S-trans Conformation Research Articles

Two additional 1,9-diarylnonanoid 3-glucosides from Erica cinerea among which an unusual α-dione in conformational equilibrium

Continuation of the phytochemical analysis of Erica cinerea fresh aerial parts resulted in the isolation of two new compounds with a very close chromatographic behavior. On the basis of spectroscopic evidence (UV, MS and NMR), the structures were established as (−)-(3S,6E)-1,9-bis(p-hydroxyphenyl)-3-hydroxynon-6-en-5-one 3-O-β-d-glucoside named 6,7-anhydroericaone 3-O-β-d-glucoside and (−)-(3S)-1,9-bis(p-hydroxyphenyl)-3-hydroxynonan-5,6-dione 3-O-β-d-glucoside named α-ericadione 3-O-β-d-glucoside. Unanticipated by its α-alkadione type structure —a very unusual feature in the plant kingdom— the latter metabolite was found as a mixture of the minor s-cis and the major s-trans conformers. Moreover, the GC–MS analysis of the aglycone moieties of such glucosides, pointed out the permanent absence of the molecular ion and the constant hydroxybenzylium base peak, as well as the conversion of the α-dione into the major α-keto-enol tautomer.

The monoepoxidation of acyclic, conjugated aliphatic dienes by dimethyldioxirane: kinetics

Abstract Kinetic data for the reaction of dimethyldioxirane (1) with a series of acyclic, aliphatic conjugated dienes 2a–f are reported. Monoepoxidation is the preferred process and the monoepoxides are formed in high yield as long as the dienes are in excess. The results show that epoxidation of the dienes is less sensitive to structural effects than that for simple alkenes. Furthermore, these monoepoxidations show little sensitivity to cis/trans substitution in marked contrast to those of simple aliphatic alkenes. The relative reactivities were modeled using a density function approach. Consistent with previous calculations on alkene systems, a concerted process with a spiro transition state was obtained for diene epoxidation regardless of the initial orientation of the reactants. The relative reactivities for monoepoxidation of 2c–e were calculated based on the weighted average of the calculated differences between Ecalc for reaction of the s-cis and s-trans conformer of each diene with 1. The predicted relative reactivities were in reasonable agreement with those from the kinetic data.

On the UV spectrum of cross-conjugated 2,3-diphenyl-1,3-butadiene

The UV spectrum of the cross-conjugated compound 2,3-diphenyl-1,3-butadiene, which can occur in both an s-gauche and s-trans conformation, is reported. The spectrum is interpreted with the use of ZINDO/S calculations and compared to the spectrum of styrene. In both conformations the difference in electronic transition energy with styrene is negligible, despite a shift in HOMO and LUMO energy. This is the consequence of the decrease of Coulomb and exchange energies by which extension of the π-system is accompanied. As such, the UV spectrum of 2,3-diphenylbutadiene, as compared to that of styrene, does not reflect extension of the π-system.

Gas-Phase Raman Spectra of s-trans- and s-gauche-1,3-Butadiene and Their Deuterated Isotopologues

The gas-phase Raman spectra of 1,3-butadiene and its 2,3-d(2),1,1,4,4-d(4) and d(6) isotopologues have been recorded using intense (6 W) green laser excitation and sensitive CCD detection. Hundreds of bands have been observed and assigned for each isotopologue. These spectra provide the best data to date for the s-trans conformer and also provide the first direct observation of the gas-phase Raman bands of the s-gauche conformer. Spectra recorded at elevated temperatures up to 250 °C for the d(0) and d(6) species help confirm the assignment of bands for the gauche rotamer. DFT computations were utilized to complement the studies. For the most part, the observed gas-phase gauche bands are in good agreement with previous matrix isolation studies. A best set of frequencies are reported for the fundamentals of the gauche rotamer of the d(0) and d(6) species.

Asymmetric induction in 8π electrocyclizations. Design of a removable chiral auxiliary.

The pseudo C(2) symmetric trans diphenyl oxazoline group acts as an effective chiral auxiliary in the 8π, 6π tandem electrocyclization of a substituted tetraene 1-carboxylic acid. Assignment of absolute stereochemistry to the [4.2.0] bicyclooctadiene product supports a model in which both s-cis and s-trans conformations favor the transition states with the same helical twist. This assignment prefaces the development of analogs of SNF4435 C and D. These natural products demonstrate activity as androgen receptor antagonists and as multidrug resistance (mdr) reversal agents.

