Synthesis, Structure, and Ring Conversion of 1,2-Dithiete and Related Compounds

3,4-Bis(methoxycarbonyl)-1,2-dithiete (6), 3,4,7,8-tetrakis(methoxycarbonyl)-1,2,5,6-tetrathiocin (8), and (Z,Z,Z,Z)-3,4,7,8,11,12,15,16-octakis(methoxycarbonyl)-1,2,5,6,9,10,13,14-octathiacyclohexadeca-3,7,11,15-tetraene (9) were synthesized by oxidation of titanocene dithiolene complex 4 with sulfuryl chloride, and the crystal structures of 6 and 8 were characterized by X-ray crystallographic analysis which revealed planar and twist geometries, respectively. Tetrathiocin 1, 11, and 12, and 16-membered cyclic compound 3 were also obtained by oxidation of the corresponding dithiolate or dithiolene complex. Dithiete 6 underwent tetramerization selectively to give 16-membered cyclic product 9 even at room temperature. Ring conversion reactions also proceeded among the 4-, 8-, and 16-membered unsaturated cyclic compounds possessing disulfide bonds under various conditions, such as in polar solvent or in the presence of silica gel. The ring-size selectivity of these ring conversion reactions was studied using ab initio molecular orbital calculations.