Catalyst-free synthesis of novel dimeric β-carboline derivatives via an unexpected [2 + 2 + 2] annulation.

We report here an unexpected catalyst-free [2 + 2 + 2] annulation reaction which allows access to novel complex dimeric β-carboline derivatives in a single step. Various substituted ynones could react with dihydro-β-carboline imines to deliver interesting [2 + 2 + 2] annulation products in moderate to good yields. Alkynoates can also be tolerated in this system.