Dithiins and Related Compounds

4- Dithiins and Related Compounds.—Diacetylenic sulphides react with Na2X (X = S, Se, or Te) to give the mono- and di-substituted heterocycles (24). Other syntheses of 1,4-dithiins from acetylenes are the reactions of amidines of phenylpropiolic acid with sulphur (or selenium) in DMF, and of dimethyl acetylenedicarboxylate with hydrogen bromide in liquid sulphur dioxide. In the latter reaction, sulphuric acid is formed by disproportionation. 

3-Amino-2-thiocyanatocyclohex-2-enone, on treatment with acid, gave the dithiin (27). Derivatives of acyclic ketones yielded different products. The dithiin 1,1-dioxide (28) has been prepared by treatment of the anion of dibenzyl sulphone with two equivalents of carbon disulphide followed by methylation. 

The structure of an alleged stereoisomer of 2,3 5,6-bis(trimethylene)-l,4-dithiin has been revised to that of the positional isomer (29), thus disproving the existence of stereoisomers due to ring inversion in doubly co-ordinated sulphur compounds. 

Several more syntheses of dihydro-1,4-dithiins by ring expansion of ethylene-thioketals or the derived -monoxides have been reported. One of these involves extensive rearrangement of a steroid. 

Numerous derivatives of (30) have been prepared and shown to display a wide spectrum of biological activity, e.g. acaricidal, sedative, etc. Further examples may be found in the patent literature. 

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