Heterodiene cycloadditions of 3-acylchromones with enol ethers

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Abstract
3-Formylchromones behave as heterodienes in stereoselective (4π+ 2π)cycloadditions with enol ethers at 20 °C, the formal endo-addition product predominating. An electron-donor substituent (OH, OMe) in the aromatic ring retards the reaction and slightly increases endo selectivity, while an electron-withdrawing group (NO2) has the reverse effect. 3-Acetyl- and 3-benzoyl-chromone react much more slowly but with similar stereoselectivity. The reaction of 3-formylchromone (3a) with t-butoxyethene is almost non-selective, while with dihydropyran it forms novel isomeric pyrano[2′,3′ : 6,5]pyrano[4,3-b][1]benzopyranones (23) and (24) in which the enol ether geometry is retained. Reaction rates and stereoselectivity vary slightly with solvent and temperature, and the results indicate a concerted cycloaddition mechanism with kinetically controlled stereoselection.