15N‐NMR. Studies of Aminopyridines, Aminopyrimidines and of Some Diazine N‐Oxides

Abstract 15N‐NMR. spectra of mono‐ and diaminopyridines, and mono‐, di‐ and triaminopyrimidines including trimethoprim and other dihydrofolate reductase inhibitors have been studied in neutral and acidic media. Complete chemical shift assignments are given. Ring‐nitrogen shifts are discussed in terms of β‐, χ‐ and δ‐substituent effects of amino and alkyl groups. Protonation states in TFA‐ and FSO3H‐solution and protonation increments for the 15N‐shifts of ring and amino N‐atoms are determined. A linear correlation is observed between amino substituent effects (Δδ(15N)) on the ring N‐atom in aminopyridines and corresponding Δδ (13C) values in aminobenzenes and, similarly, between Δδ(15N) values in aminopyrimidines and Δδ(13C) values in aminopyridines. Assignment of the 15N‐NMR. spectra of pyrimidine N‐oxides, pyrazine N‐oxides and pyridazine N‐oxides is achieved by comparison with 14N‐NMR. data and with the aid of Yb(fod)3‐induced shifts. One‐bond 15N, 1H‐coupling constants are reported for aminopyridines and aminopyrimidines and discussed in terms of conjugative interaction between NH2‐group and ring system.