The reaction of a series of (arylmethylene)azol-5-ones with phenacylpyridinium salt 3 in glacial acetic acid/ammonium acetate mixture gives different results depending on the starting azolone. The isoxazol-5-ones 1 give the unusual 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 arylideneimidazol-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.
Michael Addition versus 1,3-Cycloaddition Reactions of Pyridinium Ylides with (Arylmethylene)isoxazol-5-ones: Diastereoselective Formation of 4-[1-Aryl-2-(1-pyridinio)ethyl]isoxazolium-5-olates
The reactions of (arylmethylene)isoxazol-5-ones 1 with pyridiniumylides 2 lead to the diastereoselective formation of olates 4. The reasons for the favored Michael addition and the unsuccessful subsequent cyclization to fused O-heterocycles are discussed on the basis of X-ray analysis as well as 1H-NMR and chemical evidences.