Abstract
Two modes of cationic cyclization reactions are described: the direct and rearrangement cyclization. In the direct cyclization, products result from olefin capture of partially positive nitrogen; and in the rearrangement cyclization, products result from olefin capture of a nitrilium ion produced by an initial molecular rearrangement of the imine functionality.In order to outline general conditions directed towards the construction of aliphatic nitrogen heterocycles, several types of alkenyloximes were prepared. Specifically, olefin functionalities with alkyl, alkoxy, aryl and halogeno substituents; and imine functionalities bearing hydroxy, alkoxy and disubstituted amino groups were synthesized.The direct cyclization reaction has been found to be of limited usefulness in organic synthesis. The best results are obtained with unsubstituted alkenyloximes using zinc metal in refluxing glacial acetic acid.By contrast, the rearrangement cyclization has been found to be broad in scope and stereospecific in design. Five member ring nitrogen heterocycles are formed in moderate to high yields using various aprotic acids (phosphorous pentoxide, phosphorous pentachloride, aluminum chloride, stannic chloride and zinc bromide) in several anhydrous solvents (toluene, diethyl ether, glyme, nitromethane, acetonitrile, carbon tetrachloride and ethylene chloride).Also described are synthetic methods to functionalize five membered nitrogen rings by a regioselective alkylation-stereospecific cyclization sequence.