¹³C nuclear magnetic resonance studies. 58. ¹³C spectra of a variety of bicyclo[2.2.2]octane derivatives. Further definition of the deshielding δ effect

Abstract

J. B. STOTHERS and C. T. TAN. Can. J. Chem. 54, 917 (1976).The 13C nmr spectra of 35 bicyclo[2.2.2]octane and -octene derivatives have been determined to extend our examinations of the effects of stereochemistry on the shieldings of closely neighboring carbons. This series includes a variety of methyl substituted bicyclooctanols and -0ctenols as well as the corresponding hydrocarbons and some bicyclooctanones. With the bicyclo[2.2.2]-octane skeleton it is possible to examine an array of systems having substituents separated by three and four bonds in a variety of orientations. The interactions, termed y and 6 effects, respectively, produce distinctive shielding variations which are useful for stereochemical elucidations. Particularly interesting are the pronounced shifts observed for the carbons bearing closely neighboring substituents. Characteristically, for vicinal substituents, these carbons are shielded while for syn-axial 6 interactions, these carbons are deshielded by as much as 8.6 ppm. The results are compared with the trends found in other sterically crowded systems. Introduction As an extension of our examination of the effects of molecular geometry on 13C shieldings, we have obtained the spectra of a series of bicyclo[2.2.2]octane derivatives for comparison with the results for other bicyclic systems (1). The principal virtue of such systems for an investigation of stereochemical effects is their relatively rigid skeleton which permits one to study the shielding variations as a function of fairly well-defined substituent orientations. The shifts produced by substituents separated by three and four bonds are of particular interest because of their potential application as probes for conformational analysis in mobile systems and as a tool for stereochemical elucidations. The trends associated with interactions between

Effect of a Methoxy Substituent on the Vinylcyclobutane Carbon Migration

“…The NMR spectral data for the major epimer 6b, which eluted second from the column, closely match those reported in the literature for the endo alcohol. 22,23 Conversion of the alcohols to the methyl ethers was accomplished using standard Williamson ether synthesis conditions of sodium hydride followed by methyl iodide. Differentiation between the 5-exo-(2a) and 5-endo-methoxybicyclo[2.2.2]oct-2-ene (2b) epimers was based on 1 H and 13 C NMR characterization of each after preparative GC separation and purification.…”

Thermal Reactions of 8-Methylbicyclo[4.2.0]oct-2-enes: Competitive Diradical-Mediated [1,3] Sigmatropic, Stereomutation, and Fragmentation Processes

“…Differentiation between the 5- exo - ( 2a ) and 5- endo -methylbicyclo[2.2.2]oct-2-ene ( 2b ) epimers is based on 1 H and 13 C NMR characterization. − Although the end o -methyl hydrogens experience a typical shielding environment, the 13 C NMR chemical shift for the end o -methyl in 2b is 23.1 ppm compared to 20.5 ppm for the ex o -methyl in 2a …”

13C n.m.r. spectra of steroids —a survey and commentary

“…61 Further systematic investigations of the 13C spectra of functionalized steroids and related polycyclic systems are to be welcomed as a means of providing more reliable data for predicting shieldings, and hence, ultimately, of increasing the confidence level for structural assignments from the spectra of unknown steroids and related compounds.…”