Synthesis, Properties, and Reactions of 6a-Thiathiophthens

The synthesis of the thiathiophthens (13) from 4ZT-thiopyran-4-thiones (11), involving ring opening with sodium sulphide and oxidation of the resulting dianion (12) with ferricyanide, has been described in detail. The method provides a convenient route to the parent compound and succeeds with the symmetrically substituted thiopyrans (11 = Ph, 

Numerous routes to 1,2-dithiolylidene ketones (see p. 507), and hence to thiathiophthens, are described in a paper which is chiefly concerned with comparisons, particularly in relation to their electronic spectra, of 6a-thiathiophthens, the corresponding dithiolylidene ketones, and certain other structurally similar systems, for example (15 X = O or S), (16 X = O or S), and (17 X = O or S). From their survey of an extensive collection of spectroscopic data, the authors conclude that varying degrees 

In the same paper, the conversion of 6a-thiathiophthens into 1,2-dithiolylidene ketones is discussed. The use of sulphuric acid as a reagent for this purpose, with unsymmetrically substituted compounds, may lead to only one or both of the possible isomeric ketones. With 2-methyl-5-phenyl-6a-thiathiophthen, treatment with sulphuric acid gives the ketone (18), confirming an earlier report which had been questioned. 

Further and more definitive examples of electrophilic substitution reactions in the thiathiophthen series have been described. In the parent compound, in which both 2- and 3-positions are available for attack, formylation has been proved to occur at the 3-position, as would be anticipated from earlier experimental and theoretical work. 2-t-Butyl-6a-thiathiophthen formylates in the 4-position. The thiathiophthen aldehydes show characteristic absorption bands in the i.r. at ca. 1670 cm analogous to that shown by thiophen-2-aldehyde (1673 cm ), implying electronreleasing properties at the 3- (or 4-) position. 

Vilsmeier-Haack formylation of 2,5-diaryl-6a-thiathiophthens leads to the 3-formyl compounds, which are also obtainable from 3-aryl-l,2-dithiolium salts by treatment with triethylamine, and details have been given for the bromination of 2,5-disubstituted thiathiophthens. Nitration of 2-methylthio-5-phenyl-6a-thiathiophthen gives the 3-nitro-compound in poor yield, and attempted nitrosation experiments lead, in several examples, to rearranged structures of type (19), although the first stage of these reactions is presumed to be electrophilic attack on the 3-position. 

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