N 89 Novel chemical modifications at the 4-position of chromones. Synthesis and reactivity of 4H-chromene-4-spiro-5-isoxazolines and related compounds [Text] / V. Ya. Sosnovskikh, B. I. Usachev, A. Yu. Sizov, M. I. Kodess // Tetrahedron Letters. - 2004. - Vol. 45, № 39. - P7351-7354 : ил. - Библиогр.: с. 7353-7354 (22 ref.) Рубрики: ХИМИЧЕСКИЕ НАУКИ Кл.слова (ненормированные): CHROMONES -- DILITHIOOXIMES -- 4H-CHROMENE-4-SPIRO-5'-ISOXAZOLINES; -- ALFA,BETA-UNSATURATED OXIMES -- BECKMANN REARRANGEMENT -- NITROSATION -- BROMINATION Аннотация: Reactions of chromones with dilithiooximes proceed via nucleophilic 1,2-addition to give, on acidification, 4H-chromene-4-spiro-5-isoxazoline derivatives in high yields. On treatment with concentrated H2SO4 the isoxazoline ring of this novel spiroannulated heterocyclic system opens to give alfa,beta-unsaturated oximes, which undergo nitrosation, bromination, and the Beckmann rearrangement to the corresponding spiroisoxazolines and alfa,beta-unsaturated amides, respectively. The latter can be obtained directly by the Beckmann rearrangement of 4H-chromene-4-spiro-5`-isoxazolines \\\\Expert2\\nbo\\Tetrahedron Letters\\2004, v. 45, p.7351.pdf |