Discussion
The major product of the following reaction is :
- A.
- B.
- C.
- D.
Answer: D
The keyed product is the open-chain amino ketone CH3COCH2CH(NH2)CH3, option D.
The weakest bond of an isoxazole is the N-O bond, and H2 over Pd/C cleaves it. Opening the ring gives a β-amino enone (an enaminone), and with the hydrogenation continuing the C=C and C=N are reduced as well, so the carbonyl and an amine on a saturated chain remain. That is D, 4-aminopentan-2-one. The options A, B and C all keep the N-O bond, which a Pd/H2 reduction does not leave intact: B is the unchanged starting material, and A and C are ring-saturated isoxazolines that would themselves be opened by the same conditions.
The enaminone intermediate is not offered, so D is the best of the choices; whether the reduction stops at the enaminone depends on conditions, and the key reads it as going all the way.
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