Peri-and regio-selectivity in cycloaddition reactions of 1-substituted pyridinium-3-oxide

1-methyl- and 1-phenylpyridinium-3-oxides [Ia and Ib] reacted with benzalacetophenone [II] to yield three types of bicyclic adducts. The first proceeded via 2 pi + 4 pi cycloaddition across the 2 6-positions of the pyridine ring to give single regioisomers in favor of endo-6-substituted azabicyclo [3.2.1] octenones [III] and [IV]. The second involved 2 pi + 8 pi, by addition across the C[4]-O, to give the dihydrofuro [2,3-c] pyridine [VII], and the involved C[2]-O to give substituted dihydrofuran V and VI. 1-[4,6-dimethylpyrimidin-2-yl] pyridinium-3-oxide [Ic] reacted with 4-nitrophenylisothiocyanate by addition across the C[2]-O to give the oxazole-2-thione derivative X. Structural assignments were deduced by elemental analysis and spectral evidence