α-Oxo-ketenes have been shown to be very useful intermediates in organic synthesis and their physico-chemical properties were studied from theoretical as well as experimental points of view [1]. In contrast, relatively little is known about their sulfur analogous [2], which should be interesting in the thio-heterocyclic synthesis. We focused our attention on the thermal decomposition of thiones of selected N-, O-, and S- heterocycles under FVP-conditions. In the case of six-membered 4H-3,1-benzoxatthiin-4-thione 1 the course of the reaction depends mainly on the substitution pattern at C(2). For the non-substituted derivative 1a, the reaction led to the unstable product, which upon treatment with MeOH was converted quantitatively into methyl ortho-mercaptobenzoate.

An analogous reaction with the 2,2-dimethyl derivative 1b yielded 2-methylthiochromen-4-thione as an exclusive product. A mixture of three different compounds was identified as a result of benzo[e][1,3]thiazine-4-thione 2 thermolysis. Two of them are supposed to be formed by dimerization of initially generated 6-(thioxomethylene)cyclohexsa-2,4-dienthione. UV-Photoelectron Spectroscopy (PES) in conjunction with quantum calculations were applied to provide fundamental information about the structures and bonding of highly reactive intermediates formed during these transformations and to elucidate the reaction pathways in the gas phase.

[1] a) C. Wentrup, W. Heilmayer, G. Kollenz, Synthesis 1994, 1219. b) G. I. Yranzo, J. Elguero, R. Flammang, C. Wentrup, Eur. J. Org. Chem. 2001, 2209.

[2] T. Drewnowski, S. Leśniak, G. Mlostoń, R. Siedlecka, J. Skarżewski, Helvetica Chim. Acta in press.

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