Dithiins are a class of heterocyclic compounds, with the parent members having the formula (C<sub>2</sub>H<sub>2</sub>)<sub>2</sub>S<sub>2</sub>. Two isomers of this parent are recognized, 1,2- and 1,4-dithiins. Planar dithiins are 8àe<sup>âÂÂ</sup> systems, which would lead to antiaromaticity if the structure was planar. Akin to the behavior of cyclooctatetraene, they instead adopt nonplanar structures. Vinyldithiin, a common component of garlic, is a misnomer for 3-vinyl-4H-1,2-dithiin. 1,3-dithiins are unknown.
1,4-Dithiins have been more extensively studied. They are usually prepared by condensation of the equivalent of ñ-mercaptocarbonyls. For example, the acetal HSCH<sub>2</sub>CH(OEt)<sub>2</sub> converts upon heating to the parent 1,4-dithiin. They are nonplanar and can be oxidized to their radical cations. Photolysis leads to dimerization via a [2+2] cycloaddition. Thianthrene is dibenzo-1,4-dithiin.
1,2-Dithiins are isomers of but-2-ene-dithials. They tend to be unstable with respect to loss of sulfur and formation of the thiophene derivative:
They are often claret-colored. Some occur as flower pigments in plants of the family Asteraceae.