A new synthetic approach to the formation of 5-5,5-6,s-5-5, and 6-5-6 fused heterocycles employing cycloadditions with functionalized dipoles is described and its scope and utility explored through representative examples. The regiochemistry and stereochen~istry of the additions is established by specific deuterium labelling, n.m.r. spectroscopy, and by chemical means.On dCcrit une nouvelle approche synthetique a la formation des hetCrocycles condenses 5-5, 5-6, 5-5-5 et 6-5-6; la generalit6 et I'utilite de cette methode impliquant des cycloadditions de dipoles fonctionnalises est examine par l'intermidiaire d'exemples reprisentatifs. L a regiosClectivitC et la stereochimie des additions est Btablie i I'aide de marquage par le deuterium de faqon specifique, par r.m.n. et par des moyens chimiques.[ basic reaction to a scheme for the preparation of thermal decomposition of 3-aroylaziridines in linked heterocycles (2) as illustrated in eq. 2 via acetonitrile (3, 4) and in their reaction with consecutive condensation and 1,3-dipolar addi-p-nitrosophenols (4) when 3,5-dihydro-2H-pyrtion. A similar general synthetic scheme for the rolo[3,4-d]oxazoles were obtained in good yield.preparation of fused ring heterocycles would be a valuable extension of the general cycloaddition principle. One approach we wish to explore in this paper is the scope of the reactions of 1,3-dipoles bearing additional functionality (i.e. besides their basic 4n electron reactive system) to form bicyclic heterocycles, as shown in eq. 3. The additional functionalities f and g on the 1,3-dipole and dipolarophile, respectively, should give the potential second fused five-or sixmembered ring provided the reaction has the desired regiochemistry and stereochemistry. Indications of the usefulness of this approach had been obtained during an examination of the O = C H --COArOur initial approach to systematize these findings has been to examine the reaction of aroylazomethine ylides (generated by the thermal electrocyclic cleavage of aziridines) with a range of suitably functionalized dipolarophiles). Exploratory reactions of azomethine oxides (nitrones) are also discussed in this context. For personal use only.