[4 + 21 Cycloadditions of Halogeno Thiazyls with 2-Chloropentafluoro-1,3-butadiene ' ' Reactions of (XSN), (n = 3, 1; X = C1, F) with 2-chloropentafluoro-1,3-butadiene provide the l)i4,2-thiazines 1 a and 1 b. Fluorination with KF converts l a in 88% yield to 1 b. CH30H reacts with 1 b to form the 1-methoxy derivative 1 c. The structure of 1 c was determined by single-crystal X-ray diffraction analysis. With water l a forms the 1,2-thiazin-3-one 1-oxide 2.Mit Hexafluorbutadien reagiert (XSN), (n = 3, 1; X = C1,