Conformation of substituted five-membered rings

Proton magnetic resonance chemical shifts and coupling constants for some 5-substituted dihydrofurans, dihydrofuran and dihydrothiophene fused rings are reported, the substituents being methyl, ethyl and phenyl. The observed coupling constants are consistent with buckled rings, in which it appears that a conformation with pseudo-equatorial substituents is predominant.     

The determination of sulfoxide configuration in five-membered rings using NMR spectroscopy and DFT calculations

Cyclic five-membered ring sulfoxides and sulfones were prepared by a stepwise in situ oxidation of the corresponding sulfides with meta-chloroperbenzoic acid in an NMR tube. The oxidation was followed by NMR and both ¹H and ¹³C NMR data were collected. The geometries of all of the compounds were optimized using the DFT B3LYP/6-31G⁷ method and the ¹³C and ¹H chemical shifts were calculated for geometry-optimized structures with the DFT B3LYP/6-31++G⁷ method. The calculated ¹³C chemical shifts induced by oxidation (Δδ values) were in very good agreement with the experimental data and could be used to determine the oxidation state of the sulfur atom (–S–, –SO–, –SO₂–). The characteristic differences of the induced oxidation chemical shifts of the carbon atoms in the α-position and β-position to sulfur were successfully used to distinguish between the diastereoisomeric sulfoxides and allowed configuration determination.     

The estimation of the aromaticity of five-membered and benzo fused five-membered rings by the hybrid DFT computed magnetic properties

The structures of thiophene, pyrrole, furan, and their benzo derivatives were generated with the hybrid B3LYP density functional theory (DFT) methods employing a 6-31G* basis set. Their magnetic susceptibility anisotropics were calculated on these geometries with a continuous set of gauge transformations. The aromaticity of the heterocycles was discussed in light of their structural uniformity and magnetic susceptibility. The computed structural parameters, order of aromaticity, stability, and reactivity is in excellent agreement with the experimental results. The usefulness of this approach to determine the reactivity is discussed.     

Thia-acyliminium cyclisations : Synthesis of five-membered rings and macrocycles

Cyclisations of carbinollactams 1b–3b afford macrocycles 4–6 in good yield by the intermediacy of N-acyliminium species. A similar process applied to lactams 21b–25b affords the pyrrolizidine type compounds 26–35 through consecutive azonium-Cope rearrangement and N-acyliminium ring closure. Additional results are concerned with the use of aromatic rings as π-nucleophiles.     

Stereostructure assignment of flexible five-membered rings by GIAO 13C NMR calculations: prediction of the stereochemistry of elatenyne

The stereochemistry of conformationally mobile five-membered rings is often hard to assign from NMR data, and [2,2']bifuranyl systems are even more challenging. GIAO (13)C NMR chemical shifts have been calculated for a series of [2,2']bifuranyl and pyranopyran species, taking into account their conformational flexibility using weighted averages of the data for all low energy conformers. We show that calculation of (13)C NMR chemical shifts using the geometries obtained using molecular mechanics greatly reduces the computational expense without a significant loss of accuracy, even in this demanding system. The results were sufficiently accurate to distinguish not only the pyran and furanyl isomers but also between all the diastereoisomeric forms. As a result of this validation, we predict the stereochemistry for the recently proposed revised structure of the natural product elatenyne, which contains a [2,2']bifuranyl core.     

Pseudohelical and helical primary structures of 1,2-spiroannelated four- and five-membered rings: syntheses and chiroptical properties

The pseudohelical hydrocarbons (R)-6, (S)-7, and (R)-8 and the helical hydrocarbon (P)-9, formally derived from the helical hydrocarbon (P)-4 by stepwise replacement of each of the four-membered rings by a five-membered ring, have been prepared. Their optical rotations vary systematically, both in magnitude and sign. Of the extremes, (P)-4 represents the usual case of a right-handed dextrorotatory helix, while (P)-9 represents the unusual case of a right-handed levorotatory helix. To rationalize these facts, DFT calculations of the rotatory power of (P)-helices of three-, four-, and five-membered rings have been performed. The results show a very good agreement with the experimental data for the rigid helices of three-membered rings and always show the correct sign and order of magnitude for the flexible helices of four- and five-membered rings for which Boltzmann-averaged optical rotations of up to six conformers had to be used. Within the conformers of the latter, a set of large dihedral angles for the bonds of the inner sphere correspond to a high specific rotation, and a set of small dihedral angles correspond to a low specific rotation. As a consequence, the Boltzmann-averaged values markedly depend on the geometry and weight of the conformers involved.     

