Analysis of periodicals has showed that there is no information on the behavior of hybrid heterocyclic systems containing several pharmacophore fragments based on oxazol-5(4H)-ones and chromen-4(4H)-ones in reactions with thionizing reagents under various conditions. The interaction of 4-((4-oxo-4H-chromen-3-yl)methylene)-2-phenyloxazol-5(4H)-one with Lawesson’s reagent (LR) (2,4-bis-[pmethoxyphenyl]) has been studied for the first time – 1,3- dithiaphosphetane-2,4-disulfide) under conditions of thermal activation of the reaction mixture and the use of a closed reactor in the environment of non-polar solvents. Lawesson’s reagent is used as a mild thioniation agent. The scheme of the interaction has been discussed. Initially, it is assumed that the Lawesson reagent (LR) molecule dissociates into the particles of ylide structure, then the interaction with the carbonyl group of the chromen-4-one fragment of the initial substrate takes place, resulting in the formation of a spirocyclic intermediate, the subsequent decomposition of which produces the final product. It has been established that the use of a closed reactor makes it possible to reduce the transformation time and achieve an increase in the yield of the target product compared to the conventional type of activation of the reaction mixture. It has been shown that under the chosen conditions the transformation proceeds with the preservation of the oxazol-5(4H)-one ring. The composition and structure of the resulting compound have been established on the basis of complex data from elemental analysis, IR and NMR spectroscopy.