α-Acyliminium ion-acetylene cyclisations

Cyclisation of acetylenic ethoxylactams 1b-12b leads to bridgehead nitrogen bicyclic ketones in excellent yields. The reaction is weakly acid-catalysed and proceeds at ambient temperature. The observed regioselectivity effect is discussed in terms of stability of exo vs endo vinyl cations and ring strain effects. The high yield conversion of N-(5-hexynyl)-ethoxy lactams 7b and 8b in the 5/8 and 6/8 fused bicyclic ketolactams 7c and 8c deserves special attention.