Intramolecular hypervalent interaction in the conjugate five-membered rings

The intramolecular hypervalent interaction between the electron abundant atomic centers X and Y belonging to the IV-VI groups and second and fourth periods has been computationally studied on a model quasi-cyclic conjugate five-membered ring system 9 using the CCSD/6-311+G** and DFT B3LYP/6-311+G** methods. Electronic and structural factors affecting the strength and geometrical characteristics of the hypervalent X←Y interaction were analyzed based on the quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analyses. The donor-acceptor n(Y)→σ*(XR) interaction has been shown to be the central factor correlating all important properties of the studied hypervalently bonded compounds 9.     

Synthesis of enantiopure helical cyclophanes containing five-membered heterocyclic rings

The preparation of optically active (S)-(+)-4-(2-propenyl)[2.2]paracyclophane 1b is described. Cycloaddition reactions of this diene with N-phenylmaleimide 2 and maleic anhydride 5 were carried out under high pressure conditions. The cycloadducts were converted into optically active heterohelicenophanes. A systematic ¹H and ¹³C NMR spectroscopic analysis of the reaction products is also presented.     

On the configuration of five-membered rings: a spin-spin coupling constant approach

Five-membered rings are clearly among the most common structural motifs found in chemistry and biology. Nevertheless, the configuration of conformationally mobile five-membered rings is often difficult to assign from nuclear magnetic resonance (NMR) data. A simple, reliable, and efficient approach for the stereochemical analysis of five-membered rings based on the measurement of NMR coupling constants is presented. Density functional theory calculations using representative conformations of the full conformational space available to rings with different substitution patterns were used to identify differences between the accessible coupling constant values for cis and trans relative orientations of the substituents. The calculations were assessed experimentally using NMR data obtained from a number of models. This approach can be easily used to analyze different five-membered rings, such as oxolanes, cyclopentanes, furanosides and pyrrolidines, and their relative configuration can be determined without the need for making further conformational considerations.     

Photochemistry of diethynyl sulfides: a cycloaromatization for the formation of five-membered rings

The first five-membered ring cycloaromatization reaction has been demonstrated. Photoirradiation of bis(phenylethynyl) sulfide in hexanes/1,4-cyclohexadiene produces 3,4-diphenylthiophene through the presumed intermediacy of 2,5-didehydrothiophene. In addition, phenylacetylene is produced in this reaction consistent with competing direct carbon-sulfur cleavage. For reactions in ethanol or 2-propanol production of the thiophene is accompanied by the formation of phenylacetylene and a thionoester of the corresponding alcohol. Thiophene products also result from the irradiation of other diethynyl sulfides. [reaction: see text]     

Chemical thermodynamics of polyaromatic compounds containing heteroatoms and five-membered rings

An analysis of chemical thermodynamic properties of gaseous polynuclear aromatic compounds containing N-, S-, and O-heteroatoms and five-membered rings is presented. Standard enthalpies of formation, entropies and heat capacities taken from the literature, or derived from appropriate data, serve as the data base for this analysis. Enthalpies of formation are used to compute empirical resonance energies for polynuclear heteroaromatic structures which, in turn, allow critical examination of this property. An additive method is described for estimating entropies and heat capacities. New or revised values for thermodynamic properties for a number of polyaromatic substances are presented and recommended values for some common polyaromatics are given.     

Dearomatizing annelation of five-membered rings to naphthalenes by organolithium cyclization

[reaction: see text] gamma-Lithiopropylnaphthalenes and their oxa- and aza-tethered analogues cyclize by nucleophilic addition of the organolithium to the naphthalene ring. The resulting benzyllithiums react stereoselectively with electrophiles to give dearomatized tricyclic products with structural similarity to the arylnaphthalene lignans.     

Electrochemical annulation of five-membered rings through dearomatization of furans and thiophenes

A new methodology for the annulation of five-membered carbocyclic rings onto enones through the dearomatizing electrochemical cyclization of furans and thiophenes has been developed.     

