Denitrative Formal [5+1] Cycloaddition of 1‐(2‐Nitrophenyl)‐2‐Yn‐1‐Ones with Thioacetamide under Transition‐Metal‐Free Conditions: Synthesis of Thiochromen‐4‐Ones

A new strategy for thiochromen‐4‐one synthesis that relies on the denitrative formal [5+1] cycloaddition of 1‐(2‐nitrophenyl)‐2‐yn‐1‐ones with thioacetamide is described. Achieved under reaction conditions that did not require transition metal catalysis, nor the exclusion of air or moisture, the reaction is suggested to involve exploiting the propensity of thioacetamide to undergo hydrolysis under basic conditions. This is follows by addition of the ensuing sulfide anion to the 1,3‐alkynone and a nucleophilic aromatic substitution and denitrative rearomatization cascade to give the S‐heterocyclic product in yields of 41–99 %. Thiochromen‐4‐ones were synthesized by the denitrative formal [5+1] cycloaddition of 1‐(2‐nitrophenyl)‐2‐yn‐1‐ones with thioacetamide without the need for transition‐metal catalysis or the exclusion of air or moisture. The reaction is thought to exploit the propensity of thioacetamide to undergo hydrolysis under basic conditions. A nucleophilic aromatic substitution and denitrative rearomatization cascade gave the S‐heterocyclic products in yields of 41–99 %.