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2005
Isoxazole derivatives
R 0240
Studies on the Catalytic Hydrogenation of Baylis—Hillman Derivatives of SubstiIsoxazolecarbaldehydes. Unusual Retention of Isoxazole Ring During
08- 126 tuted
Pd—C-Promoted hydrogenation of Baylis—Hillman Adducts. — Raney-Ni catalyzed hydrogenation of substrates (I) and (VII) results in stereoselective formation of
the enaminones (II) and (VIII). They are potent precursors of α-hydroxy-1,3-diketones
and furanones. The stereoselectivity can be improved by addition of boric acid. In contrast, under Pd-C catalysis only reduction of the methylene group occurs. The expected
isoxazole ring cleavage is not observed. Hydrogenation of the acetate analogues proceeds with concomitant elimination of the acetate group. Starting from the
Baylis—Hillman adducts (XIV) the expected enaminones cannot be isolated. They
spontaneously undergo cyclization to the pyridones (XVI) and decomposition to the
enaminones (XV). Their acetates give the unsaturated enaminones (XIX) both in the
presence of Raney-Ni and Pd-C. — (SAXENA, R.; SINGH, V.; BATRA*, S.;
Tetrahedron 60 (2004) 45, 10311-10320; Div. Med. Chem., Cent. Drug Res. Inst.,
Lucknow 226 001, India; Eng.) — Jannicke
2005
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