Flash vacuum thermolysis (FVT) of 6-aryl-1,3-dioxine-4-thiones 9 leads to t
he formation of acylthioketenes 10, which are characterized by Ar matrix IR
spectroscopy as well as on-line tandem mass spectrometry. The thioketenes
10 undergo a 1,3-shift of the aryl group to generate thioacylketenes II. Ke
tenes 11 cyclize to 3-aryl-thiet-2-ones 12, which are also characterized by
matrix IR spectroscopy and tandem mass spectrometry. The thiet-2-ones 12 u
ndergo two kinds of reaction under the FVT conditions: (i) cheletropic CO e
xtrusion with formation of arylthioketenes 13. and (ii) cycloreversion to C
OS and arylacetylene.