PURPOSE: To easily and economically obtain 1,3,7-trialkyl xantine processing a neuroleptic effect in high yield by allowing theopromine to react with alkylating agent using a phase-transfer catalyst.

CONSTITUTION: The target compound of formula [R2 is R or (1'- theopromyl(mthyl; R3 and R7 are each n-1-6C alkyl] containing new compound such as 1-isopropyl-3,7-dimethyl xantine, etc., is obtained by allowing theopromine to react with an alkylating agent represented the formula, R-X (R is n-1-6C alkyl, branching 3-4C alkyl, benzyl, t-butyl-benzyl or aryl; X is sulfonate, sulfate or halogen) in the presence of a phase-transfer catalyst of quaternary ammonium salt, etc., such as tetrabutylammonium bisufate, etc., and in the presence of excess solid alkaline metal hydroxide such as NaOH, etc., optionally using a solvent such as CH2CH2, etc., at a temp. of 20 to 100°C for 5-20 hrs.