Modification to an earlier procedure that allows students to observe the results of a hydride shift mechanism.

Using the methods described in parts I-VI of this series, we have examined the long-range shielding effects of the cyano group upon the C-18 and -19 methyl protons in several cyano-steroids and the ring protons in 2-endo-and 2-exo-cyanonorbornenes.It was found that the most important shielding mechanism of the cyano group is its electric field effect, with the magnetic anisotropy effect making at most a minor contribution to the screening. However, only in the cases of the cyano-steroids was good agreement obtained between the calculated and the experimental shifts.En utilisant les methodes decrites dans les parties I-VI de cette serie, nous avons examine les effets de blindage a longue distance du groupe cyano sur les protons des methyles 18 et 19 de plusieurs cyano steroides et sur les protons du cycle des exo et endo cyano-2 norbornenes. Le mecanisme le plus important de l'effet de blindage du groupe cyano se trouve ttre son effet de champ electrique, l'anisotropie magnetique apportant une contribution mineure a l'effet d'tcran. Cependant, seules dans le cas des cyano steroides, les valeurs calculees et experimentales pour les deplacements chimiques ont kt6 obtenues avec un bon accord.

No abstract

The calculation of chemical shift values by the method used in parts I–V (1–5) has been extended to a derivation of the shielding effects of the ethylene–ketal and −thioketal groups. For these studies ketal and thioketal derivatives of monoketoandrostanes were prepared. The chemical shifts of the C-18 and -19 methyl protons in these compounds are reported for the solvents CDCl3, CCl4 and benzene.Representing both groups by point dipoles, values for the anisotropies and for K, a parameter descriptive of the electric field effect, were derived for various, coincidental, locations of the magnetic and electric dipoles along the symmetry axes of the two groups.