Synthesis and reactions of methylbenzo[c]quinolizinium salts

Six isomers of the methylbenzo[c]quinolizinium salt 3 including four new monomethyl derivatives were synthesized by thermal-intramolecular quaternization of the cis-methyl-substituted 2-[2-(2-chlorophenyl)-vinyl]pyridines 4 or by the irradiation of trans-4 with selected wavelengths (290 400 nm) in acetonitrile. Among the regioisomeric monomethyl derivatives 3, the 1-, 3-, and 6-methyl derivatives 3b, 3d, and 3g reacted with p-methoxybenzaldehyde in the presence of bis(l-piperidino)-(p-methoxyphenyl)-methane 7 to yield trans-(p-methoxystyryl)benzo[c]quinolizinium salts 6. The reactivity of 3 and methylbenzo[a]quinolizinium salts 1 was discussed on the basis of their π-electron energy.