DIASTEREOSELECTIVE RADICAL BROMINATION OF 5,6-DIHYDRO-4H-1,2-OXAZINESAND SUBSEQUENT SUBSTITUTION-REACTIONS WITH NITROGEN NUCLEOPHILES

5,6-Dihydro-4H-1,2-oxazines 1a-b, 2, and 3 are easily brominated at C- 4 with N-bromosuccinimide/dibenzoyl peroxide in tetrachloromethane. Th e bromo substituent is incorporated with surprisingly high diastereose lectivity trans to the substituent dt C-6.4-Bromo-5,6-dihydro-4H-1,2-o xazines are useful reagents for substitution reactions with N-nucleoph iles such as primary amines and azide ions. Inversion of configuration at C-4 provides derivatives of 4-amino-1,2-oxazines with uniform rela tive configuration. As a minor byproduct the dibromo adduct 7 is obtai ned by bromination of 3. The dehydrohalogenation of this compound allo ws the synthesis of the 4-bromo-6H-1,2-oxazine 12. The presented react ion sequence thus constitutes an 'umpolung'' reaction that allows the introduction of nucleophiles into a position of the oxazine ring that so far was accessible only for electrophiles. The diastereoselectivity of the bromination reaction is discussed.