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-amino-phenyl)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 constant was shown to occur as an equilibrium of trans- and cis-fused forms A and B.