Relative stability of 5-methyl- and 3,5-dimethyltetrahydro-1,3-oxazine conformers

According to HF and DFT quantum chemical calculations and ¹H NMR data, the chair–chair conformational equilibrium of 5-methyltetrahydro-1,3-oxazine shifts toward the conformer with equatorial orientation of the methyl group on C⁵ and axial orientation of the NH proton. 3,5-Dimethyltetrahydro-1,3-oxazine exists preferentially as chair conformer with equatorial orientation of the 5-methyl group; orientation of the N-methyl group almost does not affect the conformational equilibrium.     

Conformational analysis of 1,3-oxathiane

Computer simulation of pathways of conformational isomerization of 1,3-oxathiane molecule carried out with the help of HF/6-31G(d), MP2.6-31G(d)/HF/6-31G(d), and PNE/3z quantum-chemical approximations showed that interconversion between the degenerate in energy chair conformers proceeds through seven independent pathways: directly and via six flexible forms. Potential energy surface contains eight minimum points including chair conformers and enantiomeric pairs of twist forms, and also five transition states, among them different modification of semi-chair, symmetric and unmmetrical boat. Molecular dynamics methods show that flexible forms at room temperature convert into one another and into the chair conformers.     

Conformational analysis of 2,2-dimethyl-5-alkyl-1,3-dioxa-2-silacyclohexanes

Study of conformational isomerization of 2,2-dimethyl-5-alkyl-1,3-dioxa-2-silacyclohexanes using quantum-chemical HF/6-31G(d) and PBE/3z approximations showed that its route involves an equilibrium between the chair conformers with different orientation of substituent at C⁵ ring atom and proceeds through a transition state corresponding to the 2,5-twist conformation. Molecular dynamics method showed that at room temperature this conformation transforms into the equatorial or axial chair conformers through 1,4-twist or sofa forms. Based on the experimental and theoretical values of vicinal ¹H NMR coupling constants we determined quantitative conformational composition of the molecules of these compounds and the values of ??G ⁰ of the conformational equilibrium.     

Conformational analysis of 5-bromo-5-nitro-1,3-dioxane

Conformational analysis of 5-bromo-5-nitro-1,3-dioxane has been performed by computer simulation in terms of HF/pVDZ, hybrid DFT PBE/3ξ, and RI-MP2/λ2 quantum chemical methods. The global minimum on the potential energy surface corresponds to the chair conformer with axial nitro group, and the other two minima have been identified as chair conformer with equatorial nitro group and 2,5-twist. All transition states on the route of conformational transformations of the title molecule have been revealed.     

Synthesis and Structure of 1,9-Dinitro-5-oxa-11-azatricyclo[6.4.0.04,9]dodecan-2-one

A series of 6,11-disubstituted 1,9-dinitro-5-oxa-11-azatricyclo[6.4.0.0^4,9 ]dodecan-2-ones were prepared from anionic adducts of 2,4-dinitrophenol with propanone and 2-phenylethanone carbanions by successive selective reduction with sodium borohydride and aminomethylation with formaldehyde and primary amines. By spectral methods and by quantum-chemical calculations following PM3 method the structure of the molecules synthesized was shown to contain all the three rings in a chair form with equatorial substituents in positions 6 and 11.     

Thiophosphorylated derivatives of meta- and ortho-hydroxybenzaldehydes in one-step syntheses of novel calix[4]resorcinols

Thiophosphorylated derivatives of meta- and ortho-hydroxybenzaldehydes take part in one-step, acid-catalyzed condensations with resorcinol and its derivatives. As a result, new calix[4]resorcinols, which contain four 2-thioxo-1,3,2-dioxaphosphorinane fragments on the aromatic substituents of the calixarene matrix, have been synthesized. In this case, macrocyclic products are formed as rccc- (cone or boat conformation) and/or rctt- (chair conformation) diastereomers, the ratio of which depends on the structure of the starting reagents. The obtained products have been isolated and their structures determined by NMR methods and single-crystal X-ray diffraction. The spectral differences of conformational isomers of the prepared macrocycles have been demonstrated.     

Stereochemistry of seven-membered heterocycles: XLVI. Synthesis and dynamic <Superscript>13</Superscript>C NMR spectroscopy of spiro[cyclohexane-1,4′-[3,5]dioxabicyclo[5.1.0]octanes]. DFT calculations of structurally related formaldehyde and acetone acetals

Spiro[cyclohexane-1,4′-[3,5]dioxabicyclo[5.1.0]octanes] were synthesized, and their conformational behavior was studied by dynamic ¹³C NMR spectroscopy. Anancomeric displacement of conformational equilibrium toward two nonequivalent twist conformers with close energies was revealed. The relative Gibbs energies ΔG ^o and enthalpies of formation ΔH ^o of twist and chair-like conformers with endo and exo orientation of the three-membered ring of structurally related formaldehyde and acetone acetals were calculated in terms of the density functional theory at the B3LYP/6-31G(d,p) level. Like spiro-cyclohexane analogs, they were shown to have a non-chair conformation.     

