Helmut Beierbeck

A reinterpretation of beta, gamma, and delta substituent effects on 13C chemical shifts

It is shown that β-, γ-, and δ-substituent effects on 13C chemical shifts may be better understood by assuming that the upfield shifts caused by gauche γ-substituents are not due to non-bonded interactions introduced with the γ-group, but that they are due to the removal of the hydrogen on the β-substituent.

Conformational analysis of ketoximes by the application of carbon-13 nuclear magnetic resonance spectroscopy

The carbon-13 chemical shifts for a number of relatively rigid ketoximes are presented. It is shown that the chemical shift difference, A6 (sj~rr-onri), for the carbons a to the oxime carbon depends on the dihedral angle between the C=N and C.-H bonds. This stereochernical dependence is then used to determine the preferred conforn~ation of substituted cyclohexanone oximes.

Lanthanide Shift Reagents in 13C Nuclear Magnetic Resonance: Quantitative Determination of Pseudocontact Shifts and Assignment of 13C Chemical Shifts of Steroids

The l3C n.m.r. chemical shifts of some monofunctional keto- and hydroxy-steroids were monitored as a function of the concentration of added shift reagent, Pr(fod)3 or Eu(fod)3, and the observed induced shifts were fitted to the geometric quantities in the one-term pseudocontact shielding equation. The correlation between experimental and calculated shifts was quantitative for all carbons except those closest to the complexing site, where contact interactions are most noticeable. With Pr(fod)3 the carbonyl and hydroxyl carbons and the alpha carbons usually had to be excluded from the calculations in order to optimize the correlation between predicted and observed induced shifts for the remaining nuclei and to derive reasonable adduct geometries. With Eu(fod)3, contact shifts were also noticeable at the beta carbons and the europium shift reagents are therefore less useful in 13C n.m.r. spectroscopy. It is shown that the application of shift reagents, in conjunction with off-resonance decoupling, permits th...

The conformational analysis of piriprost, an inhibitor of leukotriene synthesis, based on high-field nuclear magnetic resonance studies

The analysis of proton nuclear Overhauser effect data for piriprost, (6,9-deepoxy)-6,9-(phenylimino)-Δ6,8 prostaglandin I1, indicates that the cyclopentene ring has the 11E conformation. A long-range nOe effect indicates that the α-chain is folded near the pyrrole and the phenyl rings.