Complex tauto‐ and rotamerism of 2‐(R‐phenyl)‐1,2,3,4‐tetrahydroquinazolines

Detailed NMR spectral analysis of CDCl3 solutions of 2‐(R‐phenyl)‐1,2,3,4‐tetrahydroquinazolines reveals three or four tautomeric forms. Apart from 2‐[(benzylideneamino)methyl]aniline, the other chain tautomeric forms are present only in minor quantities. In general, electron‐donating substituents increase the contribution of all chain forms. Lowering the temperature of the CDCl3 solution of 2‐(R‐phenyl)‐1,2,3,4‐tetrahydroquinazolines decreases the content of the 2‐[(benzylideneamino)methyl]aniline form. At the same time, the amount of the ring form increases. Opening of the tetrahydropyrimidine ring in 2‐(R‐phenyl)‐1,2,3,4‐tetrahydroquinazolines was found to be an endothermic process especially for less electron‐donating substituents.