15N and17O NMR spectra of vinyl ethers of pyridine and quinoline

Conclusions ¹⁵N (¹⁴N) and¹⁷O NMR spectra have been obtained on vinyl ethers of pyridine and quinoline, with unenriched samples. The principal factor determining the chemical shifts is p- conjugation of the unshared pairs of the heteroatom with the unsaturated fragments of the molecule; an additional contribution in the case of the¹⁵N signals comes from interaction of the nitrogen atom with a proton through space.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 769–773, April, 1987.     

17O NMR spectra of alpha-diesters

17O NMR spectra of title compounds were measured at natural abundance in acetonitrile solutions. Intercarbonyl dihedral angles have been estimated by molecular mechanics, which show invariance except in one case. Because of this invariance, contrary to other alpha-dicarbonyl compounds, a correlation between chemical shifts and dihedral intercarbonyl angles could not be developed. Spectroscopic and computational results allowed us to evaluate other conformational features.     

17O NMR Investigation of cyclic aromatic ethers

A linear relationship between the C-O-C angle and the molecular dihedral angle in a series of phenoxathi-ins and azaphenoxathiins is reported. ¹⁷O nmr spectroscopic data (natural abundance in acetonitrile at 75°C) were obtained on eight cyclic aromatic ethers 1-8 , including phenoxathiins, and two model compounds, acyclic aromatic ethers 9 and 10. The chemical shifts of the cyclic aromatic ethers were very sensitive to structural variations and were dependent upon electonic and conformational effects; however, no quantitative relationship between ¹⁷O chemical shift and geometric parameters was found.     

Carbon-13 NMR spectra of some tetra- and pentacyclic triterpene methyl ethers

The carbon-13 NMR spectra have been recorded for representatives of three previously unexamined classes of pentacyclic triterpenes—lupanes, fernanes and arboranes. In addition to the spectra of lupeol methyl ether, cylindrin and arundoin, the spectra of a number of other triterpene methyl ethers are also recorded. The effect of methyl ether formation on the shilding values for carbons in the 3β-hydroxy-4,4-dimethyl- ring A system is defined.     

Synthesis of 5-alkenyl-1,3-dioxanes

Cyclic acetals containing alkenyl groups were obtained from 5-methyl-5-acyl-1,3-dioxane. The 1,3-dioxane fragment does not interfere in reduction of the carbonyl group, nor in participation of the carbonyl group in Wittig and Grignard reactions.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 37–39, January, 1991.     

New syntheses and13C NMR spectra of the methyl ethers of methyl α-L-arabinopyranoside

A method is described for obtaining the methyl ethers of methyl -L-arabinopyranoside that is based on the partial methylation of methyl -L-arabinopyranoside followed by the liquid chromatography of the methyl ethers. The¹³C NMR spectra of the methyl ethers of methyl -L-arabinopyranoside have been studied.Pacific Ocean Institute of Bioorganic Chemistry, Far Eastern Scientific Center, Academy of Sciences of the USSR, Vladivostok. Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 178–181, March–April, 1985.     

Conformational analysis of 5-substituted 1,3-dioxanes

Quantum-chemical study on the potential energy surface of 5-alkyl- and 5-phenyl-1,3-dioxanes at the RHF/6-31G(d) level of theory revealed two pathways for conformational isomerizations of the equatorial and axial chair conformers. Potential barriers to this process were estimated. The Gibbs conformational energies ΔG° of substituents at C⁵ in the 1,3-dioxane ring were determined on the basis of experimental (¹H NMR) and theoretical vicinal coupling constants, which turned out to be consistent with published data.     

NMR experiments on acetals—57 : Conformational study of 5-alkylidene-1,3-dioxanes

From an extensive ¹H-NMR comparative study (homo-INDOR-, and nmdr-assisted) of a series of 2-, 4- and 2,4-substituted 5-alkylidene-1,3-dioxanes it is concluded that the basic conformation is a flattened chair (Fig. 5c,f). This is substantiated by the low barrier to ring reversal in 5-methylidene- and 5-propylidene-1,3-dioxane of ?G^≠ ～ 5·5 kcal/mole (T_c resp. ?158° and ?161°).     

