Conformational analysis. 42. Monosubstituted tetrahydropyrans

Eliel, Ernest L.; Hargrave, Karl D.; Pietrusiewicz, K. Michał; Manoharan, Muthiah

doi:10.1021/ja00377a015

Cited by 107 publications

(57 citation statements)

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“…The calculated preference of 3.4 kcal/mol is larger than the observed equatorial preference of 1.5 kcal/mol for the ethyl group in cyclohexane. 28 This indicates an additional stabilizing factor in the equatorial form of CTHP. It has been shown that interactions of the oxygen lone pair with the antibonding C-X orbitals destabilize conformers with less electronegative X.…”

Section: Resultsmentioning

confidence: 96%

Ab Initio Molecular Orbital Calculation of Carbohydrate Model Compounds. 5. Anomeric, Exo-Anomeric, and Reverse Anomeric Effects in C-, N-, and S-Glycosyl Compounds

Tvaroŝka

Carver

1996

J. Phys. Chem.

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An ab initio study of the conformational behavior of R-and -anomeric linkages in C-, N-, and S-glycosyl compounds has been carried out on axially and equatorially 2-substituted derivatives (2-ethyl, 2-methylamino, 2-thiomethyl, and 2-methylammonio) of tetrahydropyran as models. The geometry of the conformers about the anomeric C-X bond was determined by gradient optimization at the SCF level using the 6-31G* basis set. The potential of rotation has been calculated using the 6-31G* and 6-31+G* basis sets. Vibrational frequencies were calculated at the 6-31G* level and used to evaluate zero-point energies, thermal energies, and entropies for minima. Variations in calculated valence geometries for the compounds, display structural changes distinctive for the anomeric and exo-anomeric effects. Differences between bond lengths and bond angles for different conformers correlate with the importance of the lone pair delocalization interactions. The calculated conformational equilibria have been used to estimate the magnitudes of the anomeric, reverse anomeric, and exo-anomeric effects. It was found that the anomeric effect decreases in the following order: chlorine > methoxy ∼ fluorine > thiomethyl > methylamino > ethyl > methylammonio, with the methylamino, ethyl, and methylammonio groups exhibiting reverse anomeric effects. The sc preference of the methyl group over the ap orientation around the C1-C bond in 2-ethyltetrahydropyran is assumed to be entirely on basis of steric interactions. The exo-anomeric effect is expected to be present when the preference for the sc conformation is larger than that for the ethyl group. Thus, the exo-anomeric effect decreases in the order methoxy ∼ methylamino > thiomethyl. The methylammonio group does not show an exo-anomeric effect.

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Section: Resultsmentioning

confidence: 96%

Ab Initio Molecular Orbital Calculation of Carbohydrate Model Compounds. 5. Anomeric, Exo-Anomeric, and Reverse Anomeric Effects in C-, N-, and S-Glycosyl Compounds

Tvaroŝka

Carver

1996

J. Phys. Chem.

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“…'~ Similarly, the cis-2,3-dibromo compound 70 exists almost exclusively in the conformation with axial Br(2) and equatorial Br(3) (A in Scheme 2), as evidenced by the upfield shift at C-6 and unchanged (relative to the parent oxane) shift at C-5; this is in accord with previous work indicating that Br(2) in 2-bromooxane is largely axial17 whereas Br(3) in 3-bromooxane prefers to be equatorial by 0.53 kcal rnol-1. 18 The trans isomer 71 similarly exists largely in the diaxial conformation (A), as evidenced by the high-field position of both C-5 and C-6. The extent of the preference should, of course, be very large, since the tendency of Br(2) to be axial is only slightly opposed by Br(3) and the dipole repulsion of the bromine substituents should also favor the diaxial conformation.…”

Section: Cyclohexyl-x Values Indicated In Parentheses (From Ref 15 Umentioning

confidence: 99%

Carbon‐13 NMR spectra of saturated heterocycles: XI—Tetrahydropyrans (oxanes)

et al. 1983

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The 13C N M R spectra of 6 2 oxanes (tetrahydropyrans) with and without methyl substituents at various ring positions, some of them bearing in addition (or instead) ethyl, vinyl, ethynyl, carbomethoxy and methylol substituents at C-2, have been recorded, and the 294 resulting chemical shifts have been correlated by multiple h e a r regression analysis. Axial and equatorial a-, p-, y-, S-, gem-and uic-parameters for shifts caused by methyl groups at all ring positions, and similar parameters for Et, -CH=CH,, -C=CH, COzMe and CH,OH groups at C-2, are reported. Standard deviations of the parameters are, in most cases, within 0.3ppm and the agreement of calculated and experimental shifts is excellent. This is probably the largest parameter set of this type extant. 13C NMR spectra of a number of additional substituted tetrahydropyrans, and of 3,6-dihydro-2H-pyrans and 3,4-dihydro-ZBpyrans, are tabulated and discussed.

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“…In the context of aldohexopyranoses, these include (i) the Hassel-Ottar effect 18,23,41,66,67,71,72,83,116 and other 1,3-syn-diaxial repulsions; 18,41,50,[67][68][69]71,72,102,[116][117][118][119][120][121][122][123][124][125][126][127] (ii) the D2 effect 23,41,128,129 and other vicinal-gauche repulsions. 18,23,50,67,69,71,72,128,129 Note, however, that the above effects may not be entirely steric in nature, that is, they may also involve a significant dipolar 126,130,131 (electrostatic) or hyperconjugative 124,[132][133]…”

Section: Introductionmentioning

confidence: 99%

“…These effects may also involve a dipolar component 165 and thus be somewhat solvent sensitive, 166,167 but this dependence should be limited compared to that of electrostatic effects. Stereoelectronic effects of relevance for aldohexopyranoses include (i) the endo-anomeric effect; 18,67,120,127,129,132,[165][166][167][168][169][170][171][172][173][174][175][176][177] (ii) the exo-anomeric effect; 166,[171][172][173]175,176,[178][179][180] (iii) the gauche effect; 119,172,[181][182][183][184][185][186][187][188] and (iv) other effects. 124,132,135,137,184,[189][1...…”

Section: Introductionmentioning

confidence: 99%