Syntheses of Thiophens from other Ring Systems

Syntheses of Thiophens from other Ring Systems.—A new synthesis of thiophens starts from meso-ionic 1,3-dithiolones (65)—(67) (see Chapter 8), which as illustrated by resonance hybrid (66) can be considered as thiocarbonyl ylides. They 

An adduct of this type (77) reacted with dimethyl acetylenedicarboxylate at room temperature by a 1,3-dipolar addition to yield the unstable intermediate (78), which in a very interesting fragmentation gave diphenylcyclopropene (79) 

Syntheses of Thiophens from other Ring Systems.— There has been continued interest in the use of 1,3-oxathiazolium salts for the synthesis of thiophens in high yields. From the 2-aryl-substituted derivatives (56), 3-amino-thiophens (57) are obtained in the reaction with malononitrile in the presence of base, and 3-hydroxythiophens (58) and (59) with methyl cyanoacetate and 

The 2-dialkylamino-1,3-oxathiazolium salts (62) react with -diketones such as acetylacetone or benzoylacetone, or malononitrile and other active-methylene derivatives in the presence of bases to give thiophens with 

The condensation product from thiacyclopentan-3-one and methyl cyanoacetate (64) yields, in the base-catalysed reaction with sulphur, either (65) or (66) depending upon reaction temperature. Compound (67), prepared through base-catalysed condensation between acrylonitrile and ethyl thioglycolate, was condensed with anilines to (68) and aromatized to (69) with chloranil.  

Photolysis of the thiadiazoles (70) leads to the thiophens (71) as main product, in addition to smaller amounts of (72) in the reaction of (70a) and (70c). It was shown that (72) was not formed by photochemical rearrangement of (71). It is assumed that (71) is formed by dimerization of the biradical (73) to (74), followed by sulphur extrusion, while (72) is formed uia the dimer (75). 

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