Keteniminium Ion-Initiated Cascade Cationic Polycyclization

A novel and efficient keteniminium-initiated
cationic polycyclization
is reported. This reaction, which only requires triflic acid or bistriflimide
as promoters, affords a straightforward entry to polycyclic nitrogen
heterocycles possessing up to three contiguous stereocenters and seven
fused cycles. These complex, polycyclic molecules can be obtained
in a single operation from readily available ynamides which were shown
to be remarkable building blocks for multiple, consecutive cationic
transformations.