Conformational Preferences of Modified Nucleoside N2-methylguanosine (m2G) and Its Derivative N2, N2-dimethylguanosine (m 2 2 G) Occur at 26th Position (Hinge Region) in tRNA

Conformational preferences of the modified nucleosides N(2)-methylguanosine (m(2)G) and N(2), N(2)-dimethylguanosine (m(2)(2)G) have been studied theoretically by using quantum chemical perturbative configuration interaction with localized orbitals (PCILO) method. Automated complete geometry optimization using semiempirical quantum chemical RM1, along with ab initio molecular orbital Hartree-Fock (HF-SCF), and density functional theory (DFT) calculations has also been made to compare the salient features. Single-point energy calculation studies have been made on various models of m(2)G26:C/A/U44 and m(2)(2)G26:C/A/U44. The glycosyl torsion angle prefers "syn" (χ = 286°) conformation for m(2)G and m(2)(2)G molecules. These conformations are stabilized by N(3)-HC2' and N(3)-HC3' by replacing weak interaction between O5'-HC(8). The N(2)-methyl substituent of (m(2)G26) prefers "proximal" or s-trans conformation. It may also prefer "distal" or s-cis conformation that allows base pairing with A/U44 instead of C at the hinge region. Thus, N(2)-methyl group of m(2)G may have energetically two stable s-trans m(2)G:C/A/U or s-cis m(2)G:A/U rotamers. This could be because of free rotations around C-N bond. Similarly, N(2), N(2)-dimethyl substituent of (m(2)(2)G) prefers "distal" conformation that may allow base pairing with A/U instead of C at 44th position. Such orientations of m(2)G and m(2)(2)G could play an important role in base-stacking interactions at the hinge region of tRNA during protein biosynthesis process.

Ab Initio Study of Conformational Properties of Heteroatom-Containing 1,2-Bisketene Analogues

Minimum-energy and transition-state geometries of 4-oxobuta-1,3-diene-1-thione, buta-1,3-diene-1,4-dithione, 4-selenoxobuta-1,3-diene-1-thione, 4-selenoxobuta-1,3-diene-1-one, and buta-1,3-diene-1,4-diselenone were calculated using HF, B3LYP, and MP2 levels of theory and 6–31 + G* basis set by rotation around the related ‒C‒C‒ single bonds. In all of the above-mentioned molecules, the s-trans conformation was obtained as the most stable conformer with the 180° dihedral angle. In buta-1,3-diene-1,4-dithione, 4-selenoxobuta-1,3-diene-1-thione, and buta-1,3-diene-1,4-diselenone, the s-cis form of these compounds corresponded to the other energy-minimum geometry. Their skew geometries, with torsional angles approximately 100°, were a transition state for conformational interconversion between the two global minima forms. In 4-oxobuta-1,3-diene-1-thione and 4-selenoxobuta-1,3-diene-1-one, geometries with the C˭C‒C˭C dihedral angles about 51 and 43° (respectively) were attributed to the second energy-minimum geometry. Transition-state structures from both molecules were found in the torsional angles at about 0 and 100°. Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file. GRAPHICAL ABSTRACT

Structure of Methyl Pyruvate and α-(1-Naphthyl)ethylamine on Pd(111)

The adsorption of methyl pyruvate and α-(1-naphthyl)ethylamine (NEA) on Pd(111) is explored using first-principles density functional theory (DFT) calculations. It is found that methyl pyruvate adsorbs in a flat-lying geometry with a heat of adsorption of ∼44 kJ/mol, in good agreement with the experimentally measured desorption activation energy of 41 ± 6 kJ/mol. For a perpendicular methyl pyruvate geometry, the s-cis conformer is the most stable, while previous experimental results suggest that it adsorbs in the s-trans conformer. Since the s-trans conformer is the most stable in the gas phase, this may be due to the formation of a metastable species on the surface. The calculated heat of adsorption (∼29 kJ/mol) is in good agreement with the experimental desorption activation energy (33 ± 4 kJ/mol). NEA adsorbs on Pd(111) predominantly through the naphthyl ring with a heat of adsorption of ∼224.3 kJ/mol in an exo conformation and 2.1 kJ/mol less stable in an endo conformation. Including effects of van de...

