Reaction of aromatic N-oxides with dipolarophiles. III. Cycloadditions of substituted phenyl isocyanates to 3,5-dimethyl and 3,5-dibromopyridine N-oxides and X-ray crystal structures of the isomeric cycloadducts.

Abstract

Three β-substituted pyridine N-oxides (I, II and III) were subjected to 1, 3-dipolar cycloaddition with phenyl isocyanates having an ortho, meta or para substituent group. 3-Methyl- (I) and 3, 5-dimethyl-pyridine N-oxide (II) afforded the 2, 3-dihydropyridine derivatives, and the 3, 5-dibromo compound (III) afforded a 2, 3-dihydro-2-oxo-oxazolo-[4, 5-b] pyridine (IX) by the elimination of hydrogen bromide from the 2, 3-dihydropyridine thus formed. The presence of an o-substituent group or nitro group in the phenyl isocyanate resulted in a reduced cycloaddition yield. The structures of the 2, 3-dihydropyridine adducts were determined by X-ray crystallographic analysis.