Synthesis of 2-phenylethynyl-1,3-dioxanes and their hydrolysis

2-Phenylethynyl substituted 1,3-dioxanes were obtained by the reaction of 1,3-dioxanium salts with an lotsich reagent. It was shown that they are readily hydrolyzed with the formation of a-acerylenic ketones. A simple new method is proposed for the synthesis of the latter without isolating the intermediate 1,3-dioxanes using the reaction of 4 substituted 2-methyl(phenyl, furyl)-1,3-dioxanium salts with hydroxymethyl- and phenylethynylmagnesium bromide as examples.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 193–197, February, 1996.     

Mass spectra of alkenyl methyl ethers

Electron impact ionization mass spectra of numerous alkenyl methyl ethers C_nH_2n-1OCH₃ (n = 3–6) recorded under normal (4 kV, 70 eV, 175°C) and low-energy, low-temperature (8 kV, 12 eV, 75 °C) conditions are reported. The influence of the position and stereochemistry of the double bond on the dissociation of ionized alkenyl methyl ethers is discussed. The mechanisms by which these ethers fragment after ionization have been further investigated using extensive ²H-labelling experiments and by studying the energy dependence of the reactions. Ethers of allylic alcohols show spectra that are distinct from those of the isomeric species in which the double bond is separated by one or more sp³ carbon atoms from the carbon atom carrying the methoxy group. Three principal primary fragmentations are observed. The most common process, especially for ionized ethers of allylic alcohols, is loss of an alkyl group. This reaction often occurs by simple α-cleavage of radical-cations of the appropriate structure; however, alkyl groups attached to either end of the double bond are also readily lost. These formal β- and γ-cleavages are explained in terms of rearrangements via distonic ions and, at least in the case of γ-cleavages, ionized methoxycyclopropanes. Ionized homoallyl methyl ethers tend to eliminate an allylic radical, particularly at high internal energies, with formation of an oxonium ion (CH₃ ⁺O?CH₂ or CH₃ ⁺O?CHCH₃). The ethers of linear pentenols and hexenols show abundant [M - CH₃OH]⁺? ions in their spectra, especially when a terminal methoxy group is present Methanol loss also takes place from ionized ethers of allylic alcohols in which there is a Δ-hydrogen atom; this process is significantly favoured by cis, rather than trans, stereochemistry of the double bond.     

Reactivities and NMR spectra of vinyl ethers

Copolymerizations of n-butyl vinyl ether (M₁) with other vinyl ethers were carried out in toluene at ?78°C with EtAlCl₂ catalyst and the monomer reactivity ratios were determined. It was found that the relative reactivity of alkyl vinyl ether log 1/r₁ is higher when the alkyl group is more electron-donating and the reactivity correlates linearly with the Taft σ^* of alkyl group in the monomer. The NMR spectra of vinyl ethers and of vinyl ether–trialkylaluminum complexes were investigated. Close correlations were found between the spectral characteristics and the relative reactivity of vinyl ether in the copolymerization. The degree of resonance contribution in alkyl vinyl ether was also discussed on the basis of NMR data.     

Structure and spectra of 1,3-dioxanes. microwave spectrum, structural parameters, and ab initio calculations of 5-methyl-1,3-dioxane

In the microwave spectrum of a 5-methyl-1,3-dioxane sample, the rotational transitions of a and c types with 4 ≤ J ≤ 11 for five isotopomers of a molecule with ¹³C and ¹⁸O isotopes in different sites are identified in a frequency range of 18 GHz to 42 GHz. The spectroscopic constants of isotopomers are found. The substituted r _s and effective r _o structural parameters of 5-methyl-1,3-dioxane are determined. By the B3PW91/aug-cc-pVDZ density functional method the equilibrium structure of the molecule is calculated. The results of quantum chemical calculations are compared to the experimental data.