We are performing systematic studies aimed at the ap plication of novel reagents, such as selenium dichloride and dibromide, in the chemistry of organoselenium com pounds.1—6 Although these reagents cannot be isolated in pure state and undergo disproportionation in solutions7, it has been shown1 that freshly prepared selenium dichloride and dibromide can be used as selective electrophilic re agents in the synthesis of organoselenium compounds. The reactions of selenium dichloride and dibromide with acet ylene,2 diphenylacetylene,3 divinyl sulfide,4 divinyl sul fone,5 divinyl selenide,6 propargyl alcohols,8 2,3 di methoxybuta 1,3 diene9 have been studied. In contrast to selenium tetrahalides, whose reactions with organic sub stances often afford mixtures of products10, the reactions of selenium dihalides proceed selectively to afford the tar get compounds in high yields.1—6,8,9 The use of selenium dihalides for annulation of phenyl ethers containing unsaturated groups by combination of the addition to the unsaturated bond and electrophilic sub stitution in the ortho position of the benzene ring is of the utmost interest. There are no data on annulation of phenyl propargyl ether with selenium halides. We studied the reaction of selenium dichloride with phenyl propargyl ether and found the conditions that al low selective preparation of previously unknown E 3 chloromethylidene 2,3 dihydro 1,4 benzoxaselenine (1). The highest yield of product 1 (80%) was obtained upon mixing reagents at low temperature (–60 °C) in chloro form followed by heating the reaction mixture to the boil ing temperature of the solvent. One can assume that the addition of selenium dichlo ride to the propargyl group occurs at low temperature and the second step, viz., electrophilic aromatic substitution, requires heating of the reaction mixture. The structure of product 1 was established by 1H, 13C (including JMOD), and 77Se NMR spectroscopy and con firmed by the data from mass spectrometry and elemental analysis. The value of spin spin coupling constant (100 Hz) between the selenium atom and the double bond carbon atom having no protons corresponds to the C—Se direct constant (JC—Se), which is evidence of the addition of the selenium atom to the central carbon atom of the propargyl group. The E configuration of product 1 was established using the NOESY spectra. The regio and stereospecificity of the process can be explained by the fact that the reaction proceeds via the intermediate selenirenium ion А, which is attacked by the chloride ion at the sterically unshielded terminal car bon atom. Thus, we showed for the first time the possibility of annulation of the selenium containing heterocycles to the benzene ring based on the reaction of selenium dichloride with unsaturated organyl phenyl ethers, where the addi tion of selenium halides to the unsaturated bond of orga nyl phenyl ethers and electrophilic aromatic substitution is combined.