Acid-catalyzed aza-Diels-Alder versus 1,3-dipolar cycloadditions of methyl glyoxylate oxime with cyclopentadiene

The acid-catalyzed 1,4- and 1,3-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 adduct, (+/-)-methyl (1R,4R,5R)-(2-oxa-3-azabicyclo[3.3.0]oct-7-ene)-4-carboxylate (4), whose formation can be explained by a 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 (H-1 and C-13 NMR) and by X-ray crystallography, were used to analyze and propose a mechanistic explanation for both cycloadditions. (C) 2008 Elsevier Ltd. All rights reserved.