Generation of 1,3-chalcogenaza-1,3-butadienes by thermal cycloreversion of2,4,6-trisubstituted 6H-1,3,5-oxachalcogenazines

1,3-Thiaza- and 1,3-selenaza-1,3-butadienes bearing several substituents at the C-2 and C-4 positions were generated through thermal cycloreversion of 6H-1,3,5-oxnthiazines or 6H-1,3,5-oxaselenazines, respectively, and the he terodienes were efficiently trapped by using acetylenic dienophiles. When 6 H-1,3,5-oxathiazines or 6H-1,3,5-oxaselenazines were heated in the presence of nucleophiles, such as alcohols or thiols, the corresponding 1,4-adducts of the heterodienes with the nucleophiles were obtained in good yields. On the other hand, heating of 6H-1,3,5-oxathiazines or 6H-1,3,5-oxaselenazine s in the absence of trapping agents afforded several products which origina ted from the in situ generated 1,3-chalcogenaza-1,3-butadienes; also the he terodienes were not isolated or observed directly as the monomeric forms at all.