Preparation and Conformational Analysis of 6,10-Diethyl[1,2,3]trithiolo- [4,5-h]benzopentathiepin Monoxides

6,10-Diethyl[1,2,3]trithiolo[4,5-h]benzopentathiepin (1) was oxidized by mCPBA in dichloromethane to produce the corresponding four monoxides. The structures of the monoxides 2, 3, 4, and 5 were determined by X-ray crystallography to be 6,10-diethyl[1,2,3]trithiolo[4,5-h]benzopentathiepin 8-oxides (1,2,3-trithiole 2-oxides) for 2 and 3, and 6,10-diethyl[1,2,3]trithiolo[4,5-h]benzopentathiepin 7-oxides (1,2,3-trithiole 1-oxides) for 4 and 5, respectively. Compounds 2 and 3 are isomers with respect to the conformation of the 1,2,3,4,5-pentathiepin ring, and 4 and 5 are also the conformational isomers. These compound pair members, 2 and 3, and 4 and 5, were found to isomerize each other by inversion of the pentathiepin ring. The activation parameters of the isomerization, δG298≠, δH≠, and δS≠, were determined by 1H NMR spectroscopy. Because of the slow inversion of the pentathiepin ring, isolation of unsymmetrically substituted benzopentathiepin as a chiral molecule is possible. Asymmetric oxidation of 1 was performed by a Sharpless reagent [1/Ti(OiPr)4/R,R-DET/t-BuOOH = 1 : 2 : 4 : 4] to produce optically active monoxides 4a and 5a. The configuration of 4a and 5a was confirmed as R configuration on the sulfinyl sulfur atom, respectively. The specific rotation and the circular dichroism spectra of 4a and 5a were measured in chloroform; such data are apparently affected by the conformation of the pentathiepin ring.