Pyridinium N-ylides and (arylmethylene)azol-5-ones. Reaction cascade leading to an unusual spiroisoxazolinone ring

The reaction of a series of (arylmethylene)azol-5-ones with phenacylpyridin ium salt 3 in glacial acetic acid/ammonium acetate mixture gives different results depending on the starting azolone. The isoxazol-5-ones 1 give the u nusual spirans 6 in a reaction cascade involving Michael- and retro-Michael reactions, C-alkylation, aldol addition, and diastereospecific cyclization . The reaction performed with oxazol-5-ones 11 has shown that a literature re po rt has to he corrected since no oxazolopyridines 12 hut rather arylid eneimidazol-5-ones 13 are produced. In the case of pyrazolin-5-ones 14, the unavoidable formation of bis-adducts 15 always prevents any other type of reaction. (C) 2000 Elsevier Science Ltd. All rights reserved.