Asymmetric [3 + 2] Cycloaddition
of Chiral N‑Phosphonyl Imines with Methyl
Isocyanoacetate for
Accessing 2‑Imidazolines with Switchable Stereoselectivity
posted on 2017-03-02, 00:00authored byShuo 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.