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The Cu(II)-catalyzed remote enantioselective addition of 3-cyano-4-methylcoumarins to tryptanthrin-derived N-Boc ketimines is reported. The protocol tolerates variations in both the tryptanthrin and cyanomethyl coumarin parts, and the corresponding products are obtained with up to 99% ee and up to 96% yield. In addition to stereofacial selective activation of ketimines, the catalyst should also meet the steric demand of the nucleophiles to achieve high levels of enantioselectivity. A gram-scale reaction was possible, and X-ray diffraction confirmed the absolute configuration of the products. Moreover, the derivatization reaction of the product proceeded smoothly.
Xue et al. (Thu,) studied this question.