An entry into the novel tetracyclic system 6H-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 .