Design and Preparation of 3,3'-Disubstituted 2,2'-Bis(oxazolyl)-1,1'-binaphthyls (boxax): New Chiral Bis(oxazoline) Ligands for Catalytic Asymmetric Wacker-Type Cyclization

J Org Chem. 1999 Mar 5;64(5):1620-1625. doi: 10.1021/jo982104m.


New optically active 2,2'-bis(oxazolyl)-1,1'-binaphthyls (boxax) bearing various substituents (methoxycarbonyl, trimethylsilyl, (dimethylamino)carbonyl, formyl, and iodo) at their C3 and C3' positions were prepared via ortho-lithiation promoted by the oxazolyl groups. The 3,3'-disubstituted boxax ligands showed much higher enantioselectivity than the 3,3'-unsubstituted ones in the palladium(II)-catalyzed Wacker-type cyclization of an o-allylphenol. The cyclization of (E)-2-(2-methyl-2-butenyl)phenol (10) in the presence of a cationic palladium catalyst coordinated with (S)-2,2'-bis(4,4-dimethyloxazol-2-yl)-3,3'-bis(methoxycarbonyl)-1,1'-binaphthyl gave (S)-2-ethenyl-2-methyl-2,3-dihydrobenzofuran of 96% ee.