Carbon-13 nuclear magnetic resonance spectra of 5-alkyl-5-(1-methylbutyl)barbituric acids

Abstract

The natural-abundance ¹³C n.m.r. spectra of 5-alkyl-5-(1-methylbutyl)barbituric acids have been obtained. All resonances were assigned by chemical shift comparisons including the analysis of deuteriated analogues and the use of single-frequency, off-resonance decoupling. Chemical shift nonequivalence was observed between the carbonyl C-4 and C-6 atoms. This chemical shift difference is very sensitive to changes in the 5-alkyl substituent, thus indicating that ¹³C n.m.r. spectroscopy should be of considerable value in detecting steric effects and conformational changes in other optically active 5,5-dialkylbarbituric acids.