A formal [3+3] cycloaddition strategy for constructing a variety of novel fused and spiro heterocycles is described here. This formal cycloaddition involve condensations of α, β-unsaturated iminiums with 1,3-dicarbonyl equivalents. These reactions consists of a Knoevenagel condensation [an amine-assisted C-1,2-addition-reversible β-elimination] followed by a reversible 6π-electron electrocyclic ring-closure. It constitutes a step-wise formal [3 + 3] cycloaddition protocol useful for synthesis of complex heterocycles. The net result of this process is the formation of two σ-bonds in addition to a new stereocenter adjacent to the heteroatom. This formal [3 + 3] cycloaddition method can also be classified as a sequential anionic-pericyclic strategy. This review describes this strategy in details [not comprehensive] regarding its precedents, problems, solutions, stereochemical issues, mechanistic issues, and synthetic scope in natural product synthesis.
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