Photochemical reactions of conjugated cyclohexadienones and their aza analogues, e.g., 2-p@_dones, have been extensively studied. 1) However, those of diaza snalogues, e.g., 2(lH)pyrimidin-2-ones (I),draw little attention. 2,3) As part of a series of nodel studies of nucleoside base~,~) wewere interestedinphotoch&calbehaviours ofpyrimidinones. Inthispaper we report on the photDchemi&ry of l-substituted-4,6-dine~l-2(lH)-pyrimidin-2-ones (Ia and b).
Irradiation of a solution of l-phenyl-4,6-dimethyl-2(lH)-~imidin-2-one (IaJ in benzene in aPyrexvesselwith ahighpressuremarcurylampunder argon for15 hr atrocmtemperatwe afforded in 67% yield an iscrrer, 3-phenyl-4,6-dimethyl-2-oxo-l,3-diazabicyclo[2,2,0]hex-5-ene (IIa), to which structure was assigned on the basis of elenental analysis as well as chemical and physical data cited below. The IR spectrum shawed carbonyl absorption at 1760 cn-' and theband at 1640 an -' due to C%C stretching. The NMR spectrum shcwad a singlet at 6 1.83 (3H), a doublet at 6 2.08 (3H, J=l.5 Hz), and a quartet at 6 6.00 (IH, J=1.5 Hz) assignable to 4+le, 6-I%?, and