Add time:08/05/2019         Source:sciencedirect.com

Asymmetric synthesis of the novel highly sterically constrained (2S,3S)-3-methyl-3-trifluoromethyl- and (2S,3S,4R)-3-trifluoromethyl-4-methylpyroglutamic acids has been developed via diastereoselective Michael addition reactions between a Ni(II) complex of the chiral non-racemic Schiff base of glycine with (S)-o-[N-(N-benzylprolyl)amino]benzophenone (BPB) and the corresponding trifluoromethyl-containing crotonates. Of particular synthetic interest is the reaction of the glycine Ni-complex with ethyl 3-trifluoromethyl crotonate featuring excellent diastereoselectivity (>98% de) as a result of complete stereochemical discrimination between the methyl and trifluoromethyl groups. A mechanistic rationale for the observed kinetically controlled stereochemical outcome is discussed.

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