Structural studies on bio-active compounds. Part 6. Determination of the sites of protonation on three 2,4-diaminopyrimidines of pharmaceutical importance by proton-coupled 13C and 1H nuclear magnetic resonance spectroscopy

Abstract

The protonation of three 2,4-diaminopyrimidines of interest as antitumour agents has been studied by proton-coupled ¹³C n.m.r. and by ¹H n.m.r. spectroscopy in solution in dimethyl sulphoxide. Assignment of ¹³C resonances was achieved through chemical-shift considerations and through ¹³C–¹H coupling patterns, supported in one case by a DEPT experiment and by heteronuclear decoupling by selective irradiation in the proton spectrum. In each case, first protonation occurs at pyrimidine N-1 with the second, much weaker, basic site of 2,4-diamino-5-(3-amino-4-chlorophenyl)-6-ethylpyrimidine being at the aniline amino group. The observations are consistent with current hypotheses of structural requirements for inhibitory activity against dihydrofolate reductase.