The synthesis of 1H- and 5H-thieno[1,2]diazepines by the electrocyclisation of α-(2-alkenylthienyl)diazoalkanes, and some observations on their photochemical reactivity and ring inversion

Abstract

The cyclisation of the α-(2-alkenylthienyl)diazoalkanes (4) and (5) provides the first route to thieno[3,2-d]- and thieno[2,3-d]-[1,2]diazepines, (10) and (11) respectively. In contrast, 3-diazomethyl-4-(trans-2-phenylethenyl)thiophen (6) did not cyclise but gave carbene-derived products. The thieno[3H-1,2]diazepines (10) and (11) were converted by base into the isomeric thieno[4H-1,2]-diazepines (15) and (16) respectively and by u.v. irradiation into the diazeto[1,4-a]thieno[c]pyrroles (21) and (22). Variable-temperature proton n.m.r. studies have shown that the energy barrier to ring inversion is lower for the thienodiazepines than for analogous benzodiazepines.