A route to 1,4-prop[2]enoisoquinolines via the cyclisation of triene-conjugated nitrile ylides1

Abstract

Triene-conjugated nitrile ylides of the types 18 and 20 with α,β aromatic and γ,δ; ε,ζ olefinic unsaturation undergo intramolecular cyclisation on to the γ,δ bond via a formal 1,1-cycloaddition reaction to give respectively endo- and exo-1-alkenylcyclopropa[c]isoquinolines 23 and 25 as primary products. The endo-isomers 23 rearrange spontaneously at 0 °C to give 1,4-dihydro-1,4-prop[2]enoisoquinolines 24 via an aza-Cope reaction. The exo-isomers 25 give the same product on heating at 70 °C via equilibration with the endo form 26 as the primary step, provided that the concentration of the latter at equilibrium is not strongly disfavoured by the steric effect of substituents R¹ and R². If it is then an alternative rearrangement also occurs to give the azabenzo[3,4]barbaralane derivative 29.