Catalytic Asymmetric Aza Diels-Alder Reactions Promoted by Chiral Ytterbium Catalysts

3 Catalytic Asymmetric Aza Diels-Alder Reactions Promoted by Chiral Ytterbium Catalysts 

The authors assumed the transition state of the reaction was that shown in Sch. 5, in which the imine was fixed by bidentate coordination to Yb and the additive (DTBP) interacted with the phenolic hydrogen of the imine. Because the top face of the imine was shielded by DBU, the dienophile would approach from the bottom face, affording high selectivity. 

4 Catal54ic Asymmetric 1,3-Dipolar Cycloaddition Reactions Promoted by Chiral Yb Catalysts 

3- Dipolar cycloadditions between nitrones and alkenes to give isoxazolines are very important reactions in organic synthesis, because 1,3-amino alcohols can be synthesized from isoxazoline derivatives. Jprgensen and co-workers reported catalytic asymmetric 

3- dipolar cycloaddition reaction by use of a Ln-PyBOX complex [39]. The catalyst was prepared from Yb(OTf)3, 4-A molecular sieves, and PyBOX, and the reaction was performed between alkenes and nitrones. Enantiomeric excesses were up to 73% (Sch. 6) combination of Yb(OTf)3 and binaphthol gave a racemic product. 

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