Ring Expansion of 1,2,3,4-Tetra­hydro­iso­quino­lines to Dibenzo[<i>c,f</i>]azonines. An Unexpected [1,4]-Sigmatropic Rearrangement of Nitrile-Stabilized Ammonium Ylides

When the products of a Strecker reaction of 1,2,­3,4-tetra­hydro­iso­quino­lines with aromatic aldehydes are quaternized with alkyl triflates and subsequently treated with base, a ring expansion to 6,7,­8,13-tetra­hydro-5H-dibenzo­[c,f]­azonine-5-carbo­nitriles takes place. The nine-membered cyclic products can be obtained in good yields (78–89%) in a process involving the [1,4]-sigmatropic rearrangement of a nitrile-stabilized ammonium ylide. The reaction sequence provides a new, simple, and efficient method for the synthesis of these unusual N-heterocycles.