Thermal or Lewis acid-promoted electrocyclisation and hetero Diels–Alder cycloaddition of α,β-unsaturated (conjugated) carbodiimides: a facile synthesis of nitrogen-containing heterocycles
β-styrylcarbodiimide was entirely unreactive toward either the electrocyclisation or the Diels–Alderreaction even under severe reaction conditions. 4-Coumarylcarbodiimide underwent an inverse electron-demand Diels–Alderreaction with an enamine either thermally or in the presence of a Lewis acid catalyst to afford chromenopyridines. Thus experimentally observed reactivity differences of the substituted