Product selectivity of thermal Buchner reaction of methyl 2-(3-arylisoxazol-5-yl)-2-diazoacetates with benzene, naphthalene and mesitylene, and ring-opening/closing reaction of products

Methyl 2-(3-arylisoxazol-5-yl)-2-diazoacetates were synthesized and the product selectivity of their thermal Buchner reaction with benzene, naphthalene and mesitylene was studied. The reaction of the isoxazolyl-substituted diazoacetates with benzene gave a mixture of cycloheptatriene/norcaradiene isomeric adducts, which are in a rapid equilibrium. Heating the same diazo compounds with naphthalene led exclusively to stable norcaradiene adducts. In contrast, their thermal reaction with mesitylene gave only the products of carbene insertion into C–H bond of mesitylene. According to DFT calculations, such a difference in the product selectivity of the thermal Buchner reaction of the diazoacetates with benzene, naphthalene and mesitylene is associated with the fact that the first two reactions proceed under kinetic control, and the last one under thermodynamic control. Isoxazoles synthesized were converted to a new family of functionalized enaminones, (Z)-methyl 5-amino-2,5-diaryl-3-oxo-2-phenylpent-4-enoates, by reaction with Mo(CO)₆/H₂O hydrogenative reagent, wherein Mo(CO)₆ catalyzes also the retro-Buchner reactions of the norcaradiene moiety of the starting compounds. The obtained enaminones are convenient precursors of 3,6-diaryl-4-hydroxy-3-phenylpyridin-2(1H)-ones, which were prepared from them under thermal conditions in high yields.