Stereochemistry. 82. Conformation of fused five-membered heterocyclic rings derived from the intramolecular oxime olefin cycloaddition reaction
摘要:
Thermal intramolecular oxime olefin cycloaddition Of alpha-allylamino aldoximes and ketoximes 4 led stereospecifically to formation of oxadiazabicyclo[3.3.0]octanes 6. The presence of heteroatoms in these bicyclic fused 5-membered rings permits for the first time an evaluation of the conformation of this system by means of NMR. We found that some substituents in 6 restrict the conformational mobility of these five-membered rings to the extent that only one conformer was detected at 20-degrees-C. In other cases an equilibrium between two major conformers was revealed by NMR. Equilibrium measurements indicated a free energy of conversion of 13.2-13.4 kcal/mol, apparently a manifestation of the N-inversion in the isoxazolidine ring. NMR studies also showed that the NH-proton in isoxazolidines 6 prefers an axial orientation. Empirical force field data were adjusted for MM calculations in these bicyclic heterocycles. The MM2 force field with AMBER charge gave the best fit between calculated and experimental coupling constants.
NH-Isoxazolo-bicycles; new molecular scaffolds for organocatalysis
作者:Linda Doyle、Frances Heaney
DOI:10.1016/j.tet.2011.01.034
日期:2011.3
A new scaffold for organocatalysis of the Diels-Alder reaction is disclosed; an isoxazolidine ring forms the core of the catalyst and its activity is enhanced by judicious fusion of a second five-membered heterocycle to the c-edge. The catalyst performance is improved by the incorporation of an endo-cyclic electron withdrawing group adjacent to the fusion point. The organic core is effective only in the presence of an acid co-catalyst and whilst the two-component system shows potential as an enantioselective catalyst it is demonstrated that stereocontrol is a feature of the organic core and is fully independent of the choice of co-catalyst. (C) 2011 Elsevier Ltd. All rights reserved.
HASSNER, ALFRED;MAURYA, RAKESH;MESKO, ESZTER, TETRAHEDRON LETT., 29,(1988) N 41, C. 5313-5316