Condensation of ethyl 3-phenyl-2-oxocyclopentanecarboxylate 5 with 2-(S-methylthio) isourea followed by hydrolysis with HCl gave 6, 7-dihydro-7-phenylcyclopenta [e][1, 3] oxazine-2, 4 (3 H, 5 H [hair space])-dione (10a). 7, 8-Dihydro-8-phenyl-6 H-cyclohexa [e][1, 3] oxazine-2, 4 (3 H, 5 H [hair space])-dione (10b) was synthesised by reacting 2- phenylcyclohexanone (9b) with N-(chlorocarbonyl) isocyanate. The oxazines 10a, b were ...