Synthesis of 6a-Thiathiophthens

Two methods have been recommended for the preparati m of the parent member of the series. -y-Pyrone is converted, via the thione (11) and the dianion (12), into the dithiolylidene aldehyde, l-oxa-6,6a-dithiapentalene (13), and the sequence is completed by treatment with phosphorus pentasulphide (overall yield from the readily accessible -y-pyrone is 12%). In the second method, the mixture of acids (14), obtained by addition of S 

The Vilsmeier salt synthesis has been extended to the preparation of 2-methylthio-6a-thiathiophthens (15 R = H or Me). Full details have been published of the route to dithiolylidene ketones and thiathiophthens by way of 3-phenacylthio-l,2-dithiolium salts and related compounds. The 2-dimethylamino-6a-thiathiophthen (16), prepared by this method, is also obtained when the dithiolium salt (17) is treated with phosphorus pentasulphide, and the resulting mixture of isothiathiophthens (18 R = H, R = Ph) and (18 R = Ph, R = H) heated with thioacetamide. 

Difficulties have been encountered in attempts to prepare dibenzo- and dinaphtho-6a-thiathiophthen. The tetrahydro-derivative (19) could not be dehydrogenated, and the dithiolylidene ketone (20) was not converted into the corresponding thiathiophthen with phosphorus pentasulphide. Compounds (21 R = Me or Ph) and (22), in which the thiathiophthen system is 

Perhaps the most intriguing of the new structures which have been reported is the tris-dithiolethione (26), with the molecular formula C9S9. This structure is assigned to the highly insoluble product obtained when the mesitylene derivative (25) is heated with sulphur in dimethylformamide. 

A systematic study has been made of the mass spectra of 6a-thiathio-phthens. Common features are the appearance of intense molecular ion peaks, and prominent peaks corresponding to the loss of hydrogen, and of sulphur and hydrogen. 

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Details have been given for the synthesis of 6a-thiathiophthens (see Chapter 8) from 4 -thiopyran-4-thiones. Analogous reactions with 4H-thiopyran-4-selenones lead to 6a-selenathiophthens. ... 

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