Natural and synthetic 2H-pyran-2-ones and their versatility in organic synthesis☆

In 1894 Knoevenagel published a paper in the journal Chemisehe Berichte on the reaction of formaldehyde and diethyl malonate in the presence of diethylamine as catalyst.1 He obtained the bis adduct (1). Two years later he was able to show that the reaction of benzaldehyde and ethyl acetoacetate in the presence of piperidine gives the bis adduct (2) at room temperature and at 0 °C the benzylidene-1,3-di- carbonyl (3; Scheme 1).2

1.11 – The Knoevenagel Reaction

A Formal [3 + 3] Cycloaddition Approach to Natural-Product Synthesis

Syntheses of chromenes and chromanes via o-quinone methide intermediates

A detailed account regarding a formal [3 + 3] cycloaddition method using 4-hydroxy-2-pyrones and 1,3-diketones is described here. This formal cycloaddition reaction or annulation reaction is synthetically useful for constructing 2H-pyranyl heterocycles. The usage of α,β-unsaturated iminium salts is significant in controlling competing reaction pathways to give exclusively 2H-pyrans. Most significantly, experimental evidence is provided to support the mechanism of this reaction that involves a sequential Knoevenagel condensation and a reversible 6π-electron electrocyclic ring-closure of 1-oxatrienes.

A Formal [3 + 3] Cycloaddition Reaction. Improved Reactivity Using α,β-Unsaturated Iminium Salts and Evidence for Reversibility of 6π-Electron Electrocyclic Ring Closure of 1-Oxatrienes

A. Trius

Papers

The reactivity of 4-hydroxy-6-methyl-2-pyrone towards aliphatic saturated and α,β-unsaturated aldehydes

Reactions between 4‐hydroxy‐6‐methyl‐2‐pyrone and aldehydes both in the absence and the presence of added thiols