期刊
TETRAHEDRON
卷 58, 期 15, 页码 2929-2935出版社
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/S0040-4020(02)00201-6
关键词
asymmetric catalysis; exo-Diels-Alder reaction; lanthanides; pyridine-bis(oxazolines)
The typically endo-selective Diels-Alder reactions of cyclopentadiene with acryloyl- and (E)-crotonoyl-oxazolidin-2-ones have been studied to find exo-selective catalysts. Whereas bis(oxazoline)-based catalysts promote high degree of endo-selectivity (with high ee), those derived from 2,6-bis[(4'R,5'R)-diphenyl-1',3'-oxazolin-2'-yl]pyridine (pybox) and the triflates of Eu-III La-III and Ce-IV induce different stereo selectivity. Not only the exo products are obtained with Eu-III in more than 50% yield, but the enantioselectivity is excellent (more than 99% ee). The absolute configuration of the previously unknown exo cycloadduct was unambiguously determined. A stereochemical model is proposed for the activated substrate-catalyst complex which suggests that the excellent efficiency of diphenyl-substituted pybox, compared to that of the corresponding 4-phenyl-substituted ligand, is due to the substituent in the 5-position, suitably placed to blind the Re-face of the coordinated reagent. (C) 2002 Elsevier Science Ltd. All rights reserved.
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