Proton and carbon‐13 nuclear magnetic resonance studies of substituted pyrimidines. V  13 C, 1 H coupling contsants for some methyl‐ and aminopyrimidines

J ( 13 C 1 H) coupling constants for some methyl‐ and aminopyrimidines have been determined by 13 C NMR. Both the one‐bond and long‐bond and long‐range coupling constants follow general trends which can be summarized in a few simple rules. In particular, the 3 J (C‐i,H) coupling constants between a ring carbon C‐i and the ring protons are larger than the 2 J (C‐i,H) coupling constants. The opposite is observed for the couplings between the ring carbons and the methyl protons: 3 J (C,Me). These general rules are very useful for the assignment of resonances in complex 13 C spectra of pyrimidines and seem to be valid for other 6‐membered aromatic nitrogen heterocycles. Furthermore, the additivity of substituent effects on 1 J (CH) for monosubstituted pyrimidines allows the estimation of 1 J (CH) for polysubstituted pyrimidines with a very good accuracy.