Stereoselectivity of cyclisations via N-acyliminium ions to form pyrido[2′,3′:3,4]pyrrolo[2,1-a]isoindole, -isoquinoline and -benz[c]azepine ring systems

Pyrido[2′,3′:3,4]pyrrolo[2,1-a]isoindole, -isoquinoline and -benz[c]azepine derivatives are obtained by heating in polyphosphoric acid (PPA) appropriate hydroxy lactam precursors derived from pyridine-2,3-dicarboximides. The stereoselectivity of ring closure is rationalised by considering the development of A(1,3) strain in the cyclisation step from N-acyliminium ion intermediates.