(5S)-5-Phenyl-3, 4-dehydromorpholin-2-one [(5S)-5-phenyl-5, 6-dihydro-2H-1, 4-oxazin-2- one] 2a is prepared by a one-pot bromination/dehydrobromination of (5S)-5-phenyl morpholin-2-one and undergoes regio-and diastereocontrolled catalysed Diels-Alder reactions.