Effect of aromatic ring anisotropy on the 1H NMR shielding constants and conformational equilibrium of sterically strained aryl vinyl ethers

According to the 1H and 13C NMR data and quantum-chemical calculations, phenyl vinyl ether exists mainly in the s-trans conformation which is characterized by concurrent p-π* interaction of the oxygen atom with both unsaturated fragments. Introduction of two methyl groups into the ortho positions of the benzene ring forces the latter to go out from the vinyloxy group plane, leading to loss of p-π* conjugation with the aromatic ring, enhancement of p-π* conjugation with the vinyl group, and transition of the molecule to s-cis conformation. The 1H and 13C NMR data indicated that replacement of both o-methyl groups by tert-butyl makes the s-cis conformer sterically overcrowded even when the aromatic ring is oriented orthogonally with respect to the vinyl group; as a result, conformational equilibrium is displaced again toward s-trans rotamer.

Gas-phase reduction of cyclic and acyclic α,β-unsaturated ketones by hydrogen transfer on MgO. Effect of the ketone structure

The gas-phase hydrogen transfer reduction (HTR) of cyclic and acyclic α,β-unsaturated ketones to the corresponding unsaturated alcohols (UOL) using 2-propanol as hydrogen donor was studied on MgO as an alternative to the less selective conventional hydrogenation using high pressure H2. The HTR of 2-cyclohexenone and mesityl oxide were used as model reactions. The MgO activity and selectivity toward the unsaturated alcohol depended on the ketone chemical structure. Cyclic 2-cyclohexenone was in fact less reactive but more selective to UOL formation than acyclic mesityl oxide, yielding about 85% UOL (91% selectivity) at 573K. The rigid structure of 2-cyclohexenone enforces a s-trans conformation that favors selective reduction of the CO bond and thereby enhances the UOL formation. In contrast, the less rigid structure of the acyclic ketone affords the simultaneous reduction of both unsaturated bonds, CC and CO, forming also the saturated alcohol; as a consequence, maximum UOL yields of about 45% (47% selectivity) were obtained at 573K from HTR of mesityl oxide.The unsaturated ketone conversion pathways toward UOL and other compounds also depended on the ketone structure. UOL formed on MgO as a primary product from both reactants 2-cyclohexenone and mesityl oxide, via a cyclic six-membered intermediate according to the Meerwein–Ponndorf–Verley mechanism. However the saturated alcohol was produced by consecutive UOL reduction in 2-cyclohexenone reactions but directly from mesityl oxide reduction. Reduction of the CC bond toward the saturated ketone was negligible regardless of the reactant structure whereas competing reactions such as the CC bond shift were more likely to contribute during reduction of the acyclic reactant.

Microwave rotational spectrum and ab initio equilibrium structure of fumaric acid: Anharmonicity bridging the molecular characterizations

The rotational spectrum of fumaric acid was studied in a pulsed supersonic jet expansion using Fourier-transform microwave (FT-MW) spectroscopy. The ground-state rotational constants, centrifugal distortion constants and the electric dipole moment of the molecule were determined at high accuracy. Agreement of experimental values with those predicted by ab initio methods for the s-cis,s-trans conformer, one of the three plausible conformers in the gas-phase, has unambiguously confirmed the presence of this conformer in the cold jet. The semi-experimental equilibrium rotational constants B e ( i ) derived from the experimental values B 0 ( i ) and the rovibrational corrections from the MP2/cc-pVTZ cubic force field calculation are in excellent agreement with the high-level ab initio values. The ab initio geometries of low-energy conformers s-cis,s-cis, s-cis,s-trans, and s-trans,s-trans were optimized at the CCSD(T) level of theory. It was shown that the structural and spectroscopic parameters (i.e. bond lengths, rotational constants, and vibrational frequencies) from experimental and theoretical methods are compatible when anharmonic effects are taken into account.

