Studies on the catalytic hydrogenation of Baylis–Hillman derivatives of substituted isoxazolecarbaldehydes. Unusual retention of isoxazole ring during Pd–C-promoted hydrogenation of Baylis–Hillman adducts

Results of the catalytic hydrogenation of Baylis–Hillman adducts obtained from substituted 3-, 4- and 5-isoxazolecarbox-aldehydes and their corresponding acetates in the presence of Raney-Ni and Pd–C are presented. The hydrogenation of Baylis–Hillman adducts of substituted 5-isoxazolecarbaldehydes and 3-isoxazolecarbaldehydes in the presence of Raney-Ni furnishes diastereoselectively syn enaminones over anti and in the presence of boric acid as an additive further enhancement of diastereoselectivity in favor of syn isomer is observed. The Pd–C-promoted hydrogenation of these substrates is also diastereoselective in favor of syn isomer but occurs without the hydrogenolysis of isoxazole-ring. The presence of boric acid as an additive in this hydrogenation exhibits no pronounced effect on diastereoselectivity. The Raney-Ni-mediated hydrogenation of Baylis–Hillman adducts of substituted 4-isoxazolecarbaldehydes yield pyridone derivatives and Pd–C-promoted hydrogenation of the same substrate is diastereoselective to afford the anti isomer of the resulting products. The enaminones derived from Baylis–Hillman adducts of 3- and 5-isoxazolecarbaldehydes serve as versatile precursors for α′-hydroxy-1,3-diketones, which undergo acid-catalyzed ring-closure reaction to afford the furanone derivatives in excellent yields.