Asymmetric Michael addition of ketones to nitroolefins: pyrrolidinyl-oxazole-carboxamides as new efficient organocatalysts
Chiral pyrrolidinyl-oxazole-carboxamides were synthesized and used as efficient new organocatalysts for the asymmetric Michael addition of ketones with nitroalkenes under solvent-free conditions. Gratifyingly, the corresponding Michael adducts were obtained in higher yields (up to 99%) and excellent stereoselectivities (up to >99/1 dr and 99% ee). Transition state models have been proposed to account for the high enantio- and diastereoselectivity of these Michael addition reactions and also the energetics have been investigated using density functional methods. These results support the preferential formation of syn-products by the approach of trans-β-nitrostyrene through the re-face of anti-enamine.