Structure and Conformational Analysis of 5,5-Bis(bromomethyl)-2-(4-methoxyphenyl)-1,3-dioxane

The structure of 5,5-bis(bromomethyl)-2-(4-methoxyphenyl)-1,3-dioxane has been studied by ¹H and ¹³C NMR and X-ray diffraction. Molecules of the title compound exist in the chair conformation with equatorial orientation of the methoxyphenyl substituent. The dioxane ring inversion path, free conformational energy, and optimal conformation of the aryl group have been determined by computer simulation in terms of the DFT PBE/3ζ method. The calculation results are consistent with the X-ray diffraction data.     

Single step preparation and conformational analysis of novel thiophosphorylated ligands based on calix[4]resorcinol matrix

We have synthesized novel calix[4]resorcinols with four 2-thioxo-1,3,2-dioxaphospholane groups introduced in aromatic substituents in the methylidene bridges of macromolecules by the condensation of new thiophosphorylated benzaldehyde with resorcinol and its derivatives in acidic media with high yields. The formation of only one rctt isomer with corresponding chair conformation is observed, which was determined by 2D NMR-experiments.     

Conformational analysis of 4-methyl-2-trimethylsiloxy-1,3,2-dioxaphosphinane

Conformational analysis of 4-methyl-2-trimethylsiloxy-1,3,2-dioxaphosphinane was performed by the dipole moment method and quantum-chemical calculations. The 1,3,2-dioxaphosphinane heteroring was found to adopt a chair conformation with equatorial orientation of the 4-methyl group and axial orientation of the irregular trimethylsiloxy substituent. The conformational equilibrium involves non-eclipsed gauche and trans conformers (the latter prevailing) interconvertible through rotation about the exocyclic P-O bond.     

Sterically oriented synthesis and structure of new perphosphorylated resorcinarenes

Tetraarylresorcinarenes in a chair conformation of C_2h symmetry were synthesized by sterically oriented condensation of aromatic aldehydes with resorcinol and 2-methylresorcinol. By further phosphorylation of resorcinarenes with phosphorous amides perphosphorylated derivatives were obtained with rctt configuration of substituents at internuclear methylidene bridges. Structure of these compounds was proved using NMR spectroscopy and X-ray diffraction analysis.     

Conformational analysis of 5,5-dinitro-1,3-dioxane

Study of conformational transformations of 5,5-dinitro-1,3-dioxane by the method HF/6-31G(d), and also hybrid DFT PBE/3z and RI-MP2/λ2 indicated the only inversion route chair-chair through an intermediate minimum corresponding to the 2,5-twist-form. The potential barrier to this process was estimated and also the features of the intramolecular rotation of the axial and equatorial nitro groups in the chair conformer were investigated.     

Synthesis and conformational study of P‐heterocyclic androst‐5‐ene derivatives

The reactions of (20R)‐3β‐acetoxy‐21‐hydroxymethylpregn‐5‐en‐20‐ol ( 2 ) and (20R)‐3β‐acetoxypregn‐5‐ene‐20,21‐diol ( 11 ) with phenylphosphonic dichloride 3 and aryl dichlorophosphates 4–6 afforded novel types of P‐heterocyclic androst‐5‐ene derivatives 7–10 and 12 as epimeric pairs. The diastereomers were separated by column chromatography and were characterized by NMR spectroscopy. Estimation of the stereostructures of the corresponding epimers by B3LYP/631G(d) DFT ab initio calculations suggested that the six‐membered hetero ring in compounds 7b and 8a–10a adopts predominantly a chair conformation, with the P‐substituents in their preferred orientation. The cyclic phosphonate moiety in 7a or 8b–10b , however, seems to exist as an equilibrium mixture of chair–distorted‐boat or chair–chair forms. The theoretical calculations indicate that the conformational equilibrium is shifted toward the distorted‐boat conformer for 7a , with a pseudoequatorial P‐phenyl substituent, whereas for 8b–10b the chair conformer with an equatorial P‐phenoxy group predominates. © 2008 Wiley Periodicals, Inc. Heteroatom Chem 19:7–14, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20372     

Conformational analysis of 5,5-bis(halomethyl)-1,3-dioxanes

Investigation of stationary points on the potential energy surface of a number of 5,5-bis(halomethyl)-1,3-dioxanes using DFT-approximation of PBE/3ζ revealed the only path of chair form interconversion proceeding through an intermediate minimum corresponding to a 2,5-twist-conformer.     

