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Asymmetric [3 + 2] Cycloaddition of Chiral N‑Phosphonyl Imines with Methyl Isocyanoacetate for Accessing 2‑Imidazolines with Switchable Stereoselectivity

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posted on 02.03.2017, 00:00 by Shuo Qiao, Cody B. Wilcox, Daniel K. Unruh, Bo Jiang, Guigen Li
Asymmetric [3 + 2] cycloaddition of chiral N-phosphonyl imines with methyl isocyanoacetate has been established, enabling controllable/switchable stereoselectivity access to 21 examples of cycloadducts with good to excellent chemical yields (up to 92%) and high diastereoselectivities (up to 99:1 dr). The cycloaddition reaction promoted by Cs2CO3 resulted in diastereoenriched (4R,5S)-products with >99:1 dr. However, it showed the reverse stereoselectivity as diastereoenriched (4S,5R) products when AgF was employed as the catalyst. The synthesis followed the group-assisted purification (GAP) chemistry/technology in which the pure 2-imidazoline products were readily provided by washing the crude products with cosolvents of hexane and ethyl acetate.