Organocatalytic Intramolecular [4+2] Cycloaddition between In Situ Generated Vinylidene ortho ‐Quinone Methides and Benzofurans

Described herein is the enantioselective construction of oxygen‐containing [5‐6‐5] tricyclic heterocycles by an organocatalyzed asymmetric [4+2] cycloaddition of vinylidene ortho ‐quinone methides and benzofurans. According to this methodology, a series of oxygen‐containing [5‐6‐5] tricyclic heterocycles with various functional groups were synthesized in excellent enantio‐ and diastereoselectivities (>99 % ee , >20:1 d.r.). Furthermore, the deuterium‐labeling experiments and high‐resolution mass spectroscopy demonstrated that a vinylidene ortho ‐quinone methide intermediate was involved and possibly resulted from a prototropic rearrangement of 2‐ethynylphenol. Remarkably, a catalyst loading as low as 0.1 mol %, and a gram‐scale synthesis were achieved for this transformation.