S-Phenscylation of 6-Methyl-4-oxo-2-thio-2,3-dihydro-4H-1,3-oxazine and the Ring Transformation of the S-Phenacylated Compound

1,3-Oxazine 1 reacted with phenacyl bromide in water using sodium hydroxide as a base to give S-phenacylated derivative 2, but the homogeneous conditions such as NaH in ' IRT or NaOMe in methanol caused the base-induced cycloreversion of 1, resulting in the formation of thiocyanate 2. For the purpose of the intramolecular transformation to pyrimidine-fused heterocycle 11, ;was treatedwith amonia in refluxing ethanol, but gave amidamine ewhile & was obtained with amonim acetate in refluxing acetic acid.Although the direct ring closure failed, the target was finally synthesized by the cyclodehydration of ketamideI with phosphorus pentoxide at 250•‹C.Previously we have demonstrated the intramolecular displacement of oxygen in the 1,3,4oxadiazole ring with the imine nitrogen derived from the reaction of S-connected side chain ketone and m n i a as shown in eq 1 .' To explore such a transfomtion systematically, we have turned our attention to six membered ring, 6-methyl-4-0~0-2-thio-Z,3-dihydro-4~,3- oxazine Q ) , which possesses cyclic vinyl imino-ether moiety for the desired ring transformation and which, in fact, was reported to be converted into 6-methyl-2-thiouracil on treatment with m n i a as the intermolecular case.2For this purpose we here report at first S-phenacylation of and secondly the reaction of S-phenacyl derivative 2 with mines in 16. cornpound 6 invariably crystallized with inclusion of 1/5 mol of ethanol, which was also 1 observed by H NMR.17.I t s spectral data are in good agreement with those reported i n the literature.