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Atropisomerism in amidinoquinoxaline N-oxides: effect of the ring size and substituents on the enantiomerization barriers
Fecha
2015-01Registro en:
Díaz, Jimena Estela; Vanthuyne, Nicolas; Rispaud, Hélène; Roussel, Christian; Vega, Daniel Alberto; et al.; Atropisomerism in amidinoquinoxaline N-oxides: effect of the ring size and substituents on the enantiomerization barriers; American Chemical Society; Journal of Organic Chemistry; 80; 3; 1-2015; 1689-1695
0022-3263
Autor
Díaz, Jimena Estela
Vanthuyne, Nicolas
Rispaud, Hélène
Roussel, Christian
Vega, Daniel Alberto
Orelli, Liliana Raquel
Resumen
The atropisomerism of novel 2,3-dihydro1H-pyrimido[1,2-a]quinoxaline 6-oxides 1 bearing dissymmetric (ortho-substituted) 5-aryl residues and the homologous 1,2-dihydroimidazo[1,2-a]quinoxaline 5-oxides 2 was investigated. The existence of a chiral axis was demonstrated for compound 1a by X-ray diffraction and by DFT calculations of the ground state geometry. The resolution of the atropisomeric enantiomers on chiral stationary phases is reported. The barriers to enantiomerization were determined by off-line racemization studies and/or by treatment of the plateau-shape chromatograms during chromatography on chiral support. A clear ring size effect was evidenced. In all cases, six-membered amidine derivatives 1 showed higher barriers than the corresponding lower homologues 2, which also display lower sensitivity to the substituent size. Transition states for the interconversion of the atropisomers were located using DFT calculations, and involved the interaction of the ortho substituent with the formally sp2 nitrogen in the amidine moiety. In contrast, in the most favoured enantiomerization transition state of the 2-nitro derivative the ortho substituent is close to the N-oxide group.