NOVELTY - Preparing quinazolin-4(3H)-one derivatives (I), comprises carrying out oxidation of quinazoline derivatives (II) by using an oxidizing agent, where the oxidizing agent is a peroxide compound.    USE - (I) useful as intermediates for preparing 4-anilinoquinazoline and 4-phenoxyquinazoline anticancer drugs (claimed).    ADVANTAGE - The method: utilizes low cost and easily available raw materials, and selective oxidizing agent; and is environment friendly.    DETAILED DESCRIPTION - Preparing quinazolin-4(3H)-one derivatives of formula (I), comprises carrying out oxidation of quinazoline derivatives of formula (II) by using an oxidizing agent, where the oxidizing agent is a peroxide compound.    Either R1-R4 = H, halo, amino, hydroxyamino, hydroxy, ureido, N-(1-8C alkyl)amino, N,N-di-(1-8C alkyl)amino, pyrrolidin-1-yl, piperidin-1-yl, piperazin-1-yl, morpholin-4-yl, thiomorpholin-4-yl,4-alkyl-piperazin-1-yl, halo-alkylamino, hydroxyalkylamino, alkoxyalkylamino, carboxy-alkylamino, amino-alkylamino, 1-8C amido, 3-8C 2-alkenyl-amido, acrylamide group, amino-substituted 2-alkenylamide group, N,N-di-(1-8C alkyl)amino-substituted 2-alkenylamide group, benzamido, sulfonamido, benzene-sulfonamido, halo-alkylamido, hydroxy-alkylamido, alkoxy-alkylamido, amino-alkylamido, alkylamino-alkylamido or dialkylamino-alkylamide, 1-8C alkoxy, 3-8C cycloalkyloxy, halo-alkoxy, hydroxy-alkoxy, alkoxy-alkoxy, amino-alkoxy, alkylamino-alkoxy, dialkylamino-alkoxy, piperidin-1-yl-alkoxy, morpholin-1-yl-alkoxy, piperazin-1-yl-alkoxy, 4-alkyl-piperazin-1-yl-alkoxy, pyrrolidin-1-yl-alkoxy, difluoromethoxy, 1-8C hydrocarbyl or halo-hydrocarbyl, hydroxy-hydrocarbyl, alkoxy-hydrocarbyl, hydroxy-alkoxyhydrocarbyl, alkoxy-alkoxyhydrocarbyl, acyloxy-hydrocarbyl, amino-hydrocarbyl, alkylamino-hydrocarbyl, dialkylamino-hydrocarbyl, piperidinyl-hydrocarbyl, morpholinyl-hydrocarbyl, piperazinyl-hydrocarbyl, 4-alkyl-piperazinyl-hydrocarbyl, pyrrolidinyl-hydrocarbyl; or    R2R3 = 6-16 membered ring (optionally contains heteroatoms); and    hydrocarbyl = 1-8C alkyl, alkenyl or alkynyl.