Highly diastereo- and enantioselective additions of homoenolates to nitrones catalyzed by N-heterocyclic carbenes

The diastereo- and enantioselective addition of N-heterocyclic carbene-generated homoenolates to nitrones is reported. This formal [3 + 3] addition of alpha,beta-unsaturated aldehydes and nitrones generates unusual morpholinone heterocycles which are converted into gamma-hydroxy amino esters by addition of methanol to the reaction. The use of a chiral N-heterocyclic carbene (NHC) as the catalyst provides the amino esters in high diastereoselectivity (up to 20:1 dr) and enantioselectivity (up to 94%). The process accommodates alpha,beta-unsaturated aldehydes with beta-alkyl and beta-aryl substituents. The gamma-hydroxy amino esters can be converted to optically enriched gamma-lactams by a two-step procedure involving mild reduction of the N-O bond and acid-catalyzed amide formation.