Nitrogen bridgehead compounds. Part 80 unusual reaction of ethyl 9‐bromo‐4‐oxo‐6,7,8,9‐tetr ahydro‐4 h ‐pyrido[1,2‐ a ]pyrimidine‐3‐carboxylates and N ‐Methylaniline

The acid‐catalysed intramolecular nucleophilic addition of the phenyl ring to the C(9a) = N(1) double bond of ethyl 9‐( N ‐methyl‐ N ‐phenyl)‐4‐oxotetrahydro‐4 H ‐pyrido[1,2‐ a ]pyrimidine‐3‐carboxylates, formed in the reactions of ethyl 9‐bromo‐4‐oxo‐6,7,8,9‐tetrahydro‐4 H ‐pyrido[1,2‐ a ]pyrimidine‐3‐carboxylates and N ‐methylaniline, gave the first examples of a new tetracyclic pyrimido[1′,2′:1,2]pyrido[3,2‐ b ]indole ring system ( 7 ). X‐ray diffraction analysis of 7a revealed that the annelation of the pyrimidine and piperidine rings is transoid, while that of the piperidine and pyrroline rings is cis , the piperidine ring adopts an unusual 6 T 8 twisted boat conformation, while the pyrroline ring has a 9 T 8a conformation.