Electrocyclic aromatic substitution by the diazo group. Part 4. Periselectivity studies on the cyclisation of 1-thienyl-3-diazoalkenes: a route to 3H-thieno[1,2]diazepines

Abstract

In the cyclisation reactions of the 1-thienyl-3-diazoalkenes (19) and (23) the thiophene ring is more reactive to 1,7 (8π-electron) electrocyclic substitution than the analogous arene ring in the 1-phenyl-3-diazoalkene (5). The 3,3-di-2-thienyl-3H-pyrazole (21b) undergoes thermal ring expansion to give the thienodiazepine (20b) in contrast to its diaryl analogue (4) which reacts by a primary [1,5] sigmatropic shift of the aryl group.