Synthese von 4-Alkoxy- und 3-nitrosubstituierten Isoxazolidinen durch katalysierte 1,3-dipolare Cycloadditionsreaktionen von Nitronen mit Vinylethern und Nitroolefinen
作者:Michael Meske
DOI:10.1002/prac.19973390175
日期:——
1,3-Dipolar cycloadditions of the C-phenyl-N-alkylnitrones 1,3 and the C,N-diphenylnitrone (2) with vinyl ethers 5,6 are strongly catalyzed by chiral oxazaborolidines derived from N-arylsulfonyl substituted L-alpha-amino acids valine and tert-butyl leucine and BH3 . THF complex at -22 degrees C to room temperature to give predominantly the 3,5-trans-substituted 5-alkoxy-isoxazolidines 8b - 12b, but with moderate enantioselectivity. Thermal cycloaddition afforded predominantly the 3,5-cis-substituted 5-alkoxy-isoxazolidines 8a - 12a. The cycloaddition of (E)-1-ethyl-2-nitroethene (7) afforded a mixture of cis- and trans-substituted 4-nitro-isoxazolidines 13. The ratio depends on reaction temperature and catalyst. Mild ring cleavage after quarternization of 5-ethoxy-2-methyl-isoxazolidines (8) yields the corresponding beta-amino-esters 27, 28.