Asymmetric hetero Diels–Alder reaction of Danishefskyʹs dienes and glyoxylates with chiral bis(oxazolinyl)phenylrhodium(III) aqua complexes, and its mechanistic studies

Asymmetric hetero Diels–Alder reaction of Danishefskyʹs dienes with glyoxylates is catalyzed in high enantioselectivity and cis (endo)-diastereoselectivity by chiral (Phebox)RhCl2(H2O) complexes [Phebox=2,6-bis(oxazolinyl)phenyl], via the concerted [4+2] mechanism with perpendicular conformation of two carbonyl moieties of glyoxylates. Dibromide and difluoride complexes were newly synthesized and found to exhibit a slightly higher enantioselectivity of the hetero Diels–Alder products than the parent dichloride complex (Cl: 80% ee, Br: 82% ee, F: 84% ee).