Parallel Strategies for the Synthesis of Annulated Pyrido[3,4-b]indoles via Rh(I)- and Pd(0)-Catalyzed Cyclotrimerization.

Two different pathways for the synthesis of annulated pyrido[3,4-b]indoles are reported using metal-catalyzed cyclotrimerization reactions. A stepwise process using Rh(I)-catalysis in the final step of the synthesis and a multicomponent, tandem catalytic approach using Pd(0)-catalysis both lead to complex nitrogen-containing heterocycles in good yields. Substituent effects are investigated for both pathways, demonstrating that the Pd(0)-catalyzed approach is more sensitive to electron- withdrawing groups.