The reaction of the chiral lithium amide 4 with the dienoate 5a provides regio- and stereo-selectively the beta-amino esters in essentially quantitative yield with >99% diastereolsomeric excess, which can be converted upon sequential treatment with LiNPri2-B(OMe)3-MeCHO to the key intermediate 6 for the beta-lactam 7 having the correct absolute configuration.