(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 (SS)-5-phenyl morpholin-2-one and undergoes regio- and diastereocontrolled catalysed Diels-Alder reactions. Copyright (C) 1996 Elsevier Science Ltd