Highly efficient one-pot synthesis of 1-substituted-1,2,3,4-tetrahydropyrazino[1,2-a]indoles

A practical and general one-pot synthesis of 1-substituted-10-methyl-1,2,3,4-tetrahydropyrazino[1,2-a]indoles is described. The approach uses 2-(3-methyl-1H-indol-1-yl) ethylamine, benzotriazole and aldehydes in the presence of catalytic amount of acid catalysts (AlCl3, ZnCl2, ZnBr2, p-TsOH, CH3SO3H) and proceeds in high yields via iminium cation intramolecular cyclization. The mechanism of the observed intramolecular cyclization reaction has been investigated theoretically by means of PM3 semiempirical method and results were consistent with the experimental results.