residual discrepancies between theory and experiment, are discussed.

Review of vibrational data and force field constants for polyethylene

Abstract: The results of a critical review of vibrational data, their assignments, and force field constants of polyethylene and the related homologous series of n‐alkanes are presented. The vibrational frequencies derived from Raman spectroscopy, infrared spectroscopy, and neutron inelastic scattering were collected from the literature. We have reviewed the vibrational band assignments starting from the comprehensive treatment of the n‐alkanes by Schachtschneider and Snyder [1,2]1 and including subsequent reassignments. Theoretical calculations of the vibrational frequencies were reviewed with emphasis on the various models used for molecular structure and force fields. Lattice dynamical calculations of polyethylene were performed using a valence force field for intramolecular interactions and a force field derived from a nonbonded atom‐atom potential function for intermolecular interactions. The molecular and lattice structural parameters were taken from x‐ray and neutron diffraction studies of polyethylene and s...

13C and proton nuclear magnetic resonance studies of bradykinin and selected peptide fragments

Abstract: Complete /sup 13/C chemical shift assignments of the nonapeptide bradykinin have been made on the basis of pH titration studies and the examination of peptide fragments. Hysteresis effects previously reported for the /sup 13/C shifts of the Arg/sup 9/ resonances as a function of pH and interpreted to reflect an intramolecular salt bridge involving the Arg/sup 1/ guanido and the Arg/sup 9/ carboxyl (Ivanov, V. T., et al. (1975) Bioorg. Khim. 1, 1241; Proc. 4th Am. Pept. Symp., 151) were not observed in the present study. Chemical shifts of the proline carbons indicate that the trans configuration about the three X-Pro peptide bonds is strongly favored in all cases, although the minor cis resonances accounting for approximately 10% of the proline intensity can also be observed. In addition, there appears to be no significant pH sensitivity of the cis reversible trans equilibria. Studies of the solvent dependence (water ..-->.. methanol) of the carbonyl shifts are consistent with the possibility of an intramolecular hydrogen bond involving the Ser/sup 6/ carbonyl oxygen. Preliminary /sup 1/H nuclear magnetic resonance studies of the amide proton region of bradykinin and the C-terminal tetrapeptide Ser-Pro-Phe-Arg are also reported. In the latter peptide, the Phe/sup 8/ andmore » Arg/sup 9/ amide proton resonances appear as two sets of resonances corresponding to the cis and trans configurations of the Ser-Pro bond. The temperature dependenceof the proton shifts of the amide resonances is independent of whether the Ser-Pro bond is cis or trans, indicating that any intramolecular hydrogen bonding existing only for the trans peptide is insufficient to significantly reduce this parameter. Measurements of the /sup 13/C spin-lattice relaxation times indicate rapid internal motion for all of the peptide side chains, supporting the interpretation that bradykinin exists in solution primarily in a disordered state.« less

Critical Review of Vibrational Data and Force Field Constants for Polyethylene

Abstract: : The results of a critical review of vibrational data, their assignments, and force field constants of polyethylene and the related homologous series of n-alkanes are presented. The vibrational frequencies derived from Raman spectroscopy, infrared spectroscopy, and neutron inelastic scattering were collected from the literature. We have reviewed the vibrational band assignments starting from the comprehensive treatment of the n-alkanes by Schachtschneider and Snyder and including subsequent reassignments. Theoretical calculations of the vibrational frequencies were reviewed with emphasis on the various models used for molecular structure and force fields. Lattice dynamical calculations of polyethylene were performed using a valence force field for intramolecular interactions and a force field derived from a nonbonded atom-atom potential function for intermolecular interactions. The molecular and lattice structural parameters were taken from x-ray and neutron diffraction studies of polyethylene and selected n-alkanes. A refinement procedure was carried out by the method of least squares on intramolecular force field constants and on parameters of a phenomenological nonbonded atom-atom potential energy function. The resulting force field constants and associated standard deviations are presented. (Author)

Statistical mechanics of small chain molecules in liquids. II. Intermolecular pair correlations for liquid n‐butane

Abstract: In this article the intermolecular structure of liquid n‐butane is studied with an application of the RISM integral equation. It is found that the pair correlations between atoms in different n‐butane molecules do depend upon the nature of the intramolecular trans–gauche conformational equilibrium. An incorrect picture of the liquid structure is obtained if it is assumed that the fluid is composed only of trans conformers—the species most favored energetically. The local packing of CH3 and CH2 groups in the liquid is discussed. Particular attention is focused on the prediction of the Chandler–Pratt theory, which says that steric effects produce a solvent shift in conformational equilibria in liquid n‐butane. It is shown that x‐ray diffraction experiments can provide a direct probe of this phenomenon.