The reaction of 5-bromo-1-(2-bromoethyl)-6-bromomethyl-3-methyl-2,4(1H,3H)-pyrimidinedione (1) with the sodium alkoxide/2-nitropropane system and that of the dinitrate (2) of 5-bromo-1-(2-hydroxyethyl)-6-hydroxymethyl-3-methyl-2,4(1H,3H)-pyrimidinedione with sodium alkoxide provide convenient routes to a wide range of 3,4-dihydro-pyrimido[6,1-e][1,4]oxazine derivatives (3) in excellent yields.