Preparation of α,β‐Unsaturated γ‐Keto Aldehydes and New Tetronic Acid and Pyridazine Derivatives by Oxidative Transformations of Alkoxyallene‐Based Dihydrofurans

Oxidation of 3‐alkoxy‐substituted dihydrofuran derivatives 6 and 11 with DDQ unexpectedly provided α,β‐unsaturated γ‐keto aldehydes 10 and 12 . A mechanism for this new oxidative ring‐cleavage is presented. Since α,β‐unsaturated γ‐keto aldehydes are versatile intermediates, other 3‐methoxy‐substituted dihydrofuran derivatives 24 , 26 , and 28 were prepared from lithiated methoxyallene and the corresponding aldehydes. Oxidation of dihydrofuran derivatives with DDQ and subsequent treatment with sodium chlorite furnished hydroxy‐substituted tetronic acid derivatives, such as 30 and 31 . Condensation of 30 with hydrazine provided the unsaturated pyridazinone derivative 32 . A second route to pyridazine derivatives involves DDQ‐mediated oxidation of dihydrofurans and reaction of the products with hydrazine hydrate. This leads to 4‐methoxypyridazines 33 , 34 , 35 , and 36 in good overall yields. The oxidative transformations of dihydrofuran derivatives reported here demonstrate new examples of reactivity umpolung; the lithiated alkoxyallenes are equivalents of the unusual synthons B and C , which represent anions of malondialdehyde or malonaldehydic acid. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)