Recently, the enormous increase in synthesis of heterocyclic compounds containing chalcogen and nitrogen atoms has been leading to develop new methods. In this way, generation and synthetic application of 1,3-chalcogenaza-1,3-butadienes and their oxidized variants would be versatile building blocks because of their heterocumulene-like structure and potential reactivities. Within this earnest demand, only limited findings have been realized because of the preparative difficulties of their precursors and lack of suitable trapping methods for these highly reactive species. In the course of studies of highly-reactive chalcogenocarbonyl functionalities bearing π-conjugation systems, several novel methods were developed for the conversion of 6H-1,3,5-oxachacogenazines into five-membered heterocyclic compounds.

Compounds 4 were efficiently converted into 1,2,4-thidiazoles 5 through plausible pathway involving acid-induced hydrolytic fragmentation of 4 and the subsequent self-condensation of intermediacy, thioamide S-oxide. Novel Conversion of 6H-1,3,5-Oxathiazine S-oxides 3 into 3H-1,2,4-Dithiazoles 6 by the treatment with LR When 3 was heated with Lawesson's reagent (LR) at toluene refluxing temperature, 3H-1,2,4-dithiazoles 6 were afforded in moderate to high yields. An NMR monitoring experiment of 3a with LR in CDCl₃ at 25 ⁰C was observed quantitative formation of pivalaldehyde along with a trace amount of 6a. This result suggests that the reaction of 3a with LR might initially form intermediacy B, and the subsequent thermal cycloreversion of B would form C and which finally give 6 by eliminating PSOAnis moiety (Scheme 2). On the other hand, when 3 was treated with LR in the presence of EtOH, deoxygenation products 1 were formed.

Synthesis of 3H-1,2,4-Dithiazoles, 3H-1,2,4-Thiaselenazoles and 3H-1,2,4-Diselenazoles
Five-membered heterocyclic compounds, 3H-1,2,4-dithiazoles 6 and 3H-1,2,4-thiaselenazoles 7 possessing S-S and S-Se bonds, were synthesized by the reaction of 1 with elemental sulfur or selenium, respectively. On the other hand, 3H-1,2,4-diselenazoles 9 were prepared by the reaction of 2 with elemental selenium.
It can propose that the formation pathway of compounds 6, 7, 9 involving in situ generation of heterodienes D or E through thermal cycloreversion of 1 or 2 followed by [4+1]-type cycloaddition of heterodiene with sulfur or selenium species. Although compound 3H-1,2,4-thiatellurazoles 8 could not isolate by the reaction of 1 with elemental tellurium, it was expected that compound 8 was transiently formed which subsequently converted into 3H-1,2,4-dithiazole 6 via Tellurium-Sulfur exchange. In the reaction of 2 with elemental sulfur, the formation of 5H -1,2,4-thiaselenazole 10 was suggested by NMR data. However, compound 10 could not isolate due to inseparable mixture with compounds 6 and 9.
Reactivity and synthetic utilities of compounds 6, 7, and 9 were also explored. Oxidation of compounds 6 by mCPBA afforded the corresponding 3H-1,2,4-dithiazole S -oxides 11 in which oxygen atom was selectively introduced onto S-2 sulfur. Moreover, the relative stereochemistry of S-O moiety with t-Butyl group in compounds 11 was trans-orientation. When　6, 7, and 9 were treated with Ph₃P at room temperature, the S-2 sulfur and Se-2 selenium were extruded to generate 1,3-thiaza-1,3-butadienes D and 1,3-selenaza-1,3-butadienes E, which were efficiently trapped by EtOH to give the corresponding 1,4-adducts 12 and 13, respectively.

In conclusion, a novel method was developed for the synthesis of 5H-1,2,4-oxathiazoles 4 through thermal cycloreversion of 6H-1,3,5-oxathiazine S-oxides 3. A novel conversion of 3 into 6 by the treatment of LR was realized. A wide range of 5-membered 1,2,4-dichalcogenazoles 6-10 were also synthesized by the reaction of in situ generated 1,3-chalcogenaza-1,3-butadienes with elemental chalcogen.