Concerning stereochemical assignments from 13C spectra of systems experiencing γ substituent effects. Deshielding antiperiplanar interactions can mislead the unwary

Abstract

A variety of substituents bonded to quaternary carbons exert marked deshielding effects at antiperiplanar γ carbons. The shifts produced by methyl groups are ca. 2 ppm, while hydroxyl groups cause shifts as large as 6 ppm. These shifts are particularly clearly demonstrated by the C-19 shieldings in 5α-substituted steroids. For 9,10-disubstituted decalins, the effect is such that cis and trans isomers are not easily distinguished from their 13C spectra. These shifts contrast sharply with the corresponding effects found for substituents bonded to less highly substituted carbon skeletons.