1,2‐fused pyrimidines. V. Synthesis of 1‐alkyl or phenyl‐2 H ‐dipyrido‐[1,2‐ a :2′,3′‐ d ]pyrimidine‐2,5(1 H )‐diones

The reaction of 2‐[( N ‐acyl, N ‐alkyl or phenyl)amino]‐4 H ‐pyrido[1,2‐ a ]pyrimidin‐4‐ones 8a‐g with the N,N ‐dimethylformamide/phosphorus oxychloride Vilsmeier reagent 1 (95°, 90 minutes) afforded 1‐alkyl or phenyl‐2 H ‐dipyrido[1,2‐ a :2′,3′‐ d ]pyrimidine‐2,5(1 H ) − diones, 3‐alkyl substituted or not, 10a‐g . The starting compounds 8 were prepared by treating 2‐amino‐4 H ‐pyrido[1,2‐ a ]pyrimidin‐4‐ones N ‐alkyl substituted 7a,b or N ‐phenyl substituted 4 with excess anhydrides (130°, 7 hours) when the 2‐(alkylamino) derivatives 7 were used in the reaction, compounds 8 were obtained along with very small amounts of 3‐acyl‐2‐(alkylamino)‐4 H ‐pyrido[1,2‐ a ]pyrimidin‐4‐ones 9 .