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The first synthesis of isoxazole-4-carboxylic acid derivatives by domino isoxazole-isoxazole isomerization is reported. Fe(II)-catalyzed isomerization of 4-acyl-5-methoxy-/5-aminoisoxazoles (dioxane, 105 °C) leads to the formation of isoxazole-4-carboxylic esters and amides in good yields. 4-Formyl-5-methoxyisoxazoles give methyl oxazole-4-carboxylates under the same reaction conditions. Fe(II)-catalyzed isomerization of 4-acyl-5-methoxyisoxazoles under milder conditions (MeCN, 50 °C) allows the preparation of transient 2-acyl-2-(methoxycarbonyl)-2H-azirines. The azirines isomerize quantitatively either into isoxazoles under catalytic conditions (dioxane, 105 °C) or into oxazoles under noncatalytic thermolysis (o-dichlorobenzene, 170 °C). The mechanism of the isomerization and dependence of the reaction routes on substituents at starting isoxazole core and reaction conditions are discussed on the basis of DFT calculations.
Original language | English |
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Pages (from-to) | 15567-15577 |
Number of pages | 11 |
Journal | Journal of Organic Chemistry |
Volume | 84 |
Issue number | 23 |
Early online date | 28 Oct 2019 |
DOIs | |
State | Published - 6 Dec 2019 |
ID: 49487604