A 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)-catalyzed
cascade annulation
reaction between p-quinamines and 3-formylchromones
was developed, affording a series of benzopyrone-fused hydrobenzo[c,d]indoles in moderate to high yields
with excellent diastereoselectivity. This cascade reaction is efficient
since two new rings as well as one C–N, one CC, and
two C–C bonds are created in a single step. The scale-up synthesis
and versatile transformations of the products further demonstrated
the practicality and utility of this approach.