The use of shift reagents and 13C nuclear magnetic resonance for assignment of stereochemistry to oximes

Abstract

The effects of the shift reagents Eu(dpm)3 and Eu(fod)3 on the 1H and 13C nmr spectra of twelve oximes of diverse structure were examined. The proton data show that the use of proton shifts to assign oxime stereochemistry, as has been done in the past, is unreliable. In contrast, Eu(dpm)3 causes LIS values for the 13C signals of α carbons which are strongly dependent on stereochemistry. Signals for all α carbons anti to the oxime oxygen experience large downfield shifts while all syn carbons are either unaffected or shifted upfield. Thus the effects of Eu(dpm)3 on these 13C signals provides an unambiguous method of configurational assignment to oximes. The effects of Eu(fod)3, though similar to Eu(dpm)3, were less consistent and thus less promising for making configurational assignments.