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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.

The nitrosocarbonyls (R-CONO) are highly reactive species and remarkable intermediates toward different synthetic targets. This review will cover a research area whose impact in current organic synthesis is constantly increasing in the chemical community. This review represents the first and comprehensive picture on the generation and trapping of nitrosocarbonyls and is solidly built on more than 380 papers. Six different classes of key starting materials such as hydroxamic acids, N-hydroxy carbamates, N-hydroxyureas, nitrile oxides, and 1,2,4-oxadiazole-4-oxides were highlighted. The content of the review surveys all the methods to generate the nitrosocarbonyls through different approaches (oxidative, thermal, photochemical, catalytic, aerobic, and the less common ones) in the light of efficiency, yields, and mildness. The most successful trapping agents employed to catch these fleeting intermediates are reviewed, exploiting their superior dienophilic, enophilic, and electrophilic power. The work is completed by paragraphs dedicated to the detection of the intermediates, theoretical studies, and insights about the challenges and future directions for the field.