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Aza-Diels-Alder versus 1,3-Dipolar Cycloadditions of Methyl Glyoxylate Oxime with Cyclopentadiene
1 , 1 , * 1 , 2
1  CIQ-Departamento de Química, Faculdade de Ciências, Universidade de Porto, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal
2  Departamento de Química Orgánica, Faculdade de Farmacia, Universidade de Santiago de Compostela, E-15782 Santiago de Compostela, Spain

Abstract: The acid-catalyzed [3+2] and [4+2] cycloadditions between methyl glyoxylate oxime (1) and cyclopentadiene were investigated using various Lewis and/or Bronsted acids at different temperatures in dichloromethane as solvent. Besides the expected new adducts, (±)-methyl [(3-exo)-2-hydroxy-2-azabicyclo[2.2.1]hept-5-ene]-3-carboxylate (2) and (±)-methyl [(3-endo)-2-hydroxy-2-azabicyclo[2.2.1]hept-5-ene]-3-carboxylate (3) a third addict, (±) methyl (1R,4R,5R)-(2-ox-3-azabicyclo[3.3.0]oct-7-ene)-4-carboxylate (4), whose formation can be explained by a concerted 1,3-dipolar cycloaddition, was obtained. Yields and product ratios were found to be more dependent on the catalyst than on the temperature; these results and the stereochemistry of the adducts, confirmed by spectroscopic data (1H and 13C NMR) and by X-ray cycloadditions.
Keywords: aza-Diels-Alder reaction, 1,3-dipolar cycloaddition, glyoxylate oxiame, isoxazolidines, 2-hydroxy-2-azabicyclo[2.2.1]heptenes