A new synthesis of 5,6-dihydro-2-methyl-1,4-dithiins-3-carboxylic acid derivative $\underline{11a}$ and 5,6-dihydro-1,4-dithiin-3-acetic acid derivatives $\underline{12}$ by ring expansion of the corresponding 1,3-dithiolane-1-oxides $\underline{7a}$, $\underline{8}$ has been achieved. Structural assignments of the cis and trans isomers were made by $^1H$ NMR spectroscopy including aromatic solvent induced shift (ASIS) data. Disulfoxides $\underline{18a}$, $\underline{19a}$, $\underline{20a}$ and $\underline{20b}$ as byproducts were also formed by oxidation of 1,3-dithiolanes. In refluxing toluene the cis sulfoxide $\underline{7a}$ rearranged to sulfenic acid $\underline{9a}$, followed by cyclization to dihydro-1,4-dithiin $\underline{11a}$ in a high yield. In refluxing 1:1 DMF-$C_6H_6$ the trans sulfoxides $\underline{8}$ were converted to isomeric dihydrodithiins $\underline{12}$ in good yields via sulfenic acids $\underline{10}$. The ring opening of cis and trans sulfoxides $\underline{7a}$ and $\underline{8}$ to form respective $\underline{9a}$ and $\underline{10}$ occurs probably by [2,3] sigmatropic process involving 2-methylene hydrogens in the cis isomer and 2-methyl hydrogens in the trans isomer. With PTSA as a catalyst in benzene at reflux the cis sulfoxide $\underline{7a}$ readily afforded dihydro-1,4-dithiin $\underline{11a}$ in quantitative yield, while the trans sulfoxides $\underline{8}$ gave dihydrodithiins $\underline{11}$ and isomeric dihydrodithiins $\underline{12}$ in the ratio of 4:1 for amides and 1:5 for esters. In the presence of acid catalyst, the ring opening of the cis sulfoxide $\underline{7a}$ takes place presumably by stepwise mechanism via protonated sulfoxide $\underline{13a}$. The acid catalysed ring opening of the trans sulfoxides may proceed by stepwise mechanism involving 2-methylene hydrogens and by [2,3] sigmatropic rearrangement with 2-methyl hydrogens. Due to carbonyl inductive effect, the conversion of sulfoxide ester $\underline{8b}$ to sulfenic acid $\underline{10b}$ was faster than that of sulfoxide amide $\underline{8a}$, but the cyclization of sulfenic acid $\underline{10b}$ to form dihydrodithiin $\underline{12b}$ was slower than that of $\underline{10a}$. The structures of dihydrodithiins $\underline{11a}$ and $\underline{12}$ were identified by $^1H$ NMR spectroscopy and by independent synthesis.
