Pyridine synthesis by 4 + 2 cycloadditions of 1-azadienes: hetero-Diels Alder and transition metal-catalysed approaches

Cycloaddition reactions are powerful tools in synthetic chemistry. Incorporation of a nitrogen-containing component via cycloaddition represents an attractive strategy for assembling aza-heterocycles. One approach takes advantage of 4 + 2 cycloaddition reactions of 1-azadiene derivatives and 2-carbon π-components to access pyridines, a particularly significant subset of aza-heterocycles. This highlight discusses two distinct modes of reactivity within this class: the thermal pericyclic or hetero-Diels Alder (hDA) reaction and the transition metal-catalysed formal 4 + 2 cycloaddition reaction.