A13C and1H NMR study of diastereomeric α-methylidene-β-hydroxy-γ-alkoxy esters (original) (raw)
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Journal of Molecular Structure, 1992
The conformational behaviour of AsCH(CN)CH(Ar1)CH2COR (R = OCH,; N(CH,),; Ar' = Ph; A13 = Ph or 1-naphthyl) has been investigated by lH and 13C NMR spectroscopy. The vicinal proton-proton coupling constants were used together with those of carbon-proton couplings to provide information on the conformational preference in the compounds studied. The conformational distribution with respect to the C(2kC(3) bond is shifted to the conformation having trans Ph and COR groups. Coupling constants involving the cyano carbon atom and the methine proton are J&H = 2.0 Hz and &,a = 14.1 Hz. The conformation with the cyano group and the methine proton trans to each other, with gauche-oriented protons around the C(3)-C(4) bond is the preferred one for both isomers in the compounds investigated. The results obtained support the assumption that electrostatic (non-bonded attractive) interactions rather than steric demands operate as dominant factors in controlling the conformational equilibrium.
Journal of Physical Organic Chemistry, 2013
The experimental 1 H and 13 C NMR spectra of 13 phenyl cinnamates and four 4-methylcoumarins were investigated and their chemical shifts assigned on the basis of the two-dimensional spectra. For the unsubstituted cinnamic acid phenyl ester, optimized molecular structures were calculated at a B3LYP/6-311++G(d,p) level of theory. 1 H and 13 C NMR chemical shifts were also calculated with the GIAO method at the B3LYP/6-311 + G(2d,p) level of theory. The comparison between experimental and calculated NMR chemical shift suggests that the experimental spectra are formed from the superposition spectra of the two lowest energy conformers of the compound in solution. The most stable s-cis configuration found in our studies is also the conformation adopted for a related phenyl cinnamate in solid state. The experimental results were analyzed in terms of the substituent effects.
Tetrahedron letters, 1999
A new method has been developed for determining the absolute configurations of acyclic [5-or ~-methyl substituted secondary alcohols using their 2NMA esters. The 1H-NMR spectra of (R)-and (S)-2NMA esters of model compounds were measured, and A6 values (6R.este r-6S_este r) for the corresponding protons were compared between syn and anti compounds. Threshold values important for judging the relative stereoehemistry of the two chiral centers bearing methyl and hydroxy groups were obtained. The absolute configuration of the chiral omter bearing a secondary hydroxy group is easily determined based on the sign of A6 values as in the MTPA method and thus in the present study it was also possible to clearly determine the absolute configuration of the chiral center bearing a methyl group.
The differentiation of diastereomers by13C NMR spectroscopy
Magnetic Resonance in Chemistry, 1981
13C shifts are reported for diastereomers with halogen, oxygen and amino substituents which are mainly at vicinal chiral centres. The conformer populations underlying the 13C shift analysis can be verified by chemical means, such as change of temperature, solvent polarity (with dihalides), solvent hydrogen bonding capacity (with diols), deprotonation (with diacids) and by molecular mechanics calculations. The latter provide the basis for configurational and conformational assignments with a series of ephedrine derivatives. In accordance with the 13C shift observations, it is found that ephedrine derivatives predominate in conformations with a gauche O and N substituent arrangement, even in cases where hydrogen bonds are absent.
Organic Magnetic Resonance, 1984
Nitro-substituted 1,3-dioxanes and pyrans with potential biological activity were synthesized. The relative configurations of the threo and erythro isomers of 2-nitro-l-phenylpropane-1,3-diol (the starting materials) and of 5-nitro-2,4-diphenyl-1,3-dioxane, the products formed with benzaldehyde, were determined by 'H and =C NMR spectroscopy. Ciamaldehyde and nitroethanol give a homologue of nitrophenylpropanediol, which exists as a mixture of two diastereomeric cyclic hemiacetals. After dissolving in CDCI,, the ratio of these components changes &om approximately 10 : 1 to 3 : 2 in a few days. The ethyl acetal of the hemiacetal is stereohomogeneous, and the ethoxy group is axial to the pyran ring owing to the anomeric effect. This configuration was proved by two-dimensional NMR measurement, permitting separate observation of the originally overlapping mnltiplets and direct reading of the H,H coupling constants.