Preparation of a- l,2-Dithiol-3-ylidene carbonyl Compounds

Formation of the Dithiole Ring 1. From Pyran-i-thiones 

Pyran-4-thiones reacting with potassium hydrogen sulfide lead to a-(l,2-dithiol-3-ylidene) ketones and/or thiopyran-4-thiones (Eq. 

Thiopyran-4-thiones give adducts with mercury dichloride which, by reaction with aqueous sodium carbonate, give a mixture of a-(l,2-dithiol-3-ylidene)ketone and thiopyran-4-one (Eq. 2).9 10 

A better synthesis of a-(l,2-dithiol-3-ylidene)ketones from thio-pyran-4-thiones consists in the reaction with sodium hydroxide in dimethylformamide (DMF), followed by ferricyanide oxidation (Eq. 3).11 

At approximately 200°C, cinnamylidene malonic esters react with elemental sulfur and (l,2-dithiol-3-ylidene)malonic esters (27) are obtained (Eq. 4).14 15 

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