| Research Abstract |
We have previously reported that the chiral complex (10 mol%), which was prepared from 2,6-bis[(4S)-(-)-isopropyl-2-oxazolin-2-yl]pyridine ((S,S)-Pybox-iPr) and Sc(OTf)_3, was quite effected for obtaining high enantioselectivity in an asymmetric cycloaddition reaction of the carbonyl ylide genenrated from o-methoxycarbonyl-α-diazoacetophenone with benzyloxyacetaldehyde. However, the reaction with benzyl pyluvate under the same conditions did not show high enantioselevtivity, although high exo-selectivity was observed. From a survey of additives, trifluoroacetic acid (TFA, 10 mol%) was found to be an efficient additive for a high level of asymmetric induction. Thus, when the reaction was carried out in the presence of the catalyst (10 mol%), which was prepared from (S,S)-Pybox-i-Pr, Sc(OTf)_3, and TFA, at -40 ℃ by adding the diazo substrate over a period of 1 h, the corresponding cydoadducts were obtained with high diastereoselectivity (exo : endo=93 : 7) and high enantioselectivity (94
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% ee (exo)). The Sc(III)-(S,S)-Pybox-i-Pr-catalytic system including TFA was also applicable to the reactions with several α-keto esters mostly with high diastereoselectivity (exo : endo=68:12-93:7) and high enantioselectivity (80-94% ee (exo)).
On the other hand, in the reaction of the above carbonyl ylide with 3-acryloyl-2-oxazolidinone as an olefinic dipolarlophile, the chiral complex, which was prepared from 2,6-bis[(4,S)-(-)-phenyl-2-oxazolin-2-yl]pyridine ((S,S)-Pybox-Ph) and Yb(OTf)_3 in THF, was superior in terms of high enantioselectivity (96% ee) of exo-cycloadduct with high exo-selectivity (82 : 18). In the case of the reaction with 3-crotonoyl-2-oxamlidinone, the combination of Tm(OTf)_3 and S,S)-Pybox-Ph was found to be extremely valid as a chiral Lewis acid to give cycloadducts in high diastereo-(endo : exo=95 : 5) and enantioselective manners (98% ee). This rare earth metal-Pybox catalyzed method could also apply to anther diem substrate as a carbonyl ylide precursor. The reaction of N-diazoacetylpyrrolidinone with 3-acryloyl-2-oxazolidinone catalyzed by the chiral complex (10 mol%), which was prepared from Yb(OTf)_3 and 2,6-bis[(4S,5S)-(-)-diphenyl-2-oxazolin-2-yl]pyridine, gave cycloadducts with 71% ee of a major cycloadduct (major : minor = 84 : 16).
Binaphthyldiimine-Ni(II) catalysts as novel chiral Lewis acids were also developed and applied to asymmetric Diels-Alder reactions of cyclopentadiene, Michael addition reactions of silyloxyfurans, and exo-selective cycloaddition reactions of nitrones with high levels of enentioselevtivities. Less
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