Nickel catalysts chiral oxazoline ligands

Phenols have been condensed with alkenoylesters to give chromans by an oxa-Michael addition/electrophilic aromatic addition sequence with magnesium(II)- or copper(II)-bis-oxazoline complexes as chiral Lewis acid catalysts (Scheme 17b) [97]. This reaction may be initiated by an oxa-Michael reaction, followed by a hydroarylation of a carbonyl group. The authors suggest that the initial stereodetermining oxa-Michael addition is followed by a fast diastereoselective aromatic substimtion [97]. A nickel Lewis acid, derived from Ni(hfacac)2 (hfacac = 1,LL5,5,5-hexafluoro-3,5-dioxopentane enolate) and chiral Al-oxide ligands, catalyzes the enantioselective oxa-Michael cyclization of 2-tert-butyloxycarbonyl-2 -hydroxy-chalcones to 3-ferf-butoxycarbonyl flavanones, which can be decarboxylated to flavanons in a separate step (Scheme 17c) [98]. A Lewis acid activation of the unsaturated p-ketoester unit can be assumed. 

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