Synthesis of Fused Polycyclic 1,4-Benzodiazepines via Metal-Free Cascade [5 + 2]/[2 + 2] Cycloadditions

A metal-catalyst-free,
mild, and efficient synthetic protocol for
polycyclic 1,4-benzodiazepines via cascade [5 + 2]/[2 + 2] cycloadditions
between pyridinium zwitterions and arynes is reported. Mechanistic
experiments revealed that pyridinium zwitterions act as 1,5-dipoles
in [5 + 2] cycloadditions with arynes for the construction of 1,4-benzodiazepines,
which further undergo [2 + 2] cycloaddition resulting in the one-pot
formation of one C–N bond and three C–C bonds.