Abstract
Several aryl and functionally substituted enamines reacted with ethyl cyanoacetate and elemental sulfur to form 2-aminothiophene-3-carboxylic acid derivatives that proved to be excellent precursors for a variety of thiophenes.
Highlights
In 1966 Gewald[1] reported that aliphatic ketones, aldehydes or 1,3-dicarbonyl compounds react with activated nitriles and sulfur in the presence of a base at room temperature to give 2aminothiophenes.[1,2,3,4]
Since aminothiophenes are important intermediates in dye preparations,[5,6] in the pharmaceutical industry and as a precursors for other thiophenes employed in several high technology applications,[7] this methodology has been extensively utilized for the preparation of a variety of aminothiophenes via reacting ketones and aldehydes with active methylene carbonitriles and sulfur in presence of a base.[8,9,10,11]
As β-functionalized aldehydes are rather unstable compounds we considered the possible use of functionally substituted enamines as their synthetic equivalents
Summary
In 1966 Gewald[1] reported that aliphatic ketones, aldehydes or 1,3-dicarbonyl compounds react with activated nitriles and sulfur in the presence of a base at room temperature to give 2aminothiophenes.[1,2,3,4] Since aminothiophenes are important intermediates in dye preparations,[5,6] in the pharmaceutical industry and as a precursors for other thiophenes employed in several high technology applications,[7] this methodology has been extensively utilized for the preparation of a variety of aminothiophenes via reacting ketones and aldehydes with active methylene carbonitriles and sulfur in presence of a base.[8,9,10,11] In the context of our interest in aminothiophenes as precursors to arylazo dyes[12,13] we became interested in 2-amino-4unsubstituted thiophenes with a functional substituent at C-5. 3b-e reacted smoothly with elemental sulfur and ethyl cyanoacetate in the presence of equivalent amounts of piperidine (cf Experimental Section) to yield amino thiophenes 6b-e, 6b in 70%, 6c, 73%, 6d, 76% and 6e, 79% yields.
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