A new synthesis of 5,6-dihydro-$\underline{N}$-phenyl-1,4-oxathiin-2-acetic acid derivatives $\underline{13}$ by ring expansion of the corresponding 1,3-oxathiolane-3-oxide $\underline{6}$ is described. In DMF at 100℃ the sulfoxide $\underline{6}$, in which S→O bond and 2-methyl group are cis to each other rearranged to sulfenic acid $\underline{10}$ involving the unactivated 2-methyl group, followed by cyclisation to dihydro-1,4-oxathiin $\underline{13}$. The ring opening of the sulfoxide $\underline{6}$ to form $\underline{10}$ occurs probably by [2,3] sigmatropic process at 50℃, but its conversion to the dihydrooxathiin $\underline{13}$ requried more elevated temperature, 100℃. Recyclisation of the sulfonic acid $\underline{10}$ under natural conditions took place stereospecifically to give parent sulfoxide $\underline{6}$. This was explained by considering a probable conformation of $\underline{10}$ and its transition state maintaining hydrogen bonding between S-OH and amide or ester group. Due to carbonyl inductive effect, the conversion of sulfoxide ester $\underline{6b}$ to sulfenic acid $\underline{10b}$ was faster than that of sulfoxide amide $\underline{6a}$, but the cyclisation of sulfenic acid $\underline{10b}$ to oxonium ion $\underline{11}$ (or carbonium ion $\underline{12}$) was slower than that of $\underline{10}$a. The structure of dihydrooxathiin $\underline{13}$ was also discussed in detail. Thus, structure $\underline{13}$ was distinguished from the alternative structure $\underline{14}$ by spectroscopic data, independent synthesis and isotope labelling experiment.