Polycyclic aromatic hydrocarbons with five-membered rings: Modeling of experimental X-ray and neutron-diffraction structures

Several of the readily available theoretical programs are evaluated as tools for modeling the structures of polycyclic aromatic hydrocarbons with five-membered rings (CPAHs). The experimentally determined bond lengths and angles are compared to calculated values. Experimental bond lengths are also compared to Pauling and Huckel molecular orbital (HMO) bond orders. Previously published experimental X-ray and neutron-diffraction structures of acenaphthene, acenaphthylene, fluoranthene, cyclopent[o,p,q,r]benz[c]phenanthrene, and corannulene are modeled by the programs MMX, AM1, MNDO, and PM3, and previously reported STO-3G and 6-31G * data are also evaluated. In general, the error differences between the experimental and calculated results for all of the semiempirical programs were small. However, PM3 performed slightly better than AM1 and MMX, while MNDO generated structures which exhibited the largest deviation from experiment. Although the standard deviations for all programs are shown to be of comparable magnitude, a particular bond length or bond angle in any given theoretical calculation can exhibit significant error from the experimental data. The scatter in the bond order data computed from Huckel molecular orbital theory and valence bond theory is contrary to results obtained with alternant systems. It appears that these approaches are less successful at modeling accurately the nonalternant hydrocarbon systems described in this paper.     

Proton affinities of polybenzenoid aromatic hydrocarbons and those with five-membered rings

Proton affinities of PAHs including one five-membered ring are calculated by using the AM1 Hamiltonian for the determination of ΔH_f^o of the neutral and protonated molecules. The calculated PAs are compared to experimental PAs measured by chemical ionization mass spectrometry, using a new method based on competition between charge transfer and proton transfer occurring during the ionization process. A procedure is proposed to validate AM1-calculated PAs from experimental PAs after rescaling the calculated and measured PA values. The site of protonation is first determined on the criterion of the lowest loss of aromaticity, then on the criterion of the largest HOMO coefficient. For indene, the corrected result is compared to an ab initio calculation at the MP2/6-31G^*//HF/6-31G^* level and to a DFT calculation at the B3LYP/6-31G^* and the B3LYP/6-311 + G^** levels. Five new PAs are thus established and one published experimental PA is revised. © 1997 by John Wiley & Sons, Inc.     

Fused five-membered rings determine the stability of C60F60

The structure and stability of a set of (CF)60 isomers have been computed at the B3LYP/6-31G(d) density functional theory level. The most stable isomer (6, F4@C60F56) has tube-like structure with four endo C-F bonds and fused five-membered rings at the end of the tube, while the reported most stable cage structure (2, F8@C60F52) with eight endo C-F bonds is higher in energy by 22.6 kcal/mol. This is in contrast to the isolated pentagon rule for the stability of fullerenes. The mean bond dissociation energy of 6 is larger than those of the experimental known C60F36, C60F48, and graphite fluoride. The relative energy per CF unit of 6 to graphite fluoride (CF)n is 3.7 kcal/mol, which is smaller than that of C60 fullerene per carbon to graphite (about 9-10 kcal/mol).     

Hindered rotation in five-membered cyclic nitrosamines: A 13C NMR study

The equilibrium rotamer populations and N? N rotational barriers of N-nitrosopyrrolidine (1), N-nitrosothiazolidine (2), N-nitrosooxazolidine (3) and their 2-methyl derivatives, 4, 5 and 6, were determined by ¹³C NMR spectroscopy. While equal rotamer populations occur in 1 and 2, the E rotamers predominate in the other four compounds, with the highest percentage (92%) of E rotamer occurring in N-nitroso-2-methyloxazolidine (6). The average barrier to N? N bond rotation varies over a range of 4.1 kcal mol^?1in these compounds, decreasing in the order N-nitrosopyrrolidine > N-nitrosothiazolidine > N-nitrosooxazolidine. The compounds which contain an exocyclic 2-methyl group have average rotational barriers which are 0.1–0.9 kcal mol^?1 higher than those of the corresponding unmethylated derivatives. The results are interpreted in terms of the relative effects of steric hindrance by the 2-methyl substituents and electron induction by the heterocyclic sulfur and oxygen atoms on both the rotamer populations and the N? N rotational barriers.