Scaffold Diversity through a Branching Double-Annulation Cascade Strategy: Iminium-Induced One-Pot Synthesis of Diverse Fused Tetrahydroisoquinoline Scaffolds

A branching double-annulation cascade (BDAC) strategy for diverse and complex fused THIQ scaffolds via a highly reactive iminium-induced one-pot double-cyclization sequence involving Pictect–Spengler-type cyclization has been developed for the first time. The salient features of this protocol are that it allows direct and rapid access to unprecedented diverse fused THIQ skeletons, is metal/catalyst free, has a cleaner reaction profile, provides good to excellent yields, and is a convenient approach. This catalyst-free domino process facilitates the double annulation with a variety of scaffold building agents via two C–N and one C–X (X = C, N, O) bond formation in a single step under uniform reaction conditions. Furthermore, we reveal an unusual dual BDAC sequence leading to N–N-linked isoquinoline dimer.