Novel chiral hydrogen bond donor catalysts based on a 4,5-diaminoxanthene scaffold: application to enantioselective conjugate addition of 1,3-dicarbonyl compounds to nitroalkenes

Novel chiral hydrogen bond donor catalysts based on a 4,5-diamino-9,9′-dimethylxanthene skeleton were designed and synthesized. Among the phenylurea-amide hybrid molecules prepared from various natural/unnatural chiral amino acids, the phenylalanine-derived catalyst, and the proline-derived catalyst were successfully applied to enantioselective conjugate addition of 1,3-dicarbonyl compounds to nitroalkenes. Using 2-acetylcyclopentanone and 2-methoxycarbonylcyclopentanone as prochiral nucleophiles, asymmetric conjugate addition to β-aryl nitroalkenes proceeded with good diastereoselectivity to provide the corresponding products bearing an all-carbon quaternary stereocenter in excellent yield with up to 95% ee.