Colorimetric “Naked-eye” Highly Selective Detection of Cu(II) Ion by a Simple Chemosensor: An Experimental and Theoretical Modeling

Abstract The selectivity and sensitivity of N,N′-bis(4-diethylaminosalicylidene)hydrazine (sensor 1) toward Cu2+ ion have been established both by distinct “naked-eye” color change as well as UV–vis spectrophotometry. Sensor 1 formed 1:1 complex with Cu2+ ion where the most stable s-trans form of the sensor is trapped in the high energy conformer. Single-crystal X-ray structure of sensor 1 supports its s-trans conformation. Theoretical calculations support well the experimental result.

Distinct conformational preferences of prolinol and prolinol ether enamines in solution revealed by NMR

Enamines, which are key intermediates in organocatalysis derived from aldehydes and prolinol or Jørgensen–Hayashi-type prolinol ether catalysts, were investigated conformationally in different solvents by means of NMR spectroscopy, in order to provide an experimental basis for a better understanding of the origin of stereoselection. For all of the enamines studied, surprisingly strong conformational preferences were observed. The enamines of the diarylprolinol (ether) catalysts were found to exclusively exist in the s-trans conformation due to the bulkiness of the pyrrolidine α-substituent. For prolinol enamines, however, a partial population of the s-cis conformation in solution was also evidenced for the first time. In addition, for all of the enamines studied, the pyrrolidine ring was found to adopt the down conformation. Concerning the exocyclic C–C bond, the sc-exo conformation, stabilized by CH/π interactions, is exclusively observed in the case of diarylprolinol ether enamines. In contrast, diarylprolinol enamines adopt the sc-endo conformation, allowing for an OH⋯N hydrogen bond and a CH/π interaction. A rapid screening approach for the different conformational enamine features is presented and this was applied to show their generality for various catalysts, aldehydes and solvents. Thus, by unexpectedly revealing the pronounced conformational preferences of prolinol and prolinol ether enamines in solution, our study provides the first experimental basis for discussing the previously controversial issues of s-cis/s-trans and sc-endo/sc-exo conformations. Moreover, our findings are in striking agreement with the experimental results from synthetic organic chemistry. They are therefore expected to also have a significant impact on future theoretical calculations and synthetic optimization of asymmetric prolinol (ether) enamine catalysis.

Origin of Exo/Endo Selectivity in the Intramolecular Diels-Alder Reaction

The stereoselectivity of the intramolecular Diels-Alder reactions of 1 and its derivatives were investigated by ab initio calculations. The stereoselectivity mainly originates from the steric repulsion and the orbital interactions. The additional s-cis and s-trans conformations by introducing the carbonyl group at the neighbor of diene or dienophile may change the stereoselectivity, hence this kind of substitution can be utilized for stereoselectivive asymmetric synthesis.

ChemInform Abstract: A General Stereospecific Synthesis of γ-Hydroxy α,β- Unsaturated Esters.

Abstract A stereospecific synthesis of optically active γ-hydroxy.α,β-unsaturated esters is achieved, involving successively the Sharpless epoxidation of allylic alcohols, oxidation to glycidic acids, conversion into α,β-epoxy diazomethyl ketones, and irradiation in ethanol at 300 nm. Intermediates in the photo-induced rearrangement are epoxy ketenes, which undergo ethanolysis with simultaneous opening of the epoxide, preferably via a transition state involving the s-trans conformation.

ChemInform Abstract: Electronic Absorption and Phosphorescence Spectra of 2-Acetylfuran. Conformations in the Ground and Excited Triplet States

The electronic absorption and phosphorescence spectra of 2-acetylfuran were measured in various media at room and low temperatures. It was found that 2-acetylfuran exists as an equilibrium mixture of the O–O s-cis and O–O s-trans conformers in polyethylene (PE) film. Here, the O–O s-trans conformer was considered to be a major species, and the O–O s-cis conformer a minor species in a nonpolar medium such as PE. On the other hand, in a polar medium such as poly(vinyl alcohol), 2-acetylfuran existed almost completely as the s-cis conformer. The 0–0 phosphorescence bands of the O–O trans- and cis-species were at 447 and 427 nm in PE, respectively. The S1 states for both conformers are nπ* in character, while the T1 states are π π*.

