anhydro-2-(2-Allyloxyphenyl)-3,5-diphenyl-4-hydroxythiazolium hydroxide and related alkenic systems, together with the corresponding prop-2-ynyloxy and cyanomethyloxy derivatives, undergo ready thermal intramolecular 1,3-dipolar cycloadditions giving stable 1 : 1-cycloadducts, substituted 4H-thieno[3,2-c]-, and 4H-thiazolo[4,5-c]-chromenes, respectively; the structure of one of these has been determined by X-ray crystallography.