THERMAL OR LEWIS ACID-PROMOTED ELECTROCYCLIZATION AND HETERO-DIELS-ALDER CYCLOADDITION OF ALPHA,BETA-UNSATURATED (CONJUGATED) CARBODIIMIDES- A FACILE SYNTHESIS OF NITROGEN-CONTAINING HETEROCYCLES

alpha, beta-Diarylvinyl- and alpha-styryl-carbodiimides, prepared by t he aza-Wittig reaction of iminophosphoranes with isocyanates, underwen t either 6 pi-electrocyclisation upon heating or a Diels-Alder reactio n under thermal or Lewis acid-promoted conditions with appropriate die nophiles such as tetracyanoethylene, dimethyl acetylenedicarboxylate, maleimide, ethyl propiolate and tosyl isocyanate, to give isoquinoline s, pyridines, oxazines, pyrimidines and pyrrolopyridines. In contrast, beta-styrylcarbodiimide was entirely unreactive toward either the ele ctrocyclisation or the Diels-Alder reaction even under severe reaction conditions. 4-Coumarylcarbodiimide underwent an inverse electron-dema nd Diels-Alder reaction with an enamine either thermally or in the pre sence of a Lewis acid catalyst to afford chromenopyridines. Thus exper imentally observed reactivity differences of the substituted vinylcarb odiimides toward the pericyclic reactions were rationalised by conside ring not only the heats of formation but also the probability of the e xistence of reactive conformers that were estimated by semi-empirical (AM1) and molecular mechanics calculations.