An entry into the novel tetracyclic system 6 H ‐pyrido[1′,2′:4,5]pyrazino[1,2‐ a ]benzimidazole. Synthesis and conformational study of the 1,2,3,4,13,13a‐hexahydro‐3,3‐ethylenedioxy and 3‐ketone derivatives

The synthesis of the tetracyclic title compounds, acetal 5 and ketone 6 , is presented. The key step, formation of the imidazole ring to give compound 5 , involved the acid catalysed dehydration of the 2‐( o ‐aminophenyl)lactam 7b . This was generated from lactam 4 via N ‐substitution with o ‐nitrofluorobenzene and reduction of the nitro group. Deprotection of acetal 5 afforded ketone 6 which through a temperature dependence study of vicinal coupling constants was shown to occur as an equilibrium of trans ‐ and cis ‐fused forms A and B .