Detection of deuterium labelling by two-dimensional 1H,13C nuclear magnetic resonance shift correlation with 2H decoupling

Abstract

Methylene groups that are stereospecifically labelled with deuterium can be easily observed using deuterium-decoupled 2-dimensional proton/carbon nmr shift correlation spectroscopy in either normal or CH-selective mode. Application of the technique to [3-2H]camphors 2, 3, and 4 shows that overlap of other 13C resonances does not interfere with the 2H detection. Such experiments with [3-2H]-N-benzoylphenylalanine methyl esters 6 and 7 demonstrate that identification of labelled diastereotopic hydrogens can be done if they are separated by 0.1 ppm in the 1H nmr spectrum. Fully deuterated carbons (e.g., CD3 methyl, CD2 methylene, CD methine) cannot be observed by this approach, as seen by the study of pregnenolone acetate (9) randomly deuterated at C-17 and C-21. The technique is ideal for cases with extensive overlap of other proton and carbon resonances because high chemical shift dispersion in two dimensions separates the signals of interest. This nmr method was used to show that hydrogenation of cholesteryl acetate (10) with deuterium gas and palladium catalyst exchanges the C-7 α-hydrogen to give 12. The detection limit is estimated at 10% deuterium per site unless 13C-labelled compounds are used.