Forty Years after "Heterodiene Syntheses with α,β-Unsaturated Carbonyl Compounds": Enantioselective Syntheses of 3,4-Dihydropyran Derivatives.

This review is focused on the enantioselective synthesis of 3,4-dihydropyran derivatives, whose importance as chiral building blocks in the synthesis of bioactive molecules and natural products is well established. The review analyzes the different synthetic strategies by grouping them as a function of the atom numbers of the reagents involved. Starting from the classical [4 + 2] and [2 + 4] approaches, the [3 + 3], [5 + 1], and [6] strategies have been sequentially analyzed, and for each of them, the asymmetry induced by both chiral metal complexes and different kinds of organocatalysts has been examined. More than 400 papers have been reviewed, whose results have been described in the highest synthetic manner, in the attempt to emphasize the mechanism of the chirality transfer from the chiral messengers to the reaction products. This analysis allows the great flexibility of the diverse catalytic systems, the complementary of the results obtained from the different reaction pathways, and the very high level of control of the achievable molecular complexity to be evidenced.