Ru(II)-Catalyzed Regioselective Annulation of 2-Hydroxystyrenes with Allenyl Acetates via Vinylic C-H Activation.

Herein, we disclose an unprecedented and robust Ru(II)-catalyzed non-oxidative [5+1] annulation of 2-hydroxystyrenes with allenyl acetates to access biologically relevant chromene skeletons. The heteroatom on allene plays a pivotal role in controlling the regioselectivity of migratory insertion, and the reaction proceeds through a Ru-σ-allyl pathway, which has been elusive so far in C-H activation reactions with allenes. The protocol is sustainable in nature as it proceeds at room temperature and avoids the use of any toxic metal oxidants. In addition, the synthetic utility of the protocol was also demonstrated by late stage functionalization and modular synthesis of various natural product conjugates.