I , 9-Bis(methylthio)dibenzothiophene (la) was treated with one equivalent of bromine and pyridine i n the presence of 1-menthol and then with aqueous sodium hydroxide to give optically active I-(methylsulfinyl)-9-(methy1thio)dibenzothiophene (2a) enriched by the S isomer (ee: 57%). The configuration of optically pure sulfoxide (2a) was determined by X-ray crystallographic analysis to be the S configuration at the sulfinyl sulfur atom. On the other hand, I-(methyl-Gmenthoxysulfonio)-9-(methy1thio)dibenzothiophene tetrafluoroborate (4a) was isolated as a n intermediate of this asymmetric oxidation in an optically pure form, as yellow crystals. The absolute configuration of this sulfonium salt (4a) was verified by X-ray crystallographic analysis as the R configuration. Optically pure sulfonium salt (4a) also gave partially optically active sulfoxide (2a) with net inversion on its hydrolysis. It was suggested that the hydrolysis reaction of the sulfonium salt (4a) accordingly proceeds, not only via a sulfurane having a simple S, 2 type of geometry but also by a front side attack of hydroxide anion, with respect to the 1-menthoxy group, on sulfur, and the sequential elimination of the I-menthoxy group from the tetracoordinated intermediate. 0 I996 John Wiley & Sons, Inc.