Dearomative (3 + 2) and (4 + 2) Cycloadditions of [1,2,4]Triazolo[1,5- <i>a</i> ]pyridines: Rapid Access to Polycyclic Triazole-Fused Skeletons

1,3-Dipolar cycloaddition of nonstabilized <i>N</i>-methyl azomethine ylide with electron-poor [1,2,4]triazolo[1,5-<i>a</i>]pyridines was studied. 6,8-Dinitro derivatives afforded bis-adducts of 2 equiv of 1,3-dipole, while mononitro [1,2,4]triazolo[1,5-<i>a</i>]pyridines gave various adducts depending on the nature of substituents in the pyridine cycle. Isoelectronic (4 + 2) cycloaddition of electron-rich 1,3-dienes proceeds only with the most electrophilic 6,8-dinitro[1,2,4]triazolo[1,5-<i>a</i>]pyridines to give three- and four-nuclear polycyclic systems. Both reactions were found to be diastereoselective, generally leading to <i>trans</i>-(bis)adducts.