Synthesis of 3-Acyl-4-quinolones via Reductive Ring Transformation of 4-(2-Nitrobenzoyl)isoxazoles

4-(2-Nitrobenzoyl)isoxazoles are readily available from 3,5-disubstituted 4-iodoisoxazoles through iodine–lithium exchange and trapping with 2-nitrobenzaldeyde, followed by Jones oxidation of the obtained secondary alcohols. Reductive ring transformation by means of treatment with iron in acetic acid gives 2-substituted 3-acyl-4-quinolones. The mechanism of the cyclization reaction was elucidated by using appropriately substituted isoxazole building blocks and 2D NMR investigation of the products. In contrast, catalytic hydrogenation leaves the isoxazole ring untouched, whereas reduction with NaBH4/NiCl2 gives 2-substituted 3-acylquinolines in an unprecedented reductive ring transformation.