Rearrangements of tetrahydroimidazo[1,5-b]isoxazole-2,3-dicarboxylates to pyrrolo[1,2-e]imidazol-6-ols, precursors of 2,5-dihydro-1H-pyrrole derivatives

Isoxazolines 2 from the cycloaddition of imidazoline 3-oxides 1 with DMAD rearrange in the presence of methoxide to give cis-3-methoxy-7-(methoxycarbonyl)-2,7a-diaryl-5-oxo-2,3,5,7a-tetrahydro-1H-pyrrolo[1,2-e]imidazol-6-olates 3 with 100% de. The acidic hydrolysis of 3 led to kinetically controlled formation of methyl 1-formyl-4-hydroxy-5-oxo-2-phenyl-2-((arylamino)methyl)-2,5-dihydro-1H-pyrrole-3-carboxylates 6a–e. The intramolecular transformylations of the latter to the corresponding (E)- and (Z)-methyl 4-hydroxy-2-((N-(aryl)formamido)methyl)-5-oxo-2-phenyl-2,5-dihydro-1H-pyrrole-3-carboxylates 7a–e were shown to be substituent dependent (correlate with σ) and characterized by Hammett type equations. The effect of temperature was investigated and the ρ constants determined for the same reaction series at 50, 60 and 70 °C. The amide diastereomeric ratio [(E)-7]/[(Z)-7] is substituent dependent and can be described by the equation log[(E)]/[(Z)]x=−ρσI+log[(E)]/[(Z)]x=H.