Synthesis of new 5,6-dihydro-1,4-oxathiin derivatives $\underline{22}$ and $\underline{23b}$ has been achieved by ring expansion of the corresponding 1,3-oxathiolane-3-oxides $\underline{16}$ and $\underline{17}$ and by chlorinolysis of the corresponding 1,3-oxathiolane $\underline{14b}$. Structural assignments of the cis and trans isomers were made by $^1H$ NMR spectroscopy including ASIS data. In DMF at $100^\circ{C}$ the cis sulfoxide 16a rearranged to sulfenic acid $\underline{18a}$, followed by cyclization to dihydro-1,4-oxathiin 22a in good yield, while the trans isomer $\underline{17a}$ gave $\underline{22a}$ in fair yield. The ring opening of cis and trans sulfoxides $\underline{16a}$ and $\underline{17a}$ occurs probably by [2,3]-sigmatropic process involving 2-methylene group in the cis isomer and 2-methyl group in the trans sulfoxide to give sulfenic acids $\underline{18a}$ and $\underline{19a}$, respectively. It was foudn that sulfenic acid $\underline{19a}$ was isomerized to $\underline{18a}$ to give only $\underline{22a}$. The maxiture of cis and trans sulfoxides $\underline{16b}$ and $\underline{17b}$ (47:53), which was not separated from each other, gave phthalimido oxathiins $\underline{22b}$ and $\underline{23b}$ in the ratio of 9:37 (\%) in DMF at $100^\circ{C}$. But with PTSA as a catalyst in benzene at reflux this mixture afforded only $\underline{23b}$ as minor product. The acid catalyzed ring opening of the sulfoxides $\underline{16b}$ and $\underline{17b}$ may proceed by B -elimination of a methyl proton which is effected by the amide nitrogen or carbonyl oxygen to give sulfenic acid $\underline{19b}$. By the action of chlorine on 1,3-oxathiolane $\underline{14b}$ in methylene chloridecarbon tetrachloride solution oxathiins $\underline{22b}$ and $\underline{23b}$ were produced in the ratio fo 10:80 (%). From the initially formed chlorosulfonium salt $\underline{39}$ unobserved transient sulfenyl chlorides $\underline{40}$ and $\underline{41}$ were generated, followed by cyclization to probable oxonium ions $\underline{42}$ and $\underline{43}$ to give dihydro-1,4-oxathiins $\underline{22b}$ and $\underline{23b}$. The structure of oxathiins $\underline{22a}$, $\underline{22b}$ and $\underline{23b}$ was elucidated by $^1H$ NMR spectroscopy and by independent synthesis.