Solid state conformational preferences in transition metal complexes double bridged by phosphate and related ligands

The conformational preferences in the {M(μ-OPO)}₂ core of transition metal complexes double bridged by phosphate and related ligands have been determined using data retrieved from the Cambridge Structural Database by a probabilistic method. The preferred conformations lie on a pseudorotation pathway (chair and twist-chair conformations). The influence of intra-ring bridges, metal atom, coordination number or ligand in the conformational preferences has been investigated.     

One-pot synthesis of bis-1,5,3-dithiazepanes from ethane-1,2-dithiol, formaldehyde, and ammonium salts

An efficient one-pot procedure has been developed for the synthesis of bis-1,5,3-dithiazepanes by reaction of ethane-1,2-dithiol with formaldehyde and ammonium salts. According to the X-ray diffraction data, the heterorings in 3,3′-[ethane-1,2-diylbis(sulfanediylmethanediyl)]bis(1,5,3-dithiazepane) in crystal adopt a chair conformation with axial orientation of the substituent on the nitrogen atom.     

DFT characterization of 1-acetylpiperazinyl-dithiocarbamate ligand and its transition metal complexes

Employing DFT and handling the solvent effects with the PCM model, the 1-acetylpiperazinyldithiocarbamate acpdtc ligand and its M(acpdtc)₂ complexes, where M is Mn(II), Fe(II), Co(II), Ni(II) and Cu(II), are characterized computationally. The obtained results suggest that the piperazine ring adopts chair conformation in all the studied species. In the gas and solution phases, the chair form of the ligand is dominant. For the Mn, Fe and Co complexes the tetrahedral structure is more stable than the square form in the gas and solution phases. However, the Ni and Cu complexes adopt the square form, in which the complex has the inversion center. The calculated vibrational frequencies are in agreement with the experimental ones, confirming the suitability of the optimized geometries of the compounds. Atomic charges, electron distribution of the frontier orbitals, and stabilizing electron transfers are determined by the NBO analysis.     

Variable temperature NMR and theoretical study of the stereodynamics of 5-trifluoromethylsulfonyl-1,3,5-dioxaazinane: Perlin effect subject to heteroatom substitution

Multinuclear dynamic NMR spectroscopy of 5-trifluoromethylsulfonyl-1,3,5-dioxaazinane (4) revealed the existence of two close in energy chair conformers with differently oriented CF₃ groups with respect to the ring. Of the two alternative routes for their interconversion, the ring inversion path with intermediate formation of the corresponding 2,5-twist-conformer is preferred, with the energy barrier of 11.2 kcal/mol in excellent agreement with the experimental value (11.7 kcal/mol). The Perlin effect is studied experimentally and calculated theoretically for all CH₂ groups and found to be subject to the nature of the adjacent heteroatoms O and N, respectively.     

Cyclohexane in nanotubes: Direct chair–chair interconversion

DFT PBE/3ζ study of conformational transformations of cyclohexane inside nanotubes with chirality indices of (8,0) and (5,5) showed that the encapsulated molecule is characterized by shortened C–C bonds, some electric charge, and reduced barrier to chair–chair interconversion in comparison to the free molecule. Unlike free cyclohexane, no twist conformer as intermediate minimum on the potential energy surface was localized for the encapsulated molecule.     

Microwave spectrum, centrifugal distortion, dipole moment and conformation of 2-methyl-1,3-dioxane

Microwave spectrum of 2-methyl-1,3-dioxane has been investigated in the frequency range 8–40 GHz. Rotational a-type and c-type transitions with J≤40 have been identified. Rotational constants A = 4658.122(2) MHz, B = 2503.221(1) MHz, C = 1783.950(1) MHz and centrifugal distortion constants for the ground vibrational state have been found. Dipole moment components μ_a = 1.43 ± 0.01 D, μ_c = 1.15±0.01 D and overall dipole moment μ = 1.84±0.02 D have been determined. The data obtained are in accord with the chair conformation of the molecule having equatorial arrangement of the methyl group. Original Russian Text Copyright ? 2006 by A. Kh. Mamleev, R. V. Galeev, L. N. Gunderova, M. G. Faizullin, and A. A. Shapkin __________ Translated from Zhurnal Strukturnoi Khimii, Vol. 47, No.2, pp. 373–375, March–April, 2006.