Synthetic studies in sulfur heterocycles. One-pot synthesis of 'thioaurones' and their conversion into [1]benzothieno[3,2-b]pyrans via tandem reactions

Thioaurones were prepared one pot in high yield by the treatment of N,N-diethyl-2(methylsulfanyl)arylamides with LDA and an aromatic aldehyde through directed orthometalation. Heating the thioaurones derived from cinnamaldehyde above 200 oC resulted in [1]benzothieno[3,2-b]pyrans. Treatment of N,N-diethyl-2-(methylsulfanyl)benzamide and crotonaldehyde gave 4-(methylsulfanyl)benzothieno[3,2,-b]pyran via conjugate nucleophilic addition and ring closure in one pot. A possible mechanistic pathway is discussed.