Synthesis of polycyclic and 4,5-diacylthiophene-2-carboxylates via intramolecular Friedel–Crafts alkylations and unusual autooxidative fragmentation of the derivatives obtained from the samarium diiodide-promoted coupling reactions of thiophene-2-carboxyl

Our present study provides an expedient method for the synthesis of novel polycyclic and multi-substituted thiophene derivatives. A series of 4,5-di(hydroxyalkyl)-4,5-dihydrothiophene-2-carboxylates (e.g., 4a–c and 10) were prepared by the SmI2-promoted three-component coupling reactions of thiophene-2-carboxylate with aromatic aldehydes and 4-methoxyacetophenone. Diol 4a was oxidized by DDQ or pyridinium dichromate to give 5-acyl-4-hydroxyalkyl-4,5-dihydrothiophene-2-carboxylate 6a, which was subjected to dehydration to give either alkene 7 with terminal Cdouble bond; length as m-dashC double bond or alkene 15a having conjugation with the ester group, depending on the reaction conditions using different quantities of p-toluenesulfonic acid. Alkene 7 underwent an intramolecular Friedel–Crafts alkylation to give a tetralone-fused thiophene-2-carboxylate 9. By the similar procedure, a carbazole-fused thiophene 14 was also prepared. Alkenes 15a–c underwent autooxidative fragmentation to give 4,5-diacylthiophene-2-carboxylates 5a–c that were elaborated to pyridazine-fused thiophenes.