Acid-promoted reactions in 1-hydroxy, 1-dimethylaminomethyl and 1-methylene-4-arylmethyl-2,4-dihydro-1H-pyrazino[2,1-b]quinazoline-3,6-diones

The 1-dimethylaminomethyl or 1-methylene group of the title compounds was introduced through a Mannich or a tandem of Mannich–Hofmann reactions as the final step of a protocol that is shorter than other previously described for these precursors of N-acyliminium species. In these compounds, the acid-promoted intramolecular cyclizations of Pictet–Spengler-type were restricted to the N(2)-unsubstituted compounds, while their N(2)-methyl substituted analogues gave instead dimerization products. The cyclization was effective with 1-hydroxy-1,2-disubstituted compounds, which were obtained through addition of a Grignard reagent to 2H,4H-pyrazino[2,1-b]quinazoline-1,3,6-triones.