Synthesis of novel bicyclic nitrogen heterocycles by the intramolecular dipolar cycloaddition reaction of nitrones with allenes and alkynes

Abstract

A study of the intramolecular dipolar cycloaddition reactions of a series of allenyl and alkynyl nitrones has been carried out. Phenylhydroxylamine readily reacts with o-(1,2-propadienyloxy)benzaldehyde to give a dioxaazabicyclo[3.2.1]octene. The intramolecular 3+2-cycloaddition of 2-(1,2-propynyloxy)benzaldehyde and N-methylhydroxylamine was also studied. In this case, the major cycloadduct formed was identified as 4-methyl-2,5- endo-oxo-2,3,4,5-tetrahydro[1,4]benzoxazepin. The formation of this novel rearranged product involves an initial dipolar cycloaddition across the acetylenic π-bond to give the expected 4-isoxazoline as a transient intermediate. The next step proceeds by homolysis of the O-N linkage and this is followed by ring closure to produce an aziridine which undergoes further C-C bond cleavage. The resulting azomethine ylide is converted to the final rearranged product by means of an ortho-quinone methide intermediate. Phenylsulfonyl activated allenes were also found to react with alkylhydroxylamines to give nitrones which undergo cycloaddition with the neighboring π-bond to afford the resulting bicyclic nitrogen heterocycles.