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

New optically active 2,2'-bis(oxazolyl)-1,1'-binaphthyls (boxax) bearing va rious substituents (methoxycarbonyl, trimethylsilyl, (dimethylamino)carbony l, formyl, and iodo) at their C3 and C3' positions were prepared via ortho- lithiation promoted by the oxazolyl groups. The 3,3'-disubstituted borax li gands showed much higher enantioselectivity than the 3,3'-unsubstituted one s in the palladium(II)-catalyzed Wacker-type cyclization of an o-allylpheno l. 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-dimethyl oxazol-2-yl)-3,3'-bis(methoxycarbonyl)-1,1'-binaphthyl gave (S)-2-ethenyl-2 -methyl-2,3-dihydrobenzofuran of 96% ee.