The synthesis of N-[4-(2-hydroxyethyl)-1,2,4-oxathiazinan-3-ylidene]- benzamide (2a) and N-[4-(2-hydroxyethyl)-1,2,4-oxathiazinan-3-ylidene]-2- fluorobenzamide (2b) by oxidation of the corresponding 1,1-bis(2-hydroxyethyl)- 3-aroylthioureas with potassium iodate in aqueous solution is reported. Variable temperature 1H NMR spectra of 2a prove that the heterocyclic 1,2,4-(O, S,N) six-membered ring is involved in a dynamic chair-boat conformational interconversion. Molecular mechanic calculations show that the chair conformation is more stable than the boat conformation by 3.0 kcal/mol. The synthesis of the adduct [(2a)·0.5 HgCl2] 3 as well as the X-ray structural characterization of 2a and 3 are also reported. © 2005 Verlag der Zeitschrift für Naturforschung.