Nuclear magnetic resonance 6- membered rings

Infrared and nuclear magnetic resonance spectral evidence led Kendall and Hajos to conclude that furan-3,4-dione (37) exists as such, which is surprising since cyclic a-diketones with five-membered rings are usually monoenolized. The 2,5-dicarbethoxy derivative 38 was earlier stated to exist in the dihydroxy form. ... 

There are several systematic nuclear magnetic resonance studies of the interaction between the substituents and the protons and ring atoms of five-membered heterocycles. In some 2-substituted furans, thiophenes, selenophenes, and tellurophenes there is a linear correlation between the electronegativity of the chalcogen and several of the NMR parameters.28 As there also is a good correlation between the shifts of the corresponding protons and carbons in the four heterocycles, the shifts of unknown selenophene and tellurophene derivatives can be predicted when those of thiophene are known. This is of special interest for the tellurophene derivatives, since they are difficult to synthesize. In the selenophene series, where a representative set of substituents can be introduced in the 2- as well as in the 3-position, the correlation between the H and 13C shifts and the reactivity parameters according to Swain and Lupton s two-parameter equation... 

The (5,5) (2N2)-fused heterocyclic system contains three ring carbon atoms, one fusion carbon atom, and one additional nonfusion carbon atom in each five-membered ring. Only scattered H and 13C nuclear magnetic resonance (NMR) data are available for these systems. 

Sec. IV.C] FOUR MEMBERED RINGS WITH ONE SULFUR ATOM C. Nuclear Magnetic Resonance... 

The aromatic character of the dibenzo derivative 2 has also been calculated. Studies on the behavior of the deep red anion 3, as a potential IOji heteroaromatic system, were described previously <1996CHEC-II(9)268>. H NMR spectra (NMR - nuclear magnetic resonance) of l,3-dithiepin-2-carbodithiolate 4, a ligand obtained from the reaction of carbon disulfide with the lithium salt of 1,3-dithiepin, as well as its palladium salt suggest a good deal of Hiickel aromatic character in the seven-membered dithiepin ring <1991ICA(185)169>. 

Block, E., Bazzi, A.A., Lambert, J.B., Wharry, S.M., Andersen, K.K., Dittmer, D.C., Patwardhan, B.H., and Smith, J.H., Carbon-13 and oxygen-17 nuclear magnetic resonance studies of organosulfur compounds the four-membered-ring-sulfone effect, J. Org. Chem., 45, 4807, 1980. 

Dioxiranes 4 have equivalent oxygens (attested by isotopic labeling and 170 NMR NMR - nuclear magnetic resonance) and an t/)3-hybridized carbon (of course, the exact hybridization is influenced by the 3-membered ring), whereas carbonyl oxides 1-3 have an r/)Z-hybridized carbon atom and give rise to diastereomers la and lb when R yf R1. 

For a long time, pentazole chemistry only involved the study of the chemical and physical properties of the pentazole compounds discovered by Huisgen and Ugi. More than 40 years later, the first pentazole attached to a five-membered ring system, tetrazolylpentazole, was reported by Hammerl and Klapotke <2002IC906>. Unfortunately, this compound decomposes below — 50 °C. Imidazolylpentazole is reported to be stable up to — 30 °C. Recently, the first dipentazole, l,4-bis(/>-pentazolyl)butane, has been observed by nuclear magnetic resonance (NMR) spectroscopy <2004ZNB716>. 

In this chapter, the theoretical methods, crystal structures, characteristic nuclear magnetic resonances, reactivities, and formation of four-membered rings with two heteroatoms including selenium or tellurium that have appeared in the last decade are described. 

This difference in the AG values is especially observable in the temperature-dependent, nuclear magnetic resonance spectrum of 5,6-bis(acetamido)-l,6-anhydro-5,6-dideoxy-j8-L-idopyranose. This sugar contains a flve-membered and a six-membered ring amide in... 

When CFjCOjD is used as the acid proton, nuclear magnetic resonance (NMR) shows rapid and stereospecific exchange between the Rh hydride and the two endo positions on the six-membered ring, reflecting the reversible transfer of the Rh hydride to endo sites at the ends of the diene system . 

An important contribution to the problem of ir-electron bonding with seven-membered rings is probably made by measurements of the chemical shifts of ring protons as studied by the nuclear magnetic resonance method. It has recently been shown (117) that the displacement of the electron density of the ring protons increases along the series (CsHs)", CeHe, ( 7117)+. Perhaps this expresses the decreasing tendency to enter stable ir-bonds. 

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