Thermal or Lewis acid-promoted electrocyclisation and hetero Diels–Alder cycloaddition of α,β-unsaturated (conjugated) carbodiimides: a facile synthesis of nitrogen-containing heterocycles

Abstract

α,β-Diarylvinyl- and α-styryl-carbodiimides, prepared by the aza-Wittig reaction of iminophosphoranes with isocyanates, underwent either 6π-electrocyclisation upon heating or a Diels–Alder reaction under thermal or Lewis acid-promoted conditions with appropriate dienophiles such as tetracyanoethylene, dimethyl acetylenedicarboxylate, maleimide, ethyl propiolate and tosyl isocyanate, to give isoquinolines, pyridines, oxazines, pyrimidines and pyrrolopyridines. In contrast, β-styrylcarbodiimide was entirely unreactive toward either the electrocyclisation or the Diels–Alder reaction even under severe reaction conditions. 4-Coumarylcarbodiimide underwent an inverse electron-demand Diels–Alder reaction 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 vinylcarbodiimides toward the pericyclic reactions were rationalised by considering not only the heats of formation but also the probability of the existence of reactive conformers that were estimated by semi-empirical (AM1) and molecular mechanics calculations.