A cascade reaction with iminium ion isomerization as the key step leading to tetrahydropyrimido[4,5-d]pyrimidines.

A novel cascade reaction of aminopyrimidines 1 with N-alkyl amino acids or analogues was investigated. The keys to this cascade are the isomerization of an iminium ion formed between the aldehyde group in pyrimidine and the secondary amine of an amino acid, and subsequent cyclization to the neighboring amino group. This sequence could be useful in the synthesis of novel tetrahydropyrimido[4,5-d]pyrimidine libraries.