ChemInform Abstract: Steric Perturbation of the Conjugated Triene Chromophore. Conformational Analysis of (E)- and (Z)-3-Methyl-1,3,5-hexatriene and ( Z)-3-tert-Butyl-1,3,5-hexatriene

A theoretical and NMR-spectroscopic conformational analysis is presented of the 3-methyl-1,3,5-hexatrienes and of (Z)-3-tert-butyl-1,3,5-hexatriene. It is shown that (E)-3-methyl-1,3,5-hexatriene exists mainly as the tEt rotamer and (Z)-3-methyl-1,3,5-hexatriene as the tZt rotamer. Indications are found for steric strain involving the Me group of the E isomer. In contrast to earlier theoretical predictions, this strain does not lead to detectable amounts of s-cis rotamers. Ab-initio calculations reported here agree with this experimental finding, but MMP2 calculations underestimate the energy of the s-cis form. 1H NMR relaxation rates and Nuclear Overhauser Enhancements (NOE) are shown to allow a reliable determination of the predominant ground-state conformation of alkyl-substituted hexatrienes. NOE experiments on (Z)-3-tert-butyl-1,3,5-hexatriene reveal that a tert-Bu group changes the preferred conformation from tZt to cZt but, surprisingly, is unable to completely counteract the strong intrinsic preference for a spatially extended s-trans,s-trans conformation. We estimate the rotamer population as cZt/tZt = 80/20 (±5).

(E)-1,3-Bis(2,3,4,5,6-pentafluorophenyl)prop-2-en-1-one

In the title compound, C15H2F10O, the two perfluorinated arene rings are tilted at an angle of 66.08 (5)° with respect to each other. The olefinic double bond adopts an E configuration and the single bond between the olefinic and carbonyl double bonds has an s-trans conformation. The carbonyl group is not in a coplanar alignment with respect to the neighbouring arene ring (0.963 Å from aryl plane) while being coplanar with regard to the olefinic double bond (0.0805 Å from olefinic bond). The crystal packing does not feature significant hydrogen-bond-type or stacking inter­actions.

X-ray Crystal Structure and Time-Resolved Spectroscopy of the Blue Carotenoid Violerythrin

Violerythrin, a blue-colored carotenoid, has been investigated by X-ray crystallography and steady-state and ultrafast time-resolved absorption spectroscopy. The X-ray crystal structure of violerythrin shows that the molecule is nearly planar with the terminal rings positioned in the s-trans conformation. The steady-state and time-resolved spectroscopic data of violerythrin do not differ significantly from those of other carbonyl carotenoids with long (N > 10) pi-electron conjugated chains. This indicates that while the four carbonyl groups in violerythrin are critical for generating the bathochromic shift that leads to the blue color of the molecule, no dramatic changes attributable to a charge-transfer state known to affect the excited-state properties of carotenoids with short polyene chains occur. This may be due to the symmetric distribution of the carbonyl groups, which would preclude such an effect. The structural requirements for a blue, neutral, carotenoid are a planar, symmetric, cross-conjugated chromophore, containing at least 30 pi-electrons, a central polyene chain with 9 or 10 conjugated carbon-carbon double bonds connected at each end by an s-trans or trans bond to two identical, cyclic end groups, each possessing a conjugated keto group further cross-conjugated to another keto group, or a double bond in a quinoid type structure.

C [dbnd]N [sbnd]N [dbnd]C conformational isomers of 2′-hydroxyacetophenone azine: FTIR matrix isolation and DFT study

2′-hydroxyacetophenone azine (APA) has been studied by matrix isolation infrared spectroscopy and quantum chemical calculations. The DFT/B3LYP/6-311++G(2d,2p) calculations demonstrated the existence of two conformers for the lowest energy E/ E configuration of APA, a s- trans and a gauche ones. The conformers are characterized by similar energies and differ in the value of a C N N C angle, that was calculated to be 180° for a planar s-trans conformer and 155° for a non-planar gauche one . The calculated barrier for conformational interconversion is also very low, ca. 1 kJ mol −1 for the conversion from a gauche conformer to a trans one. The FTIR spectra of an argon matrix doped with APA from a vapour above solid sample evidence the presence of both conformers that exhibit reversible interconversion at matrix temperatures. The comparison of the theoretical spectra with the experimental ones and reversible temperature dependence of the experimental spectra allowed for unambiguous spectroscopic characterization of the trans and gauche conformers. The experiment also demonstrated that a gauche conformer is more stable than a trans one. The spectra analysis indicates that transformation from a trans conformer to a gauche one weakens the intramolecular O H⋯N bonds in the molecule.